Ecology

Class 11 Biology, unit 15, book pages 389–411

Practise 64 MCQs Take the unit quiz Review 58 cards

Ecology studies how living organisms relate to each other and to their physical surroundings. This unit follows energy through food chains, webs, trophic levels and ecological pyramids, follows materials through the water and nitrogen cycles, and shows how communities change by succession and populations change in size. It ends with the human impact on ecosystems, the greenhouse effect, ocean acidification and coral reefs, and the main ecosystems of Pakistan.

Learning outcomes (21)

Define species, population, community and ecosystem.
Distinguish between the various modes of nutrition different species possess.
Identify plants as producers for converting light energy to chemical energy.
Define trophic levels.
Discuss the loss of energy between trophic levels.
Explain the greenhouse effect with examples of gases that exhibit this behaviour.
Describe the harmful effects of greenhouse gases on the environment.
Explain with regard to ocean acidification how coral reefs are used as a barometer for the health of an aquatic ecosystem.
Define biogeochemical cycles and locate the primary reservoirs of the chemicals in these cycles.
Describe the water cycle in detail.
Define the terms aquifers and water table.
Discuss the nitrogen cycle in detail.
Describe productivity in terms of gross primary productivity and net primary productivity.
Interpret the pyramids of number, biomass and energy.
Define ecological succession as the process through which ecosystems change from simple to complex.
Describe primary and secondary succession.
Differentiate between xerarch and hydrarch succession.
Explain the xerarch succession on a bare rock starting from the small pockets of lichens to the vegetation of flowering plants.
Describe characteristics of a population, such as growth, density, distribution, carrying capacity and minimum/viable size.
Explain the effect of growth of the human population on the ecosystem.
Describe the 4 important ecosystems of Pakistan.

15.1EcosystemOften examined

The different communities of living organisms interact with their environment. The study of the relationship of living organisms with each other and with their physical surroundings is called ecology. It is a way to study the structure and function of nature.

The ecosystem is the major unit of ecology. It consists of the atmosphere, hydrosphere, soil and biological communities. It is a self-sustained geographical area where living organisms interact with their non-living environment. Plant and animal communities together with their non-separable environment form an ecosystem.

Ecology: The study of the relationship of living organisms with each other and with their physical surroundings.
Ecosystem: A self-sustained geographical area where living organisms interact with their non-living environment; plant and animal communities together with their non-separable environment.
Biodiversity: The variety of different species of plants, animals and microorganisms that coexist within an ecosystem.
Species: A group of related organisms which can interbreed among themselves freely in nature and produce fertile offspring of their kind.
Population: Members of a species living in a geographical area at a given time.
Community: Interacting populations of different species taken together. Co-evolving populations interact in several ways: they may compete for resources, one may prey over another, or they may form symbiotic relations.
Biosphere: The thin life-containing blanket surrounding the Earth. It includes all the ecosystems on Earth.

Levels of organisation in ecology: individuals of one species form a population, populations that interact form a community, a community with its non-living surroundings is an ecosystem, and all ecosystems make the biosphere.

One key characteristic of an ecosystem is its biodiversity. This diversity is essential for the stability and resilience of the ecosystem. Each species plays a specific role in the ecosystem which contributes to its overall balance and functioning. In an ecosystem different populations interact with each other and with their environment, and these interactions are essential for the survival and well-being of all organisms within it. Ecosystems vary widely in size, from small ponds to vast rainforests or even the entire planet.

Conditions that make life possible on Earth

A steady supply of energy from the Sun.
Presence of water as the medium of life.
A suitable range of temperature.
The correct proportion of minerals.
Screening of ultraviolet radiation by ozone.

15.1.1Components of ecosystemOften examined

An ecosystem comprises two basic parts: abiotic components and biotic components.

	Abiotic components	Biotic components
Meaning	The non-living factors present in the ecosystem	The living part of the ecosystem
Examples	Light, air, water, soil and the basic elements and compounds	Producers, consumers and decomposers

Components of an ecosystem.

The biotic components are further classified as producers, consumers and decomposers.

Producers

Producers are the autotrophs present in an ecosystem. They include plants, algae and photosynthetic bacteria. These organisms are able to synthesise complex organic compounds (food) from inorganic raw materials. Producers form the basis of any ecosystem. In terrestrial ecosystems, plants are the main producers. In aquatic ecosystems, the main producers are the floating photosynthetic organisms (mainly algae) called phytoplankton, and shallow-water rooted plants.

15.1.1Plants as major autotroph

Plants are the major autotrophs on land. They convert light energy into chemical energy in the process of photosynthesis.

Carbon dioxide is absorbed from the atmosphere through the stomata and then taken up by the mesophyll cells.
Water is absorbed by the roots from the soil and ascends in the xylem by transpiration pull to the mesophyll cells.
In the mesophyll cells carbon dioxide and water are used as raw materials to synthesise glucose.
This food synthesis needs energy, which is provided by light. Chlorophylls and other photosynthetic pigments trap different wavelengths of light and make it available for the energy-dependent phases of photosynthesis.

How a land plant converts light energy into chemical energy.

15.1.1ConsumersOften examined

Consumers are heterotrophs. They cannot synthesise their own food and so depend upon producers for food. Consumers include all animals, fungi, protozoans and many of the bacteria. Animals are the major consumers of ecosystems. They are further classified as herbivores and carnivores. Herbivores (for example cattle, deer, rabbit, grasshopper) feed on plants.

Classes of consumers.
Type of consumer	Feeds on	Nature	Examples from the book
Primary consumers (herbivores)	Producers directly	Herbivorous	Insects, reptiles, birds and mammals on land; crustaceans, molluscs and herbivorous fishes in water; cattle, deer, rabbit, grasshopper
Secondary consumers	Primary consumers	Carnivorous (feed on the flesh of other animals)	Many fishes, frogs, snakes, predatory birds, and mammals like lion and tiger
Tertiary consumers	Secondary consumers	Carnivores	Wolf, owl, hawk

15.1.1DecomposersOften examined

Dead bodies of living organisms are a source of energy for other organisms. The waste matter which passes out of living bodies is also a source of energy and nutrients. These materials are not wasted by the ecosystem; they are used by decomposers.

Bacteria and fungi are the major decomposers. They secrete digestive enzymes on the dead organic matter to break it down into simple molecules. They absorb some of these materials for their own use, and other substances are added to the environment. The released substances are available to producers again. This recycling of materials is necessary to maintain balance in the ecosystem.

Figure 15.1: Food chain. Energy and nutrients pass from producer to consumers; decomposers act on dead matter and return materials to producers.

Decomposers secrete digestive enzymes onto dead organic matter.
The enzymes break the matter down into simple molecules.
Decomposers absorb some of the molecules for their own use.
The rest are released into the environment, where producers take them up again.

15.1.2Food chain and food websOften examined

The energy-rich organic molecules produced by autotrophic organisms are the food source for heterotrophic organisms. Animals feeding on plants are in turn eaten by other animals. In this way energy and nutrients are transferred from one organism to another in a defined way. The sequence of organisms through which food and nutrients move in an ecosystem is called a food chain.

A single food chain taken from the grassland web: the arrows show the direction in which food and energy move.

In natural conditions food chains rarely occur as a simple linear arrangement; they are interconnected with each other, forming a food web. For example, in a grassland ecosystem, grass may be eaten by a grasshopper and a mouse. The mouse may be eaten by a snake or directly by a hawk, and the hawk also feeds on the snake. On the other hand, the grasshopper is eaten by a frog, which is eaten by a snake, and the frog is also eaten by a hawk. Food webs provide food alternatives at each trophic level. They are important to maintain balance in the ecosystem.

Figure 15.2: Food web of a grassland. Each arrow points from the organism eaten to the organism that eats it.

Food chain compared with food web.
Point	Food chain	Food web
Arrangement	One simple linear sequence of organisms	Many food chains interconnected with each other
Food choice	Only one path for food and energy	Food alternatives are available at each trophic level
Occurrence in nature	Rare in natural conditions	The usual natural condition
Importance	A single line of feeding	Important to maintain balance in the ecosystem

15.1.3Trophic levelOften examined

In a natural community each stage of the food chain is called a trophic level. Trophic levels are the hierarchical levels of the food chain through which energy and nutrients flow. They represent a position in the food chain and indicate the source of energy and nutrients of the organisms.

Trophic levels.
Trophic level	Organisms	What they feed on
First (T1)	Producers: plants, algae and some bacteria	Make their own food
Second (T2)	Primary consumers (herbivores)	Autotrophs directly
Third (T3)	Secondary consumers (carnivores)	Herbivorous animals
Fourth (T4)	Tertiary consumers (carnivores)	Other carnivores
Fifth (T5)	Quaternary consumers, present in some ecosystems	Tertiary consumers

Trophic levels of a food chain. Each higher level has less energy available and fewer organisms.

There are fewer organisms at higher levels because less energy is available there.

Worked example: trophic levels in the grassland web

Using the food web of Figure 15.2, give the trophic level and type of consumer of each organism in the chain grass → grasshopper → frog → snake → hawk. Then explain why the hawk can occupy more than one trophic level.

Organism	Trophic level	Type
Grass	T1	Producer
Grasshopper	T2	Primary consumer (herbivore)
Frog	T3	Secondary consumer (carnivore)
Snake	T4	Tertiary consumer (carnivore)
Hawk	T5	Quaternary consumer (top carnivore)

Count from the producer. Grass makes its own food, so it is T1; each animal that eats the one before it is one level higher.
The hawk is not restricted to this chain. In the web it also feeds directly on the mouse (grass → mouse → hawk), where it is a secondary consumer at T3.
It also feeds on the frog (grass → grasshopper → frog → hawk), where it is a tertiary consumer at T4.
So the trophic level of a consumer in a food web depends on the chain being followed. This is why a food web gives the hawk food alternatives at several levels.

15.1.4Primary productivityOften examined

The radiation from the Sun is the ultimate source of energy to drive life on Earth. Producers in the ecosystem trap this energy and incorporate it in the food molecules during photosynthesis. This energy flows to consumers at different levels in a non-cyclic (unidirectional) manner.

Productivity in an ecosystem is the rate at which organic matter is created by producers through photosynthesis. It is usually measured in terms of the biomass produced per unit area over a specific period of time.

Gross primary productivity (GPP): The total amount of solar energy fixed by producers in the photosynthesis process. It depends on the availability of light, water, inorganic nutrients and suitable temperature.
Net primary productivity (NPP): The amount of energy left after producers have met their respiratory needs. It appears as the organic matter or biomass of producers.

NPP = GPP - R

R = energy used by the producers in respiration

Gross and net primary productivity: NPP is what remains of GPP after the producers meet their own respiratory needs.

Worked example: net primary productivity

In a grassland the producers fix

1800 g m^{- 2} yea r^{- 1}

of organic matter (GPP) and use

700 g m^{- 2} yea r^{- 1}

in respiration. Find the NPP and the fraction of GPP that is available to the rest of the food web.

Write the relation: NPP is what is left after respiration.
$NPP = GPP - R$
Substitute the values.
$NPP = 1800 - 700 = 1100 g m^{- 2} yea r^{- 1}$
Find the fraction available as biomass.
$\frac{1100}{1800} = 0.61 \approx 61%$
This 1100 is the energy and matter available to the herbivores, carnivores and decomposers; the other 700 was used up by the producers themselves.

N P P = 1100 g m^{- 2} yea r^{- 1} (61% of GPP)

15.2Energy flow

Even under optimum conditions only about 1% of the radiation is converted in photosynthesis by producers into gross primary productivity. The energy-rich organic food prepared by producers is the source of energy for heterotrophic organisms. The energy flow in the ecosystem follows the laws of thermodynamics.

15.2.1The loss between trophic levelsOften examined

Living organisms lose energy at each trophic level. The processes responsible for this energy loss include:

respiration;
growth;
reproduction;
non-predatory death (organisms that die but are not eaten by consumers);
loss in the form of heat;
loss as excretory products or as faeces.

Thus, as energy is transferred from one level to the next there is a loss of energy. The total energy level progressively decreases down the food chain.

Figure 15.3: Energy flow in an ecosystem. Only about 10% of the energy passes to the next trophic level.

E_{next level} = \frac{10}{100} \times E_{this level}

About 10% of the energy of one trophic level reaches the next (Figure 15.3).

Worked example: following 10% down a chain

The producers of a food chain contain

50000 J

of energy. Using the 10% transfer shown in Figure 15.3, find the energy at each higher trophic level of grass → grasshopper → frog → snake, and state where the remainder goes.

Trophic level	Energy	Working
Producers (T1)	50 000 J	given
Primary consumers (T2)	5 000 J	10% of 50 000
Secondary consumers (T3)	500 J	10% of 5 000
Tertiary consumers (T4)	50 J	10% of 500

At each step multiply by $\frac{10}{100} = 0.1$ .
Total lost between T1 and T2 is $50000 - 5000 = 45000 J$ , which is 90%.
This loss is respiration, growth, reproduction, non-predatory death, heat, and excretory products or faeces, not "destroyed" energy: it leaves the chain mostly as heat.
Because energy shrinks by a factor of ten at every step, little remains for higher levels, so there are fewer organisms there.

5000 J, 500 J, 50 J

15.2.2Ecological pyramidsOften examined

Different ecosystems support different types of food chains that contain all the basic trophic levels. There is a proportionate relationship between organisms at each level. The relationships of biotic components can be represented in the form of pyramid diagrams called ecological pyramids. There are three main types, used to show the relative amounts of energy, biomass or numbers of organisms at each trophic level within an ecosystem.

(i) Pyramid of energy

It is the most useful pyramid as it represents the amount of energy at each trophic level. Producers at the bottom have the largest energy level as they fix light energy by themselves. The energy level decreases from the producers to the top consumer level. At each trophic level energy is used by organisms to perform life activities and some is lost as wastes or heat. So the pyramid of energy shows a regular pattern of decrease in energy at each level. With each step up there is an energy loss of 90 percent.

(ii) Pyramid of biomass

It represents the total biomass at each trophic level of an ecosystem. Biomass is the dry weight of all living material in the ecosystem. The largest biomass belongs to producers, and it decreases progressively in the higher trophic levels.

(iii) Pyramid of number

It represents the number of organisms at each trophic level of an ecosystem. It is not always regular in shape, but in most cases the number of individuals decreases at each level. Producers are greater in number, with fewer consumers at the top level.

Figure 15.4: Ecological pyramids of energy, biomass and numbers. Each higher trophic level has less.

The three ecological pyramids.
Pyramid	Each level shows	Pattern
Energy	Amount of energy available at that trophic level	Always decreases upwards; the most useful pyramid
Biomass	Total dry weight of living material at that level	Largest at producers, decreasing in higher levels
Numbers	Number of individuals at that level	Not always regular, but in most cases decreases upwards

15.3Biogeochemical cycleOften examined

Every organism requires nutrients for its survival. These nutrients are obtained from the environment. The movement of these nutrients in the ecosystem is cyclic. This flow of nutrients from the environment to the organisms and back to the environment is called the biogeochemical cycle.

15.3.1Primary reservoirs of nutrients

A general model of nutrient cycling includes the main reservoirs of elements and the processes that transfer elements between reservoirs. Each reservoir is defined by two characteristics: whether it contains organic or inorganic materials, and whether or not the materials are directly available for use by organisms.

Figure 15.5: A general model of the nutrient cycle. Reservoirs a to d are defined by whether they hold organic or inorganic material and whether it is directly available to organisms.

The four reservoirs of Figure 15.5.
Reservoir	Contains	Available to organisms?	Examples
a	Organic	Yes	Living organisms and detritus
b	Organic (fossilised)	No, cannot be assimilated directly	Coal, oil, peat (fossil fuels)
c	Inorganic (dissolved in water, in soil or air)	Yes, assimilated directly	Atmosphere, soil, water
d	Inorganic, tied up in rocks	No, only slowly through weathering and erosion	Minerals in rock

How nutrients move between reservoirs

Nutrients in living organisms and in detritus (reservoir a) become available to other organisms when consumers feed and when detritivores consume non-living organic matter.
Long ago, some material moved from the living organic reservoir to the fossilised organic reservoir (b) when dead organisms were buried by sedimentation over millions of years and became coal, oil or peat. Nutrients in these deposits cannot be assimilated directly.
Inorganic materials dissolved in water, in soil or in air (reservoir c) are available. Organisms assimilate them directly and return chemicals to this reservoir through the relatively rapid processes of cellular respiration, excretion and decomposition.
Elements tied up in rocks (reservoir d) cannot be tapped directly, but may slowly become available through weathering and erosion.
Unavailable organic materials move into the available inorganic reservoir when fossil fuels are burned, releasing exhaust into the atmosphere.

15.3.2Water cycleOften examined

The major reservoir of water is the ocean, which contains over 97% of the available water. The water cycle is driven by solar energy, which evaporates water, and by gravity, which draws the water back to Earth in the form of precipitation, that is rain, snow, dew and sleet (rain mingled with snow or hail).

Figure 15.6: Water cycle. Solar energy evaporates water; gravity returns it as precipitation.

Water in the environment

Water falling on land takes more varied paths. Some is evaporated from the soil, lakes and streams. A portion runs off the land back to the oceans. When rain water falls, some of the water sinks or percolates into the ground and saturates the earth to a certain level. The top of the saturation zone is called the water table.

Whenever the Earth contains a basin or channels, water appears to the level of the water table. The water within basins is called lakes and ponds, and the water within channels is called streams or rivers. Sometimes ground water is also located in underground rivers called aquifers.

Water table: The top of the zone in the ground that is saturated with percolated water.
Lakes and ponds: Water within basins.
Streams or rivers: Water within channels.
Aquifers: Ground water located in underground rivers.

Water in living bodies

The bodies of living things are roughly 70% water, so some of the water in the cycle enters the living communities of ecosystems. It is absorbed by the roots of plants and much of it is evaporated back to the atmosphere from their leaves. A small amount is combined with carbon dioxide during photosynthesis to produce high-energy molecules. Eventually these are broken down during cellular respiration, releasing the water back to the environment. Heterotrophs get water from their food or by drinking.

Steps of the water cycle (Figure 15.6).
Process	What happens
Evaporation	Solar energy turns water from oceans, soil, lakes and streams into vapour
Transpiration	Water absorbed by plant roots is evaporated from the leaves
Condensation	Water vapour changes to cloud
Precipitation	Water returns to Earth as rain, snow, dew or sleet
Run-off	A portion of the water on land flows back to the oceans
Percolation	Water sinks into the ground and saturates it down to the water table

15.3.3Nitrogen cycleOften examined

Nitrogen is required by all living organisms for the synthesis of organic molecules such as amino acids, nucleic acids and proteins. The nitrogen cycle consists of a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is a continuous cycle maintained by the decomposers and other bacteria. The cycle involves ammonification, nitrification, nitrogen fixation, assimilation and denitrification.

Figure 15.7: Nitrogen cycle. Nitrogen gas is fixed, passes through ammonia, nitrite and nitrate, is assimilated by organisms, and is returned to the air by denitrification.

1. Ammonification

The nitrogenous wastes of animals and the nitrogenous compounds of dead organisms are decomposed by saprophytic soil bacteria and fungi to form simple substances like water, carbon dioxide, amino acids and energy. The amino acids are converted into ammonia or ammonium ions. The production of ammonia or ammonium compounds in the decomposition of organic matter by microorganisms is called ammonification. It occurs in the soil, in an aerobic environment.

2. Nitrification

Some ammonia escapes into the soil, but much of it, and the ammonium ions, are converted into nitrates by nitrifying bacteria. This is accomplished by two groups of bacteria. The first group (e.g. Nitrosomonas) converts ammonia to nitrites, and the second group (e.g. Nitrobacter) converts nitrites to nitrates. This process is called nitrification. It takes place only in well-aerated soils because the bacteria responsible for it are aerobic.

NH X_{3} Nitrosomonas NO X_{2} X^{-} Nitrobacter NO X_{3} X^{-}

3. Nitrogen fixation

Nitrogen gas is composed of two atoms of nitrogen linked by a very strong triple bond. This makes it chemically unreactive, and large amounts of energy are required to break the bond. Nitrogen gas can be fixed in three ways.

Three ways of fixing nitrogen.
Type of fixation	How it happens	Detail given in the book
Atmospheric	Spontaneous, by lightning: nitrogen and oxygen combine to form various oxides of nitrogen, carried by rain into the soil where plants can use them	Only a small amount (5-8%) is fixed this way
Industrial	Synthesis of nitrogen-containing fertilizers	Manmade
Biological	Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia	Fix 60% of the nitrogen gas in the atmosphere

N X_{2} + 3 H X_{2} 2 NH X_{3}

Biological fixation: only a relatively few bacteria (the nitrogen-fixing bacteria) can carry out this reaction.

Fixed nitrogen is made available to plants by the death and lysis of free-living nitrogen-fixing bacteria, e.g. Azotobacter (aerobic) and Clostridium (anaerobic), or from the symbiotic association of some nitrogen-fixing bacteria with plants, e.g. Rhizobium.

4. Assimilation

Assimilation is the process of utilisation of nitrogenous compounds in living bodies. Many microorganisms are able to use free nitrogen directly from the atmosphere, but plants obtain nitrogen in the form of inorganic nitrogenous compounds like ammonia and nitrates from the soil, whereas animals take their nitrogen from the eating of plants or other animals.

5. Denitrification

Nitrogen can be lost as a result of the activities of certain soil bacteria. In the absence of oxygen these bacteria break down nitrates, releasing nitrogen back into the atmosphere and using the oxygen for their own respiration. This process is known as denitrification, and such bacteria are called denitrifying bacteria, e.g. Pseudomonas, which reduce nitrates in the soil to the gaseous state.

nitrate NO X_{3} X^{-} ⟶ nitrite NO X_{2} X^{-} ⟶ nitrogen oxides NO + N X_{2} O ⟶ nitrogen gas N X_{2}

Summary of the nitrogen cycle.
Process	Organisms	Starting material	Product	Oxygen
Ammonification	Saprophytic bacteria and fungi	Nitrogenous wastes and dead organisms	Ammonia or ammonium ions	Aerobic soil
Nitrification	Nitrosomonas, then Nitrobacter	Ammonia and ammonium	Nitrites, then nitrates	Well-aerated soil (aerobic bacteria)
Nitrogen fixation	Azotobacter, Clostridium, Rhizobium; also lightning and industry	Nitrogen gas	Ammonia (biological); nitrogen oxides (atmospheric)	Aerobic or anaerobic
Assimilation	Plants (from soil), animals (by eating)	Ammonia, nitrates; plants and animals	Organic nitrogen compounds in living bodies	Not stated
Denitrification	Pseudomonas and other denitrifying bacteria	Nitrates	Nitrogen gas	Absence of oxygen

Worked example: which process of the nitrogen cycle?

Name the process and, where the book names them, the organisms responsible in each case: (a) a bolt of lightning makes nitrogen oxides that the rain carries to the soil; (b) bacteria in the root nodules of a legume make ammonia from the air; (c) Nitrobacter converts nitrites to nitrates; (d) in a waterlogged soil, with no oxygen, nitrate is changed to nitrogen gas; (e) soil fungi break down a dead animal to ammonium ions.

Case	Process	Reason
(a)	Atmospheric nitrogen fixation	Lightning lets nitrogen and oxygen combine to form oxides of nitrogen
(b)	Biological nitrogen fixation (by Rhizobium)	Symbiotic nitrogen-fixing bacteria convert $N X_{2}$ to ammonia
(c)	Nitrification (second step)	The second group of nitrifying bacteria converts nitrites to nitrates
(d)	Denitrification (by denitrifying bacteria such as Pseudomonas)	Without oxygen, bacteria break down nitrates and release nitrogen gas
(e)	Ammonification	Saprophytic bacteria and fungi decompose dead organic matter into ammonia or ammonium

Use the cue words: lightning or fertilizer means fixation; nodules mean biological fixation; nitrites to nitrates means nitrification; no oxygen with nitrates means denitrification; dead matter to ammonia means ammonification.

15.4Ecological successionOften examined

A community may begin when a new habitat is created, as when a volcanic island rises out of the sea or when rock and soil are deposited by a retreating glacier. New communities may also form in regions that have been disturbed, as by fire or hurricane. Species that arrive early in a new or disturbed habitat tend to be replaced later by other species, which in turn may be replaced by other species. Earlier species are replaced by later species because the replaced species are better able to grow and reproduce under the environmental conditions of the area.

Ecological succession: The process by which species are replaced over time.
Sere: An individual succession.
Seral stages: The development phases of a sere.

The course of ecological succession: species that arrive early are replaced by later species until a relatively stable climax community forms.

15.4.1Kinds of successionOften examined

There are two kinds of ecological succession: primary succession and secondary succession.

Primary succession: The change in species composition over time in a habitat that was not previously inhabited by organisms. Bare rock surfaces, such as recently formed volcanic lava and rock scraped clean by glaciers, are sites where primary succession might occur.
Secondary succession: The change in species composition over time in a habitat already substantially modified by a pre-existing community. Soil is already present at the site. Common examples: (a) abandoned farm fields undergo secondary succession as they revert to forest; (b) succession in a forest area where vegetation has been devastated by fire, flood, cyclone and so on.

Primary compared with secondary succession.
Point	Primary succession	Secondary succession
Starting habitat	Never inhabited before: bare rock, new lava, rock scraped clean by glaciers	Already substantially modified by an earlier community
Soil	No soil at the start; it must be made	Soil is already present
Examples	Volcanic lava, glacier-scraped rock	Abandoned farm field reverting to forest; forest devastated by fire, flood or cyclone

Figure 15.8: Secondary succession: herbs and grasses, then shrubs and young trees, then a mature forest over more than 40 years.

Hydrarch and xerarch succession

Hydrarch compared with xerarch succession.
Point	Hydrarch succession	Xerarch succession
Where it begins	In ponds, lakes, marshes or elsewhere in water	On bare rocks, sand dunes, rocky slopes and similar places
Habitat condition	Water is plentiful	Deficiency of water
Stages are called	Hydroseres	Xeroseres (together, the xerosere)

Stages of succession

The first requirement of succession in any bare area is the migration of plants and animals from the surrounding areas and their aggregation.

Pioneers. The migrants that first arrive are called pioneers. They become successful in taking hold of the soil and increase in number.
By their death and decay the pioneers increase the organic matter, moisture and nitrogen content of the soil. The enriched soil becomes suitable for the next group of invaders.
Seral communities. The next groups of invaders are called seral communities, and they make up one seral stage.
By their activities, the serals modify the environment. The changed environment becomes unsuitable for their own growth, and a new group of plants and animals invades.
Climax community. After several seral communities, the end product of succession is the climax community, a relatively stable community.

15.4.2Xerarch successionOften examined

Xerarch succession begins on bare rock where water is deficient and ends in a climax community. Its stages are described below in the order in which they occur.

Figure 15.9: Xerarch succession on bare rock (primary succession): bare rock, crustose lichens, foliose lichens, mosses, herbs, shrubs, trees and the climax stage, in order of time.

1. Crustose-lichen stage

This is the pioneer stage. On bare rocks only crustose lichens (crust-like lichens) can grow. They are slow growers and can withstand extreme desiccation. When there is rain they absorb water like sponges and decompose rock by secreting acids. Important members of this stage are Licanora and Rhinodina.

2. Foliose-lichen stage

On the little soil which has accumulated on the rock there appear species of foliose lichens (leaf-like lichens), and by their activities a thin layer of soil collects. Important members of this stage are Permelia and Dermatocarpon.

3. Moss stage

When sufficient soil has accumulated in the minute crevices and depressions in the rock, xerophytic mosses begin to appear. The mosses increase the amount of soil. By their death and decay a mat may form on the rock surface. This can hold a greater amount of water and, along with the soil, makes the habitat suitable for herbs.

4. Herb stage

Herbaceous weeds, mostly annuals, invade the rock. Their roots penetrate deep down, secrete acids and enhance the process of weathering. Leaf litter and the death of herbs add humus to the soil. Shading of the soil results in a decrease in evaporation and a slight increase in temperature. As a result the xeric conditions begin to change, and biennial and perennial herbs and xeric grasses begin to inhabit the rock.

5. Shrub stage

On the soil appear xeric shrubs (low woody plants smaller than a tree, with little or no trunk; bushes). They may start from seeds or invade from adjacent areas by rhizomes. They make the condition unsuitable for herbs and overshadow them, so the herbs are unable to compete and are replaced by shrubs. Early invasion of shrubs is slow, but once a few bushes have become established, birds invade the area and help disperse the seeds. This results in dense growth, shading the soil and making conditions unfavourable for the growth of herbs, which then begin to migrate.

6. Tree stage

A change in environment favours colonisation by tree species. Tree saplings begin to grow among the shrubs and establish themselves. The trees form a canopy and shade the area. Shade-loving plants continue to grow as secondary vegetation. Leaf litter and decaying roots weather the soil further and add humus to it, making the habitat more favourable for the growth of trees.

7. Climax stage

The first species of trees are relatively xeric. As the weathering process continues and the soil deepens, the xeric trees in turn give place to mesophytic species of trees. Ultimately a forest may develop.

Stages of xerarch succession.
Stage	Members	What it does to the habitat
Crustose lichen (pioneer)	Licanora, Rhinodina	Withstands desiccation; secretes acids that decompose rock
Foliose lichen	Permelia, Dermatocarpon	Collects a thin layer of soil
Moss	Xerophytic mosses	Adds soil; a mat of dead moss holds water
Herb	Annual weeds, then biennial and perennial herbs, xeric grasses	Roots and acids speed weathering; humus; shading cuts evaporation
Shrub	Xeric shrubs	Shade out the herbs; birds disperse seeds, dense growth
Tree	Tree saplings, shade-loving secondary vegetation	Canopy shades area; humus and weathering improve soil
Climax	Xeric trees, then mesophytic trees: a forest	Soil deepens; relatively stable community

Worked example: why must lichens come before mosses?

On a bare rock, explain why mosses cannot be the first colonisers, and why each stage is later replaced by the next.

A bare rock has no soil and almost no water. Only crustose lichens can grow, because they withstand extreme desiccation, absorb water like sponges when it rains and secrete acids that decompose the rock.
Mosses need some soil in the crevices of the rock. This soil has to be made first, by the lichens decomposing the rock and by the lichens' own death and decay (foliose lichens then collect a thin layer of soil). Without it mosses cannot establish.
Each stage improves the habitat: moss adds soil and a water-holding mat, herbs add humus and shade, shrubs and trees add more shade and humus.
The same changes make the habitat unsuitable for the stage that made them. For example, shrubs overshadow herbs, which cannot compete and are replaced. So each seral stage prepares the way for the next and is then replaced by it.
The sequence ends when the community is relatively stable: the climax, a forest of mesophytic trees.

Habitat condition	Early stages (lichen, moss)	Late stages (shrub, tree)
Soil	Almost none, then thin	Deep, with humus
Water	Deficient, xeric	Held by soil, mat and shade
Light at ground level	Full light	Shaded by canopy

15.5Population dynamics

The numbers and structure of an animal population vary over time. The factors which cause these variations are described by population dynamics. Population dynamics is concerned with the study of long-term and short-term changes in population size and the factors that regulate population size.

Factors regulating population size.
Inflow (increases the population)	Outflow (decreases the population)
Births	Culling (to pick and destroy individuals, e.g. seal, deer)
Immigration	Predation
	Natural deaths
	Accidents
	Emigration

Population size changes with inflow (births, immigration) and outflow (culling, predation, natural deaths, accidents, emigration).

15.5.1Characteristics of a populationOften examined

A population is a group of individuals of a single species living in the same general area. Members of a population rely on the same resources, are influenced by similar environmental factors and have a high likelihood of interacting and breeding with one another. The characteristics of a population are: growth, density, distribution, carrying capacity and viable size.

Growth

An increase in the number of individuals of a population is called population growth. Consider what happens if a few individuals enter an unoccupied area. Assuming there is enough food and that predation and disease are not too severe, reproduction will occur and the number of individuals will increase.

At first there may be a lag phase as the individuals settle into their new environment.
As reproduction gets underway, the population shows exponential growth: the population size doubles at regular intervals. During the exponential phase the population is said to grow geometrically.

Population growth

100

120

Time

Number

K (carrying capacity)

lag phase

exponential growth

population

A few individuals enter an unoccupied area: a lag phase, then exponential growth, then the numbers level off at the carrying capacity K.

Density

Density is the number of individuals per unit area or volume, e.g. the number of mulberry trees per square kilometre in Islamabad, or the number of E. coli bacteria per millilitre in a test tube.

density = \frac{number of individuals}{area or volume}

Density is the number of individuals per unit area or volume.

Distribution

The dispersion of individuals in a population is called distribution. There are three distribution patterns found in different populations.

Distribution patterns.
Pattern	Description
Clumped	Individuals aggregate in patches
Uniform	Individuals are evenly spaced
Random	Unpredictable spacing among individuals

The three distribution patterns of a population: clumped (patches), uniform (evenly spaced) and random (unpredictable spacing).

Carrying capacity

There is a limit to the number of individuals that can occupy a habitat. The carrying capacity is defined as the maximum population size that a particular environment can support. Carrying capacity is not fixed but varies over space and time with the abundance of limiting resources.

Role of the Population Welfare Department

The Department of Population Welfare provides Family Planning and Reproductive Health Services through various programmes: (1) Family Welfare Centres, (2) Mobile Service Units and (3) Reproductive Health Services (RHS) Centres, which are being run by various NGOs with the assistance of the Population Welfare Department.

Worked example: density and doubling

(a) A park of

4 k m^{2}

has 360 mulberry trees. Find the density. (b) A test tube holds

2 mL

of broth with 400 E. coli cells. If the number doubles every 30 minutes, find the population after 2 hours and name the type of growth.

(a) Density is the number of individuals per unit area.
$density = \frac{360}{4 k m ^{2}} = 90 trees per km^{2}$
(b) Density at the start: number per millilitre.
$\frac{400}{2 mL} = 200 cells per mL$
Two hours is four doubling periods of 30 minutes. Double the population at each period.

Time (min)	Doublings	Cells in the tube	Cells per mL
0	0	400	200
30	1	800	400
60	2	1 600	800
90	3	3 200	1 600
120	4	6 400	3 200

The population doubles at regular intervals, so it shows exponential (geometric) growth. This continues only while food and space last: the carrying capacity of the test tube is the maximum number that it can support.

90 trees per km^{2}; 6400 cells after 2 h (exponential growth)

15.5.2Human impact on ecosystem

Human activities have had significant impacts on ecosystems worldwide. These impacts are often negative and have led to environmental degradation, loss of biodiversity and disruption of natural processes.

Eight ways in which human activities affect ecosystems.
Impact	Cause and effect (as given in the book)
1. Habitat destruction and fragmentation	Deforestation: large-scale clearing of forests for agriculture, urbanisation and industry destroys habitats. Urbanisation: expanding cities lead to habitat loss and fragmentation, isolating populations and reducing genetic diversity. Infrastructure development: roads, dams and other projects fragment habitats and disrupt wildlife migration patterns.
2. Pollution	Air: emissions from industries, vehicles and agriculture lead to acid rain, smog and health problems. Water: discharge of pollutants into rivers, lakes and oceans harms aquatic life. Soil: chemicals from agriculture, mining and industry contaminate soil, affecting plant and animal life.
3. Climate change	Greenhouse gas emissions: burning fossil fuels releases $CO X_{2}$ and other gases, trapping heat and causing global warming. Temperature changes affect species' distribution, migration and behaviour, often causing mismatches with food sources and habitats. Sea level rise: melting ice caps and glaciers threaten coastal ecosystems and communities.
4. Overexploitation	Overfishing reduces fish populations and disrupts marine food chains. Hunting and poaching push many species to the brink of extinction. Defaunation: the decline of large animals affects plant populations, since they disperse seeds and maintain ecosystem balance.
5. Invasive species	Introduction of non-native species: invasive species outcompete native species for resources, causing declines and sometimes extinction. Altered ecosystem dynamics: invasives can change soil composition, nutrient cycling and fire regimes.
6. Land use changes	Agricultural expansion: habitat loss, soil degradation and greater use of pesticides and fertilizers. Monoculture farming: planting a large area with a single crop reduces biodiversity and makes ecosystems more vulnerable to diseases and pests.
7. Loss of biodiversity	Extinction: human activities have driven many species extinct, disrupting ecosystems and reducing genetic diversity. Genetic erosion: reduced populations lead to inbreeding and loss of genetic diversity, making species more vulnerable to diseases and environmental changes.
8. Resource depletion	Water depletion: excessive use of water for agriculture, industry and domestic purposes depletes aquifers and dries up wetlands. Soil degradation: unsustainable agriculture leads to soil erosion, loss of fertility and desertification.

15.5.3The greenhouse effectOften examined

The greenhouse effect is a natural process that warms the Earth by trapping heat radiation.

Heat radiations from the Sun reach the Earth. Some of these are absorbed and warm the Earth's surface.
The rest are radiated back to space.
Greenhouse gases in the atmosphere trap some of this outgoing heat energy, preventing it from escaping back into space.
This trapped energy keeps the Earth's surface warmer than it would be without an atmosphere. It is due to this warm surface that the Earth is a comfortable place to live.

Figure 15.10: Greenhouse gases trapping heat radiation. Some heat leaves the Earth for space; some is trapped by the gases and warms the surface.

However, human activities, particularly the burning of fossil fuels (coal, oil and natural gas) and deforestation, have significantly increased the concentration of greenhouse gases in the atmosphere. These increased concentrations enhance the natural greenhouse effect, leading to a warming of the Earth's surface.

Greenhouse gases

The main greenhouse gases.
Gas	Sources and notes (book)
Carbon dioxide (\ce{CO2})	The most prevalent greenhouse gas emitted by human activities. Released when coal, oil and natural gas are burned for energy. Deforestation also reduces the number of trees available to absorb \ce{CO2} from the atmosphere.
Methane (\ce{CH4})	A potent greenhouse gas with a much higher heat-trapping ability than \ce{CO2}, although present in lower concentrations. Released during the production and transport of coal, oil and gas; livestock and other agricultural practices also contribute.
Nitrous oxide (\ce{N2O})	Released from agricultural and industrial activities, and during the combustion of fossil fuels and solid waste.
Fluorinated gases	Synthetic gases used in a variety of industrial applications, released during their production and use: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (\ce{SF6}) and nitrogen trifluoride (\ce{NF3}).

Effects of greenhouse gases

Effects of increased greenhouse gases.
Effect	What happens
Global warming	The most prominent effect: a long-term increase in the Earth's average surface temperature. Brings changes in weather patterns and more frequent and severe heat waves.
Climate change	Altered precipitation patterns, more intense storms and changes in seasonal temperatures; droughts, floods and other extreme weather events affect ecosystems and societies.
Melting polar ice and glaciers	Higher temperatures melt polar ice caps and glaciers faster, raising sea levels (a threat to coastal communities and ecosystems) and disrupting habitats of animals such as polar bears and penguins.
Ocean acidification	Excess \ce{CO2} absorbed by the oceans lowers the pH of seawater, harming marine life, especially organisms with calcium carbonate shells or skeletons such as corals and shellfish.
Biodiversity loss	Ecosystems and habitats are disrupted, species distributions shift, and some species may become extinct if they cannot adapt or migrate fast enough.
Impact on agriculture	Changes in temperature and precipitation reduce crop yields in some regions (drought, heat) and increase pest and disease pressure in others, leading to food shortages and higher food prices.
Health impacts	Heat-related illnesses, respiratory problems from air pollution, and the spread of diseases by insects such as mosquitoes.
Economic consequences	Costs of disaster response, infrastructure damage, healthcare and reduced agricultural productivity.

Addressing the issue is crucial to reducing these effects. Efforts to reduce emissions, a transition to renewable energy sources, and adaptation to the changing climate are essential to minimise the long-term consequences of increased greenhouse gas concentrations in the atmosphere.

15.6Coral reefsOften examined

Coral reefs are diverse and productive marine ecosystems found in shallow, tropical and subtropical waters. They are made up of colonies of tiny animals called coral polyps, which secrete calcium carbonate to form hard skeletons that provide the structure of the reef. Coral reefs are often called the "rainforests of the sea" because of their incredible biodiversity and the complex relationships between the different organisms within the ecosystem.

Coral animals are the primary organisms that build the reef structure. They have a symbiotic relationship with photosynthetic algae, which provide them with nutrients and give them their vibrant colours.

Significance of coral reefs

They are among the most biodiverse ecosystems on Earth.
They provide a habitat for numerous plant and animal species: thousands of species of vertebrates and invertebrates, including colourful reef fish, predators and herbivores.
Sea-grasses and algae provide food and shelter for various marine life.
They act as natural barriers, protecting coastlines from erosion and reducing the impact of storms and waves.
Their symbiosis with photosynthetic algae supplies the coral with nutrients.

Ocean acidification and coral reefs

The ocean plays a crucial role in regulating the climate of the Earth by absorbing large amounts of \ce{CO2}. Burning fossil fuels and deforestation have increased \ce{CO2} levels significantly. About 25% of the \ce{CO2} released is absorbed by the oceans, presently about 22 million tons per day. However, the excess \ce{CO2} absorbed by the ocean has significant consequences: it is changing the chemistry of the ocean. It combines with water to produce carbonic acid, lowering the pH of the ocean. The increase in temperature also enhances this acidification process.

\ce{CO2 + H2O -> H2CO3}

Carbon dioxide and water form carbonic acid, which lowers the pH of seawater.

How excess carbon dioxide leads to ocean acidification and damage to coral reefs.

Ocean acidification can harm marine life, particularly organisms with calcium carbonate shells or skeletons, such as corals, molluscs and some types of plankton. More than half of the world's coral is under threat due to ocean acidification. These organisms find it more difficult to build and maintain their shells and skeletons in acidic waters, leading to weaker structures and increased vulnerability to predation and disease. This can disrupt marine ecosystems and the food webs that rely on these organisms. It is predicted that by 2080 ocean conditions will be so acidic that coral erosion will outpace their rebuilding.

15.7The ecosystems of Pakistan

Pakistan is a country with diverse ecosystems because of its varied topography and climate. Four important ecosystems are discussed in the book: the grassland ecosystem, desert ecosystems, coniferous alpine and boreal forests, and temperate deciduous forests.

Four important ecosystems of Pakistan.
Ecosystem	Where	Rainfall and temperature	Productivity
Grassland (tropical and temperate)	Tropical grasslands (savannas); temperate grasslands in Balochistan and parts of Punjab	Tropical: monsoon, 200 to 400 mm (up to 600 mm); very hot summer days, mild dry winters. Temperate: 150 to 500 mm; warm summers, freezing winters	Tropical: moderate to high. Temperate: lower, limited by water and nutrients
Desert	Thar, Cholistan, Thal, Kharan and the cold desert of Skardu	100 to 250 mm, mainly in the monsoon (July to September); summers above 40 °C, sometimes 50 °C; winters below 10 °C	Relatively low: scarce water and limited plant cover
Coniferous alpine and boreal forests	Northern mountains: Western Himalayas and the Karakoram Range	Moderate to high precipitation: snow in winter, rain in summer; cold, often below freezing	Slow growth: short growing season, limited soil nutrients; biodiversity hotspots
Temperate deciduous forests	Northern Pakistan, especially Azad Kashmir and the Himalayan and Karakoram foothills	Moderate rainfall throughout the year; warm summers, cool winters	Depends on soil quality, climate and plant diversity; rich in biodiversity

Annual rainfall ranges quoted for the grassland and desert ecosystems of Pakistan.

15.7.1Grassland ecosystemOften examined

Grassland ecosystems in Pakistan are diverse and can be broadly classified into tropical and temperate grasslands, each with distinct characteristics.

Rainfall

Type	Rainfall
Tropical grasslands	Typically a monsoon climate. Rainfall varies across regions: some areas receive between 200 and 400 mm annually, while others get up to 600 mm during the monsoon season.
Temperate grasslands	Relatively lower rainfall than tropical grasslands: 150 to 500 mm annually, making these areas more prone to drought.

Temperature

Type	Temperature
Tropical grasslands	Quite high during the day, often exceeding 40 °C (104 °F) in summer, but can drop significantly at night. Winters are mild and dry, averaging around 10 to 20 °C (50 to 68 °F).
Temperate grasslands	Distinct seasons. Summers are warm, averaging between 25 and 35 °C (77 to 95 °F); winters can be cold, dropping below freezing point.

Flora

Common grass species include Cenchrus ciliaris (buffelgrass), Themeda anathera (Indian red oats), Apluda mutica (weedy rice grass), Panicum antidotale (velvet grass) and various species of Dicanthium, Stipagrostis and Heteropogon.

Fauna

This ecosystem supports diverse wildlife, including herbivores like deer, antelopes, gazelles, wild goats, rodents and various bird species. Predators like foxes, wolves, jackals, big cats and birds of prey are also present.

Productivity

Tropical grasslands in Pakistan are also known as savannas and have moderate to high productivity. The vegetation is adapted to periodic droughts and fires, allowing for the growth of grasses and other plants with varying water requirements. Temperate grasslands, found in regions like Balochistan and parts of Punjab, have lower productivity compared to tropical grasslands. The growth of vegetation is limited by the availability of water and nutrients.

Threats and conservation

Overgrazing by domestic livestock poses a significant threat to grassland ecosystems in Pakistan, leading to soil erosion and loss of biodiversity. Human activities such as agriculture, urbanisation and infrastructure development contribute to habitat loss and fragmentation. Changing climate patterns, including altered rainfall and temperature regimes, impact the distribution and health of grassland ecosystems.

Conservation efforts in Pakistan focus on sustainable land management, wildlife protection, and raising awareness about the importance of grassland ecosystems in maintaining biodiversity and supporting local communities.

15.7.2Desert ecosystemsOften examined

Deserts are the driest of all the terrestrial ecosystems. These are regions where unique flora and fauna have adapted to arid conditions. Pakistan has the deserts Thar, Cholistan, Thal and Kharan, and the cold desert of Skardu.

Climate and rainfall

Rainfall: the desert regions of Pakistan are characterised by low and erratic rainfall. Annual rainfall is very low, about 100 to 250 millimetres, and mainly falls during the monsoon season, from July to September.
Temperature range: extreme temperatures. During summer months temperatures can soar above 40 °C (104 °F) and sometimes even reach 50 °C. In winter they can drop significantly, often below 10 °C (50 °F).

Flora

Xerophytic plants have adapted to conserve water. Xerophytic plants such as cacti, thorny bushes (like Acacia and Prosopis species) and succulents are common. These plants have modified leaves or stems to reduce water loss through transpiration.

Fauna

Group	Examples
Mammals	Rodents such as jerboas and desert gerbils; carnivores like desert foxes and wildcats; reptiles including snakes and lizards are abundant
Birds	Sandgrouse, larks and raptors, with specific behaviours and physiological adaptations for the harsh environment
Insects	Ants, beetles and certain butterflies, with mechanisms to survive in arid conditions

Productivity and food chains

Primary productivity in deserts is relatively low due to the scarcity of water and limited plant cover. Plants like drought-resistant grasses and shrubs form the base of the food chain. In the food chains, herbivores feed on desert plants and predators in turn feed on these herbivores. Scavengers also play a crucial role, as they help recycle nutrients by feeding on dead organisms.

Adaptations and survival

Adaptation	Description
Water conservation	Plants have deep root systems to tap underground water sources; animals often have concentrated urine and efficient cooling mechanisms to minimise water loss
Nocturnal behaviour	Some animals, especially reptiles and insects, are active during the cooler night hours to avoid the scorching daytime heat
Burrowing	Many animals, including rodents and reptiles, are proficient burrowers; burrows give shelter from extreme temperatures and reduce water loss

15.7.3The coniferous alpine and boreal forests

The coniferous alpine and boreal forests in Pakistan are primarily found in the northern regions, particularly in the mountainous areas of the Western Himalayas and the Karakoram Range. These forests are adapted to cold temperatures and receive moderate to high levels of precipitation, mainly in the form of snow during winter and rain during summer.

Coniferous alpine and boreal forests of Pakistan.
Feature	Description
Temperature	Cold, especially in winter months; low average temperatures, often dropping below freezing point
Rainfall	Moderate to high precipitation: primarily snow in winter and rain in summer. This moisture is essential for the growth of coniferous trees and other vegetation
Flora	Dominant trees are conifers such as pine, spruce, fir and cedar, adapted to cold climates with needle-like leaves that help conserve water. Alpine plants like rhododendrons, junipers and various grass species withstand cold temperatures and high altitudes
Fauna	Mammals such as snow leopards, brown bears, Himalayan tahr, red foxes and various species of deer; birds including golden eagles, griffon vultures, chukar partridges, and various finches and warblers; butterflies and moths; some regions have amphibian species adapted to cold, mountainous environments
Productivity	Relatively slow compared to temperate or tropical forests because of harsh conditions: the growing season is short and the availability of nutrients in the soil is limited

Despite slow growth, these forests are biodiversity hotspots, supporting a variety of plant and animal species uniquely adapted to cold, high-altitude environments. The specific flora, fauna and productivity of these forests can vary with the altitude, microclimates and local environmental factors within the alpine and boreal regions of Pakistan.

15.7.4Temperate deciduous forests

Deciduous forests in Pakistan are primarily found in the northern regions of the country, particularly in the foothills of the Himalayas and the Karakoram Range, especially in Azad Kashmir.

Temperate deciduous forests of Pakistan.
Feature	Description
Rainfall and temperature	A moderate amount of rainfall distributed throughout the year. Temperature varies seasonally, with warm summers and cool winters being typical; specific patterns vary with altitude and geographical location
Flora	Trees that shed their leaves in the autumn season. Common species include oak, maple, pine, ash, walnut and deodar cedar. Undergrowth may consist of ferns, mosses and various wildflowers
Fauna	Birds such as pheasants, partridges and eagles; mammals such as Himalayan brown bear, snow leopard, deer and monkeys; a diverse range of insects, amphibians and reptiles
Productivity	Depends on factors such as soil quality, climate and plant diversity. These forests are typically rich in biodiversity and contribute to overall productivity by providing habitats, regulating water flow and supporting nutrient cycling

Human impacts

These forests play a vital role in providing habitat for wildlife species, maintaining soil fertility and regulating water flow in rivers and streams. However, deforestation and unsustainable logging practices have significantly reduced the area covered by deciduous forests in Pakistan, leading to environmental concerns and loss of biodiversity.

Human activities such as deforestation, agricultural expansion and infrastructure development have had a significant impact on temperate forests in many parts, leading to habitat loss and fragmentation. These activities can threaten various plant and animal species that depend on the forests for their survival. Conservation efforts are essential to protect the remaining temperate deciduous forests and their biodiversity.

Formula sheet

Net primary productivity

\text{NPP}=\text{GPP}-R

GPP is the gross primary productivity (total solar energy fixed by producers in photosynthesis), R is the energy the producers use in respiration, NPP is the net primary productivity (energy left as producer biomass).

Biomass per unit area per unit time, e.g. \mathrm{g\,m^{-2}\,year^{-1}}

Energy passed to the next trophic level

E_{\text{next}}\approx 0.10\,E_{\text{this level}}

E_{\text{this level}} is the energy at one trophic level; about 10% reaches the next (Figure 15.3).

Unit: Joules (J)

Population density

\text{density}=\dfrac{\text{number of individuals}}{\text{area or volume}}

Number of individuals of a population in a given area or volume.

Units: Individuals per \mathrm{km^2}, per mL and so on

Key terms

Ecology: The study of the relationship of living organisms with each other and with their physical surroundings.
Ecosystem: A self-sustained geographical area where living organisms interact with their non-living environment; the major unit of ecology.
Biodiversity: The variety of different species of plants, animals and microorganisms that coexist within an ecosystem.
Species: A group of related organisms which can interbreed among themselves freely in nature and produce fertile offspring of their kind.
Population: A group of individuals of a single species living in the same general area at a given time.
Community: Interacting populations of different species living together.
Biosphere: The thin life-containing blanket surrounding the Earth, including all the ecosystems on Earth.
Abiotic components: The non-living factors of an ecosystem: light, air, water, soil and the basic elements and compounds.
Biotic components: The living part of an ecosystem: producers, consumers and decomposers.
Producers: Autotrophs (plants, algae and photosynthetic bacteria) that synthesise complex organic compounds from inorganic raw materials.
Phytoplankton: Floating photosynthetic organisms, mainly algae, that are the main producers of aquatic ecosystems.
Consumers: Heterotrophs (animals, fungi, protozoans and many bacteria) that depend upon producers for food.
Primary consumers: Herbivores that feed directly on producers.
Secondary consumers: Carnivores that feed on primary consumers.
Tertiary consumers: Carnivores that feed on secondary consumers.
Decomposers: Bacteria and fungi that secrete digestive enzymes on dead organic matter and break it down into simple molecules that return to the environment.
Food chain: The sequence of organisms through which food and nutrients move in an ecosystem.
Food web: Interconnected food chains in an ecosystem, giving food alternatives at each trophic level.
Trophic level: Each stage of the food chain, indicating the source of energy and nutrients of the organisms.
Gross primary productivity (GPP): The total amount of solar energy fixed by producers in photosynthesis.
Net primary productivity (NPP): The amount of energy left after the producers have met their respiratory needs; it appears as producer biomass.
Ecological pyramids: Pyramid diagrams that show the relative amounts of energy, biomass or numbers of organisms at each trophic level.
Biomass: The dry weight of all living material in the ecosystem.
Biogeochemical cycle: The cyclic flow of nutrients from the environment to the organisms and back to the environment.
Water table: The top of the saturation zone, the level to which percolating water saturates the ground.
Aquifers: Ground water located in underground rivers.
Ammonification: The production of ammonia or ammonium compounds in the decomposition of organic matter by microorganisms.
Nitrification: The conversion of ammonia to nitrites by Nitrosomonas and of nitrites to nitrates by Nitrobacter, in well-aerated soil.
Nitrogen fixation: The conversion of nitrogen gas to ammonia or nitrogen oxides: atmospheric (lightning), industrial (fertilizers) or biological (bacteria).
Assimilation: The utilisation of nitrogenous compounds in living bodies: plants take up ammonia and nitrates, animals eat plants or other animals.
Denitrification: The breakdown of nitrates by soil bacteria, in the absence of oxygen, releasing nitrogen gas into the atmosphere.
Ecological succession: The process by which species are replaced over time in an area.
Sere: An individual succession; its development phases are the seral stages.
Primary succession: Change in species composition over time in a habitat not previously inhabited by organisms, such as bare rock.
Secondary succession: Change in species composition over time in a habitat already substantially modified by a pre-existing community, where soil is already present.
Hydrarch succession: Succession that begins in ponds, lakes, marshes or elsewhere in water; its stages are hydroseres.
Xerarch succession: Succession initiated on bare rocks, sand dunes and rocky slopes where water is deficient; its stages are collectively the xerosere.
Pioneers: The first migrants of plants and animals that take hold of a bare area in succession.
Seral community: A group of invaders that follows the pioneers and forms one seral stage, modifying the environment.
Climax community: The end product of succession: a relatively stable community.
Population dynamics: The study of long-term and short-term changes in population size and the factors that regulate it.
Population growth: An increase in the number of individuals of a population.
Lag phase: The initial period of growth in which individuals settle into a new environment.
Exponential growth: Growth in which the population size doubles at regular intervals (geometric growth).
Density: The number of individuals per unit area or volume.
Distribution: The dispersion of individuals in a population: clumped, uniform or random.
Carrying capacity: The maximum population size that a particular environment can support.
Greenhouse effect: A natural process that warms the Earth because greenhouse gases trap some of the outgoing heat radiation.
Greenhouse gases: Gases such as carbon dioxide, methane, nitrous oxide and fluorinated gases that trap heat in the atmosphere.
Global warming: The long-term increase in the Earth's average surface temperature.
Ocean acidification: The fall in the pH of seawater caused by excess carbon dioxide absorbed by the oceans, which forms carbonic acid.
Coral polyps: Tiny animals that live in colonies and secrete calcium carbonate to form the hard skeleton of a coral reef.
Savannas: Tropical grasslands of Pakistan with moderate to high productivity, adapted to periodic droughts and fires.
Xerophytic plants: Plants adapted to conserve water in dry conditions, such as cacti, thorny bushes and succulents.
Biodiversity hotspots: Areas, such as the coniferous alpine and boreal forests, that support many species uniquely adapted to the conditions.

Multiple choice questionsp. 410–411

Rhizobium is a group of nitrogen fixing bacteria found in the root nodules of:

AGrasses
Blegume plantsAnswer
Cferns
Dall plants

Why

Rhizobium lives in the root nodules of legume plants, where it fixes atmospheric nitrogen into ammonia for the plant (symbiotic association, section 15.3.3). Grasses and ferns do not form these nodules, and the nodules are not found in all plants, so A, C and D are wrong.

During the succession process, with the passage of time, the change in communities is:

AOrderly and sequentialAnswer
BParallel
CPerpendicular
DRandom

Why

Succession follows a fixed order: pioneers are replaced by seral communities, and each stage alters the habitat so that the next group can invade, until the climax community is reached (section 15.4). Parallel and perpendicular are not descriptions of this change, and random is wrong because each stage is determined by the one before.

Producers of an ecosystem are:

APlants only
Bplants and algae only
Cplants, algae and autotrophic bacteriaAnswer
Dplants, algae, fungi and autotrophic bacteria

Why

Producers are autotrophs. The book lists plants, algae and photosynthetic bacteria as producers (section 15.1.1). Fungi are heterotrophs (decomposers and consumers), so D is wrong, and A and B leave out the autotrophic bacteria.

What is the predictable and gradual change in the species composition of a given area called?

AEnvironmental issues
BClimax community
CPioneer community
DEcological successionAnswer

Why

Ecological succession is the predictable, gradual replacement of species in an area over time (section 15.4). The climax community (B) is only the end product of succession, a pioneer community (C) is only its first stage, and environmental issues (A) is not a process of species change.

The sequence of organisms through which energy moves in an ecosystem is called:

Atrophic level
Bfood web
Cbiomass
Dfood chainAnswer

Why

A food chain is the sequence of organisms through which food and nutrients move (section 15.1.2). A trophic level (A) is only one step of that sequence, a food web (B) is a network of many interlinked food chains, and biomass (C) is the dry weight of living material.

The role of decomposers in the nitrogen cycle is to:

Afix atmospheric nitrogen to ammonia
Brelease ammonia from organic compounds, thus returning it to the soilAnswer
Cdenitrify ammonia, thus return nitrogen to the atmosphere
Dconvert ammonia to nitrates for plant use

Why

Decomposers (saprophytic bacteria and fungi) break down nitrogenous wastes and dead matter, converting amino acids into ammonia or ammonium ions. This is ammonification (section 15.3.3). A is done by nitrogen-fixing bacteria, C is denitrification by denitrifying bacteria, which convert nitrates to nitrogen gas, and D is nitrification by bacteria such as Nitrosomonas and Nitrobacter.

Note. The book's option C reads 'denitrify ammonia'. Denitrification acts on nitrates (section 15.3.3), so the wording is loose, but the option is still not the decomposers' role.

Which of these about ecological succession is incorrect?

Afood chain relationships become more complex
Bspecies diversity increases as succession proceeds
Crole of decomposers becomes all the more important
Dis a random processAnswer

Why

Succession is orderly and directional: each seral stage modifies the habitat and is replaced by a predictable next group, ending in the climax community (section 15.4). Statements A, B and C describe the usual course of succession as organic matter and soil build up, so the incorrect statement is D, that succession is a random process.

Note. The book prints no key. The text does not state A to C in words; they are the usual features of succession, so D is the odd one out.

The nutrient cycle always involves:

Arocks as reservoir
Bmovement of nutrients through the biotic communityAnswer
Cthe atmosphere as an exchange pool
Dloss of nutrients from the biosphere

Why

Every biogeochemical cycle passes nutrients through living organisms: nutrients move from the environment into organisms and back again (section 15.3). Rocks are a reservoir for some cycles only, and the atmosphere is an exchange pool only for gaseous cycles. D is wrong because nutrients are recycled, not lost from the biosphere.

The ecosystems with very little annual rain and higher temperatures are:

Agrassland ecosystems
Bdeciduous forest ecosystems
Cboreal forest ecosystems
Ddesert ecosystemsAnswer

Why

Deserts receive only about 100 to 250 mm of rain a year, and summer temperatures often rise above 40^\circ\mathrm{C} (section 15.6.2, desert ecosystems). Grasslands receive more rain (150 to 600 mm), deciduous forests get moderate rain, and boreal forests are cold, with snow in winter.

The main cause of ocean acidification is:

Aexcessive use of water
Boffshore oil exploration drilling
Cincreasing level of carbon dioxide in the atmosphereAnswer
Dvolcanic activity in the oceans

Why

Excess CO_2 from burning fossil fuels and from deforestation is absorbed by the ocean, where it combines with water to form carbonic acid and lowers the pH of seawater (section 15.6). Water use, offshore drilling and volcanic activity are not given in the book as causes.

Short answer questionsp. 411

13 marks

Describe water in 'basins'.

Solution

A basin is a hollow in the land surface. Rain water that percolates into the ground saturates the soil and rock up to the water table, so water in a basin stands at the level of the water table.

Water in a basin is standing water, called a lake or pond.
Its surface lies at the level of the water table, the top of the saturated zone, so a basin fills wherever the ground dips below that level.
Like other surface water, water in basins evaporates back into the atmosphere and so returns to the water cycle (section 15.3.2).

Water in a basin (lake or pond) and in a channel (stream or river) stands at the level of the water table.

22026 Annual I, Set B, Q.2 (viii)3 marks

Describe the ways of nitrogen fixation.

Solution

Nitrogen gas has a very strong triple bond, so it is unreactive and must be fixed (combined into compounds that organisms can use). There are three ways:

Atmospheric fixation: lightning gives enough energy to combine nitrogen and oxygen into oxides of nitrogen. Rain carries these into the soil, where plants can use them. Only about 5 to 8% of fixed nitrogen comes this way.
Industrial fixation: nitrogen-containing fertilizers are synthesised, for example by combining nitrogen with hydrogen to make ammonia: \ce{N2 + 3H2 -> 2NH3}.
Biological fixation: nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia. Free-living bacteria such as Azotobacter (aerobic) and Clostridium (anaerobic) fix it directly, and the symbiotic Rhizobium does so in the root nodules of legume plants. This is the largest share (the book gives about 60%).

Note. The book places the equation \ce{N2 + 3H2 -> 2NH3} under biological fixation. It describes the industrial reaction, which the bacteria do not carry out, so it is placed under industrial fixation here.

33 marks

Give the role of decomposers in the nutrient cycle.

Solution

Decomposers, mainly bacteria and fungi, return materials from dead organisms, waste matter and detritus to the environment, so that producers can use them again.

They secrete digestive enzymes onto dead organic matter, breaking it down into simple molecules.
They absorb some of these simple molecules for their own use and release the rest, such as ammonia, water and carbon dioxide, into the soil and air.
In the nitrogen cycle they carry out ammonification, converting nitrogenous wastes and dead bodies into ammonia or ammonium ions, which nitrifying bacteria then convert to nitrates. This recycling keeps the cycle going and maintains balance in the ecosystem (section 15.3.3).

43 marks

What is productivity? Describe productivity in desert ecosystem.

Solution

Productivity is the rate at which organic matter is created by producers through photosynthesis. It is measured as the biomass produced per unit area over a period of time (section 15.1.4).

Productivity in the desert ecosystem is low:

Water is scarce and rain is erratic (about 100 to 250 mm a year), and plant cover is limited, so producers make little organic matter.
The plants that do grow are drought-resistant grasses and shrubs, with deep roots or reduced leaves. They form the base of the food chain.
Herbivores feed on these plants, predators feed on the herbivores, and scavengers and decomposers recycle the dead organisms.

53 marks

Explain the pyramid of energy.

Solution

A pyramid of energy shows the energy available at each trophic level, with the producers at the base.

The producers have the largest energy, because they capture light energy by photosynthesis. Energy falls at each step up. The book's example gives 10,000 J at the producers, 1,000 J at the primary consumers, 100 J at the secondary consumers and 10 J at the tertiary consumers.
Only about 10% of the energy at one level is passed on. The rest is used in respiration, growth and reproduction, lost as heat, lost in excretory products and faeces, or left in non-predatory deaths.
Because energy is lost at each step, the pyramid is always upright, few organisms live at the top, and there are only a few trophic levels.

Pyramid of energy using the values of Fig 15.3. Energy falls by about 90% at each step.

63 marks

Differentiate between primary and secondary succession.

Solution

Both are forms of ecological succession, but they start from different conditions:

Feature	Primary succession	Secondary succession
Starting habitat	Never before inhabited, such as bare rock, fresh volcanic lava or rock scraped clean by glaciers	Already modified by a pre-existing community, such as abandoned farm fields or forest after fire, flood or cyclone
Soil	Absent at the start; soil must form through weathering and the activity of the pioneers	Already present
Time taken	Long, because soil must build up first	Shorter, because soil is already there (Fig 15.8 reaches a forest in 40+ years)

Both lead to a climax community.

73 marks

What is net primary productivity?

Solution

Gross primary productivity (GPP) is the total solar energy fixed by producers in photosynthesis.
Net primary productivity (NPP) is the energy left after producers have met their own respiratory needs (GPP minus respiration). It appears as the organic matter, or biomass, of the producers.
NPP is the energy available to support the whole food web, including herbivores, carnivores and decomposers, so it is a critical ecological parameter.

83 marks

Explain the term 'biosphere'.

Solution

The biosphere is the thin layer of life surrounding the Earth. It includes all the ecosystems on Earth, from small ponds to vast rainforests.
Life is possible in it because the conditions for life are met: a steady supply of energy from the sun, water as the medium of life, a suitable range of temperature and the correct proportion of minerals.
The ozone layer screens out harmful ultraviolet radiation, so living organisms within the biosphere are protected.

Extensive answer questionsp. 411

18 marks

Describe greenhouse effect. Give the consequences of increase in greenhouse gases.

Solution

The greenhouse effect

The greenhouse effect is the natural process that warms the Earth. Heat radiation from the sun reaches the Earth. Some of it is absorbed and warms the surface, and the rest is radiated back towards space as infrared heat. Greenhouse gases in the atmosphere trap part of this outgoing heat and send it back to the surface, so the Earth stays warmer than it would be without an atmosphere (figure).

Greenhouse effect: sunlight passes through the atmosphere, Earth re-radiates heat as infrared, and greenhouse gases trap part of it and send it back, so the surface stays warm.

The main greenhouse gases

Gas	Main sources
Carbon dioxide (CO₂)	Burning coal, oil and natural gas; deforestation also reduces the trees that absorb CO₂
Methane (CH₄)	Production and transport of coal, oil and gas; livestock and agriculture. Per molecule it traps far more heat than CO₂, but it is present in lower concentrations
Nitrous oxide (N₂O)	Agricultural and industrial activities, combustion of fossil fuels and solid waste
Fluorinated gases (HFCs, PFCs, SF₆, NF₃)	Synthetic gases used in industry, released during their production and use

Consequences of an increase in greenhouse gases

Higher concentrations strengthen the natural greenhouse effect, so the Earth warms. The consequences are:

Global warming: a long-term rise in average surface temperature, with more frequent and severe heat waves.
Climate change: altered rainfall, more intense storms, droughts, floods and changed seasonal temperatures.
Melting polar ice and glaciers: rising sea levels threaten coastal communities and the habitats of animals such as polar bears and penguins.
Ocean acidification: the oceans absorb excess CO₂, the pH of seawater falls, and organisms with calcium carbonate shells or skeletons, such as corals and shellfish, are harmed.
Loss of biodiversity: species shift their distribution, and those that cannot adapt or migrate quickly enough may become extinct.
Agriculture: reduced crop yields in droughts or heat, more pests and diseases, food shortages and higher food prices.
Health: heat-related illness, respiratory problems from air pollution, and the spread of diseases by insects such as mosquitoes.
Economic costs: disaster response, damage to infrastructure, healthcare costs and lower agricultural productivity.

Reducing emissions, moving to renewable energy and adapting to climate change are the ways to limit these effects.

22026 Annual I, Set D, Q.4 OR2025 Annual I, Set B, Q.3 OR8 marks

What is succession? Give the stages of primary succession on a barren rock.

Solution

Definition

Ecological succession is the process by which the species in a habitat are replaced over time, in a predictable and gradual way, until a relatively stable community develops. Each individual succession is a sere, each development phase is a seral stage, and the end product is the climax community. Succession on bare rock is primary succession. Because the rock is dry, it is xerarch succession (a xerosere).

How succession proceeds

Pioneers, the organisms that arrive first, colonise the bare habitat and take hold of the surface.
Their death and decay add organic matter, moisture and nitrogen, so the soil is enriched.
The seral communities that follow can now grow in the modified habitat.
Their activities change the environment again, the next group invades, and this continues until the climax community is established.

Xerarch succession on bare rock (primary succession): each stage changes the habitat so that the next stage can establish, ending in the climax community.

Stages of xerarch succession on bare rock

Stage	Characteristic organisms	What the stage does
Crustose-lichen	Lecanora, Rinodina	Only crust-like lichens grow. They are slow growers, survive extreme dryness, absorb water like sponges and secrete acids that decompose the rock.
Foliose-lichen	Parmelia, Dermatocarpon	Leaf-like lichens grow on the thin soil that has collected on the rock, and their activity builds up a thin layer of soil.
Moss	Xerophytic mosses	Soil collects in crevices and depressions. The mosses add soil, and their death and decay form a mat that holds more water.
Herb	Annual weeds, then biennial and perennial herbs and xeric grasses	Roots penetrate and weather the rock, acids speed weathering and leaf litter adds humus. Shading lowers evaporation, so the xeric conditions begin to change.
Shrub	Xeric shrubs, spread by seed or rhizomes; birds disperse seeds	Dense shrubs shade the soil and overshadow the herbs, which cannot compete and migrate away.
Tree	Tree saplings, then shade-loving plants	Trees form a canopy and shade the area. Leaf litter and decaying roots add humus, making the habitat suitable for trees.
Climax	First xeric trees, then mesophytic trees	As weathering deepens the soil, the xeric trees give way to mesophytic species and a forest develops.

Note. The book prints Licanora, Rhinodina, Permelia and Dermatocarpom. The correct spellings, Lecanora, Rinodina, Parmelia and Dermatocarpon, are used here.

38 marks

Give the significance of coral reefs. Explain the effects of ocean acidification on coral reefs ecosystem.

Solution

Significance of coral reefs

Biodiversity: reefs are among the most biodiverse ecosystems on Earth, home to thousands of vertebrate and invertebrate species, which is why they are called the rainforests of the sea.
Food and shelter: sea-grasses and algae provide food and shelter for many marine organisms. The coral animals get nutrients from the photosynthetic algae they live with.
Coastal protection: reefs act as natural barriers that protect coastlines from erosion and reduce the impact of storms and waves.
Indicator of ecosystem health: corals are sensitive to ocean acidification, so reefs are used as a barometer for the health of an aquatic ecosystem.

How ocean acidification affects coral reefs

The excess CO₂ absorbed by the ocean combines with water to form carbonic acid, which lowers the pH of seawater. Warmer water speeds up this process. The effects on the reef follow these steps:

Burning fossil fuels and deforestation raise the CO₂ in the atmosphere, and the ocean absorbs part of it.
CO₂ reacts with seawater to form carbonic acid, so the pH of the seawater falls.
\ce{CO2 + H2O -> H2CO3}
Corals (coral polyps) and shellfish need calcium carbonate to build and maintain their hard skeletons. In more acidic water they find this harder, so their skeletons are weaker.
Weak skeletons are more vulnerable to predation and disease, and the reef structure erodes.
Erosion outpaces the rebuilding of the reef, so the habitat, the food webs that depend on it and the biodiversity they support are lost.

Chain of events by which ocean acidification damages a coral reef.

More than half of the world's coral is already under threat from ocean acidification. The book predicts that by 2080 ocean conditions will be so acidic that coral erosion will outpace rebuilding.

48 marks

Describe grassland ecosystems in Pakistan.

Solution

Grassland ecosystems in Pakistan are broadly classified as tropical and temperate grasslands. They differ in rainfall, temperature and productivity:

Feature	Tropical grasslands (savannas)	Temperate grasslands
Rainfall	200 to 400 mm a year, and up to 600 mm in the monsoon season	150 to 500 mm a year, so these areas are more prone to drought
Temperature	Summer days often above 40^\circ\mathrm{C}, with a drop at night; mild, dry winters of 10 to 20^\circC	Warm summers of 25 to 35^\circC; cold winters with temperatures below freezing
Productivity	Moderate to high; the vegetation is adapted to periodic droughts and fires	Lower; growth is limited by the availability of water and nutrients

Flora and fauna

Flora: grasses such as Cenchrus ciliaris (buffelgrass), Themeda anathera (Indian red oats), Apluda mutica (weedy rice grass) and Panicum antidotale (velvet grass), together with species of Dichanthium, Stipagrostis and Heteropogon.
Herbivores: deer, antelopes, gazelles, wild goats, rodents and various bird species.
Predators: foxes, wolves, jackals, big cats and birds of prey.

Threats and conservation

Overgrazing by domestic livestock causes soil erosion and loss of biodiversity.
Agriculture, urbanisation and infrastructure development cause habitat loss and fragmentation, and changing rainfall and temperature patterns alter the distribution and health of grassland ecosystems.
Conservation focuses on sustainable land management, wildlife protection, and raising awareness of the importance of grasslands for biodiversity and local communities.

Note. The book prints Dicanthium; the grass genus is Dichanthium, which is used here.

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2025 Annual I, Set B, MCQ 11 mark

When energy flows from one trophic level to the next, the amount of energy lost is about:

A10%
B50%
C90%Answer
D100%

Why

Only about 10% of the energy of one trophic level reaches the next (Figure 15.3), so about 90% is lost, as heat, in respiration, growth, reproduction, non-predatory death and excretory products or faeces. 10% is the amount passed on, not lost, which is the tempting wrong choice; 50% and 100% do not fit the 10% rule.

2026 Annual I, Set B, Q.2 (viii)1x3 = 3 marks

Explain three ways of nitrogen fixation.

Solution

Nitrogen gas (\ce{N2}) has a very strong triple bond between its two atoms, so it is chemically unreactive and a large amount of energy is needed to break the bond. It must be fixed (combined into compounds) before organisms can use it. It is fixed in three ways.

1. Atmospheric fixation (1 mark)

Lightning supplies the energy for nitrogen and oxygen of the air to combine and form various oxides of nitrogen. Rain carries these oxides into the soil, where plants can use them. This is spontaneous and fixes only a small amount (5 to 8%).

2. Industrial fixation (1 mark)

Man makes nitrogen-containing fertilizers in industry, for example by combining nitrogen and hydrogen to form ammonia.

\ce{N2 + 3H2 -> 2NH3}

3. Biological fixation (1 mark)

Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia. This is the largest share (about 60%). Free-living bacteria such as Azotobacter (aerobic) and Clostridium (anaerobic) make fixed nitrogen available to plants when they die and lyse. Rhizobium is symbiotic and lives in the root nodules of legume plants.

2025 Annual I, Set B, Q.2 (viii)1.5+1.5 = 3 marks

Write about de-nitrification and assimilation processes in nitrogen cycle.

Solution

Denitrification (1.5 marks)

In the absence of oxygen, certain soil bacteria (denitrifying bacteria, e.g. Pseudomonas) break down nitrates and use the oxygen for their own respiration. Nitrogen is released back into the atmosphere as a gas, so this is a loss of nitrogen from the soil. It completes the cycle.

\ce{NO3^- -> NO2^- -> NO + N2O -> N2}

Assimilation (1.5 marks)

Assimilation is the utilisation of nitrogenous compounds in living bodies. Plants take up nitrogen from the soil as inorganic compounds such as ammonia and nitrates and build amino acids, proteins and nucleic acids from it. Animals get their nitrogen by eating plants or other animals. (Many micro-organisms can use free nitrogen directly from the atmosphere.)

2025 Annual I, Set B, Q.3 OR7 marks

Explain different stages of xerarch succession on bare rock starting form lichens to flowering plants.

Solution

Xerarch succession is primary succession that begins on bare rock where water is deficient. It passes through seral stages and ends in a climax community.

Xerarch succession on bare rock, in order of time, ending in the climax community.

Stages of xerarch succession in order.
Stage	Members and what happens
1. Crustose lichen (pioneer)	Licanora, Rhinodina. Slow growers that withstand extreme desiccation; absorb water like sponges in rain and decompose the rock by secreting acids
2. Foliose lichen	Permelia, Dermatocarpon. Grow on the little soil that has collected; their activities add a thin layer of soil
3. Moss	Xerophytic mosses appear when soil has gathered in crevices. They add soil; their dead mat holds water, making the habitat suitable for herbs
4. Herb	Annual weeds first; roots penetrate and secrete acids (more weathering); humus is added; shading reduces evaporation; then biennial and perennial herbs and xeric grasses
5. Shrub	Xeric shrubs (from seeds or rhizomes) overshadow the herbs, which are replaced; birds help disperse seeds, giving dense growth
6. Tree	Tree saplings establish among the shrubs; a canopy shades the area; leaf litter and decaying roots add humus and weather the soil
7. Climax	Xeric trees give place to mesophytic trees as soil deepens; a forest develops, a relatively stable community

2026 Annual I, Set B, Q.4 OR3+3 = 6 marks

Explain kinds and general stages of succession.

Solution

Kinds of succession (3 marks)

Ecological succession is the process by which species are replaced over time. There are two kinds.

Point	Primary succession	Secondary succession
Starting habitat	Never inhabited before: bare rock, new volcanic lava, rock scraped clean by glaciers	Already substantially modified by a pre-existing community
Soil	None at the start; it has to be made	Already present
Examples	Succession on bare rock	Abandoned farm fields reverting to forest; a forest devastated by fire, flood or cyclone

Succession is also named by the habitat. Hydrarch succession begins in water (ponds, lakes, marshes) and its stages are hydroseres. Xerarch succession begins on bare rock, sand dunes and rocky slopes where water is deficient, and its stages are xeroseres.

General stages of succession (3 marks)

Plants and animals migrate from surrounding areas into the bare area and aggregate there.
Pioneers. The first migrants take hold of the soil and increase in number.
By their death and decay the pioneers add organic matter, moisture and nitrogen to the soil, which becomes suitable for the next group of invaders.
Seral communities. The next groups of invaders form one seral stage after another. Their activities change the environment so that it becomes unsuitable for themselves, and a new group invades.
Climax community. After several seral communities, the end product is a relatively stable community.

General stages of succession: pioneers, seral communities, climax.

2025 Annual I, Set D, MCQ 141 mark

A forest was completely destroyed by fire. After about hundred year, a new forest was evolved. The type of succession shown is:

APrimary and xerarch
BSecondary and xerarchAnswer
CPrimary and hydrarch
DSecondary and hydrarch

Why

The fire destroyed the vegetation but the soil was left, so the habitat was already modified by a previous community. That is secondary succession. It is xerarch because it is a land (dry-habitat) succession; hydrarch succession begins in water. Primary succession needs a habitat never inhabited before (bare rock, lava), so A and C are wrong.

2026 Annual I, Set D, Q.2 (viii) OR3 marks

State the events of nitrification in nitrogen cycle.

Solution

Nitrification is the conversion of ammonia and ammonium ions (formed by ammonification) into nitrates by nitrifying bacteria. It takes place in two steps, carried out by two groups of bacteria.

Ammonia and ammonium ions in the soil are converted into nitrites by the first group of bacteria (e.g. Nitrosomonas).
Nitrites are converted into nitrates by the second group of bacteria (e.g. Nitrobacter).
The bacteria are aerobic, so nitrification occurs only in well-aerated soils. The nitrate formed is the form of nitrogen that plants take up from the soil.

\ce{NH3 ->[\textit{Nitrosomonas}] NO2^- ->[\textit{Nitrobacter}] NO3^-}

2025 Annual I, Set D, Q.2 (xiv) OR3 marks

Name any three main greenhouse gases and their sources of emission.

Solution

Greenhouse gas	Sources of emission
Carbon dioxide (\ce{CO2})	Burning of coal, oil and natural gas for energy; deforestation reduces the trees that absorb it
Methane (\ce{CH4})	Production and transport of coal, oil and gas; livestock and other agricultural practices
Nitrous oxide (\ce{N2O})	Agricultural and industrial activities; combustion of fossil fuels and solid waste

(Fluorinated gases, synthetic gases used in industry and released during their production and use, are the fourth group.)

2025 Annual I, Set D, Q.45+2 = 7 marks

Describe Nitrogen Fixation, Ammonification and Nitrification stages involved in Nitrogen cycle. Also draw labelled Nitrogen cycle flow sheet.

Solution

The nitrogen cycle is a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is maintained by decomposers and other bacteria.

1. Nitrogen fixation (about 2 marks)

Nitrogen gas has a strong triple bond and is unreactive, so it must be fixed. It is fixed in three ways: atmospheric (lightning makes oxides of nitrogen that rain carries into the soil), industrial (nitrogen fertilizers are manufactured, e.g. \ce{N2 + 3H2 -> 2NH3}) and biological (nitrogen-fixing bacteria convert nitrogen to ammonia: free-living Azotobacter and Clostridium, and symbiotic Rhizobium in the root nodules of legumes).

2. Ammonification (about 1.5 marks)

Nitrogenous wastes of animals and compounds of dead organisms are decomposed by saprophytic soil bacteria and fungi into water, carbon dioxide, amino acids and energy. The amino acids are converted into ammonia or ammonium ions. This production of ammonia in the decomposition of organic matter is ammonification (aerobic soil).

3. Nitrification (about 1.5 marks)

Ammonia and ammonium ions are converted into nitrates by two groups of aerobic bacteria: Nitrosomonas converts ammonia to nitrites and Nitrobacter converts nitrites to nitrates. It occurs only in well-aerated soils.

\ce{NH3 ->[\textit{Nitrosomonas}] NO2^- ->[\textit{Nitrobacter}] NO3^-}

Labelled nitrogen cycle flow sheet (2 marks)

Nitrogen cycle flow sheet: fixation, ammonification, nitrification, assimilation and denitrification.

Plants take up nitrates and ammonia (assimilation) and animals eat plants; denitrifying bacteria return nitrogen to the air to complete the cycle.

2026 Annual I, Set D, Q.4 OR1x6 = 6 marks

Describe xerarch succession on bare rock from lichens to flowering plants.

Solution

Xerarch succession is primary succession that begins on bare rock where water is deficient. It passes through seral stages and ends in a climax community.

Xerarch succession on bare rock, in order of time, ending in the climax community.

Stages of xerarch succession in order.
Stage	Members and what happens
1. Crustose lichen (pioneer)	Licanora, Rhinodina. Slow growers that withstand extreme desiccation; absorb water like sponges in rain and decompose the rock by secreting acids
2. Foliose lichen	Permelia, Dermatocarpon. Grow on the little soil that has collected; their activities add a thin layer of soil
3. Moss	Xerophytic mosses appear when soil has gathered in crevices. They add soil; their dead mat holds water, making the habitat suitable for herbs
4. Herb	Annual weeds first; roots penetrate and secrete acids (more weathering); humus is added; shading reduces evaporation; then biennial and perennial herbs and xeric grasses
5. Shrub	Xeric shrubs (from seeds or rhizomes) overshadow the herbs, which are replaced; birds help disperse seeds, giving dense growth
6. Tree	Tree saplings establish among the shrubs; a canopy shades the area; leaf litter and decaying roots add humus and weather the soil
7. Climax	Xeric trees give place to mesophytic trees as soil deepens; a forest develops, a relatively stable community

Exam focus

Describe the nitrogen cycle: nitrogen fixation, ammonification, nitrification, assimilation and denitrification, with a labelled flow sheet. (long)
Set D 2025 Q.4 (7 marks) asked fixation, ammonification and nitrification with a flow sheet, and the cycle's single stages came as short questions in four papers. Learn all five processes and the figure.
Go to answer
Xerarch succession on bare rock, from lichens to flowering plants (crustose lichen, foliose lichen, moss, herb, shrub, tree, climax). (long)
A long question in three papers: 2026 Set D Q.4 OR, 2025 Set B Q.3 OR, and the book's long question 2. Know the order and what each stage does for the next.
Go to answer
Three ways of nitrogen fixation (atmospheric, industrial, biological) with the organisms. (short)
2026 Set B Q.2 (viii) and book short question 2; also part of 2025 Set D Q.4.
Go to answer
Nitrification in two steps (Nitrosomonas, then Nitrobacter) and denitrification and assimilation. (short)
2026 Set D Q.2 (viii) OR (nitrification) and 2025 Set B Q.2 (viii) (denitrification and assimilation).
Go to answer
Kinds of succession and the general stages (pioneers, seral communities, climax); primary compared with secondary succession. (long)
2026 Set B Q.4 OR (3+3), 2025 Set D MCQ 14 (secondary, xerarch after a forest fire) and book short question 6.
Go to answer
Loss of energy between trophic levels: about 90% is lost and 10% passes on; the six routes of loss. (mcq)
2025 Set B MCQ 1; also book short question 5 on the pyramid of energy.
Go to answer
Name the main greenhouse gases with their sources, and describe the greenhouse effect and its consequences. (short)
2025 Set D Q.2 (xiv) OR asked three gases and sources; book long question 1 asks the whole effect (8 marks).
Go to answer
Ocean acidification and coral reefs, and the grassland ecosystem of Pakistan. (long)
No paper question in 2025 or 2026 (Sets B and D) on these; they are book long questions 3 and 4 and unit SLOs, so they remain likely.
Go to answer