Ecosystem | ISC Class 12 Biology Notes

This note covers ecosystem structure and types, the pond as a functioning system, production of organic matter, breakdown of dead material, movement of energy through feeding relationships, measurements of living material and nutrients, and ecological pyramids.

What is an ecosystem, and how is it organised?

Definition: An ecosystem is a functional unit of nature in which living organisms interact with one another and with their surrounding physical environment.

An ecosystem may be as small as a pond or as large as a forest or sea. The biosphere is the part of Earth supporting life. Many ecologists regard it as a global ecosystem made up of all local ecosystems.

Which types and components can be recognised?

Terrestrial ecosystems occur on land, while aquatic ecosystems occur in water. Forests, grasslands and deserts are terrestrial examples. Ponds, lakes, wetlands, rivers and estuaries are aquatic examples. Crop fields and aquariums may also be considered man-made ecosystems.

Abiotic components are the non-living physical and chemical parts of the system, including air, water and soil. Biotic components are its living organisms. Their interactions give each type of ecosystem its characteristic physical structure.

Organic matter means carbon-based material associated with organisms, including food and dead remains. Inorganic materials include water, carbon dioxide and mineral salts. Nutrients are substances organisms require for growth and functioning.

Producers make organic food from inorganic materials. Consumers obtain food from other organisms. Decomposers break down dead organic matter into simpler inorganic substances. These groups describe functions within an ecosystem, rather than merely listing the organisms present.

A species is a group of similar organisms capable of interbreeding under natural conditions to produce fertile offspring. Species composition identifies and counts the plant and animal species present. Stratification describes their vertical distribution.

Species composition and stratification are distinct structural features. Knowing which species occur does not, by itself, describe their vertical arrangement. The forest example distinguishes the organisms making up the community from the layers in which they are distributed.

Structure becomes functional through the production of organic matter, its decomposition, energy flow and nutrient cycling. Nutrient cycling means the storage and movement of nutrient elements through ecosystem components, allowing those nutrients to be used repeatedly.

How does a pond function as an ecosystem?

A pond is a shallow water body and a fairly self-sustainable unit. Its water, organisms and bottom deposits interact, making it a useful example of how the living and non-living parts of an aquatic ecosystem function together.

What are the pond's components?

The abiotic part includes water, dissolved inorganic and organic substances, and the rich soil deposit at the bottom. Solar input, temperature cycles, day-length and other climatic conditions regulate the rate at which the whole pond functions.

Autotrophs are organisms that form organic food from inorganic materials. Pond autotrophs include phytoplankton, the microscopic photosynthetic organisms suspended in water, some algae, and floating, submerged and marginal plants. Marginal plants grow at the pond's edges.

Heterotrophs depend on organic food made by other organisms. Pond consumers include zooplankton, the animal component of plankton, as well as free-swimming and bottom-dwelling forms. Plankton are organisms that drift in water.

Fungi, bacteria and flagellates, organisms with whip-like structures called flagella, act as decomposers. They are especially abundant at the bottom of the pond. Their activity links dead material with the release of substances that producers can reuse.

How are these components connected?

1. Capture of sunlight: autotrophs use the Sun's radiant energy to convert inorganic materials into organic matter.
2. Consumption: heterotrophs feed on the autotrophs, passing food and its chemical energy into consumers.
3. Breakdown: dead organic matter undergoes decomposition, which converts complex organic substances into simpler inorganic substances.
4. Reuse of nutrients: mineralisation, the release of inorganic nutrients from organic matter, makes nutrients available again to autotrophs.

These events recur, but matter and energy do not behave identically. Nutrients can return for reuse. Energy moves in one direction through feeding levels and is dissipated as heat to the environment. Calling a pond fairly self-sustainable does not remove its requirement for an energy input.

How are primary and secondary productivity measured?

Biomass means the mass of living material. Photosynthesis is the process by which plants use light energy to form organic food from inorganic materials. Primary production is the amount of biomass or organic matter plants produce per unit area over a period through photosynthesis.

Productivity is the rate of biomass production. Production can be expressed as mass or energy per unit area; productivity also includes time. A rate allows the production of different ecosystems to be compared on a common basis.

What do the units and symbols mean?

Mass-based productivity may be expressed as g m⁻² yr⁻¹, meaning grams per square metre per year. Energy-based productivity may be expressed as kcal m⁻² yr⁻¹, meaning kilocalories per square metre per year. A kilocalorie is a unit of energy.

Gross primary productivity (GPP) is the rate of organic matter production during photosynthesis. Respiration is the cellular breakdown of organic food to release usable energy. Plants use a considerable amount of gross production in respiration.

Net primary productivity (NPP) is gross primary productivity after subtracting plant respiration losses, represented by R. The quantities must refer to the same area and time and use matching units.

GPP − R = NPP

Net primary productivity is the biomass available for consumption by heterotrophs, including herbivores and decomposers. Herbivores are animals that eat plants. Secondary productivity is the rate at which consumers form new organic matter; it is not another name for plant production.

Why does productivity vary?

Primary productivity depends on the plant species present, environmental factors, nutrient availability and the plants' photosynthetic capacity. Differences in these factors help explain why productivity varies among ecosystems. Area alone does not determine the total production of an ecosystem.

The whole biosphere's annual net primary productivity is approximately 170 billion tons of organic matter as dry weight. Oceans occupy about 70 per cent of Earth's surface but contribute only 55 billion tons; the rest is produced on land.

What processes break down dead organic matter?

Detritus consists of dead plant remains, such as leaves, bark and flowers, dead animal remains and faecal matter. It supplies the raw material for decomposition. Decomposers convert complex organic matter into inorganic substances such as carbon dioxide, water and nutrients.

Detritivores are animals that feed on detritus. Earthworms are an example: they help break down complex organic matter and loosen soil. Their physical breakdown of material should be distinguished from the chemical action of enzymes released by microbes.

How do the five named processes differ?

1. Fragmentation: detritivores, such as earthworms, break detritus into smaller particles. This is the physical subdivision of dead organic material.
2. Leaching: water-soluble inorganic nutrients move down into soil layers and become precipitated as unavailable salts. Precipitation here means formation of an insoluble solid.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances. Enzymes are biological catalysts, substances that speed up chemical reactions.
4. Humification: a dark-coloured, amorphous substance called humus accumulates. Amorphous means lacking a definite organised shape or structure.
5. Mineralisation: some microbes further degrade humus and release inorganic nutrients. Microbes are microscopic organisms, including bacteria and many fungi.

Note: The processes of decomposition operate simultaneously on detritus. The numbered list distinguishes their actions; it does not describe five compulsory, separate stages that must finish one after another.

Humus is highly resistant to microbial action and decomposes at an extremely slow rate. Its colloidal nature, involving very fine dispersed particles, enables it to serve as a nutrient reservoir. Slow decomposition does not mean that humus cannot undergo further breakdown.

A tree, a fallen green leaf, partly consumed leaves and further decomposition lead towards organic-rich soil. Arrows also show nutrients entering a food web and leaching into soil. A large arrow returns from the soil towards the tree.

See Fig. 12.1 in your NCERT textbook

The diagram connects the breakdown of leaf material with the soil supporting plant growth. Some material enters feeding relationships while other material is decomposed. This connection explains why decomposition contributes to the continued functioning of the whole ecosystem.

Which factors control the rate of decomposition?

The rate of decomposition depends on both the chemical composition of detritus and climatic conditions. Decomposition is largely an oxygen-requiring process. The word “largely” matters: the statement should not be strengthened into a claim that every decomposition process requires oxygen.

How does the composition of detritus matter?

Under a particular climatic condition, decomposition is slower when detritus is rich in lignin, a strengthening material in plant cell walls, or chitin, a structural substance found in fungal walls and arthropod coverings. Arthropods are animals with jointed limbs and an external skeleton.

Decomposition is quicker when detritus is rich in nitrogen and water-soluble substances such as sugars. The comparison assumes the climatic condition is the same. Otherwise, a difference in decay could reflect both the material's composition and its surrounding conditions.

How do temperature, moisture and oxygen act?

Temperature and soil moisture are the most important climatic factors regulating decomposition through their effects on soil microbes. Warm, moist conditions favour their decomposition activity. Low temperature and anaerobiosis, the absence of oxygen, inhibit decomposition.

Moisture and oxygen availability must therefore be considered separately. The statement that warm, moist conditions favour decomposition does not mean that oxygen-free conditions favour it too. A sound explanation identifies the actual condition and connects it with microbial activity or the composition of detritus.

How does sunlight enter an ecosystem and energy move through it?

Except for the deep-sea hydrothermal ecosystem, associated with hot-water vents on the sea floor, the Sun is the only energy source for ecosystems on Earth. This exception should remain attached to the statement about the dependence of ecosystems on sunlight.

Photosynthetically active radiation (PAR) is the part of incoming solar radiation that can be used in photosynthesis. Less than 50 per cent of incident solar radiation is PAR. “Incident” means falling on a surface.

Plants capture only 2 to 10 per cent of PAR. This percentage uses PAR as its starting quantity; it is not 2 to 10 per cent of all incident sunlight. Keeping the starting quantity clear prevents confusion between successive stages of energy capture.

What happens after producers capture light?

1. Energy capture: plants and photosynthetic bacteria use the Sun's radiant energy to make food from simple inorganic materials.
2. Entry into consumers: animals obtain that food energy by feeding on plants directly or on other animals that depend on plants.
3. Transfer through feeding: food passes between organisms, while the energy available decreases at successive feeding levels.
4. Entry into detritus: dead organisms become dead organic material that supplies energy to decomposers.
5. Heat loss: energy is dissipated as heat to the environment during ecosystem functioning, requiring a continuing energy supply.

A trophic level is an organism's feeding position in a food chain. Energy moves from the Sun to producers and then to consumers. This unidirectional flow differs from nutrient cycling: loss as heat does not represent nutrients returning to plants.

Arrows connect sunlight, a plant, a plant-eating larva, a bird and a larger bird. The four feeding levels are labelled. Branching arrows lead towards dead material below, while several outward arrows are labelled “Heat”.

See Fig. 12.3 in your NCERT textbook

The branches show why a feeding sequence alone does not describe every energy pathway. Dead material can arise at different levels, and energy loss occurs throughout the system. The energy captured by a producer does not remain within that organism indefinitely.

How do grazing food chains, detritus food chains and food webs differ?

A food chain is a sequence of organisms linked by feeding. A grazing food chain (GFC) begins with living green plants. The sequence grass → goat → human shows a producer, a primary consumer eating that producer, and a secondary consumer eating the primary consumer.

A primary consumer feeds on producers. A secondary consumer eats primary consumers; a tertiary consumer feeds on secondary consumers. Carnivores are animals that eat other animals. Primary carnivores eat herbivores, so they are secondary consumers.

Where does the detritus pathway begin?

A detritus food chain (DFC) starts with dead organic matter. Its decomposers are heterotrophs, mainly fungi and bacteria. They meet energy and nutrient requirements by degrading detritus rather than feeding on living producers at the start of the chain.

These decomposers are also called saprotrophs, organisms that obtain nourishment from dead organic material. They secrete digestive enzymes onto dead and waste material, break it down into simple inorganic materials and subsequently absorb the products.

Why are natural feeding relationships webs?

A food web is a network formed by interconnected food chains. Some organisms in the detritus food chain are prey for grazing-chain animals. Omnivores, animals eating both plant and animal food, also connect feeding pathways; cockroaches and crows are examples.

Thus, grazing and detritus chains may connect at some levels. Treating them as completely isolated systems would miss these natural relationships. A chain follows one feeding sequence, whereas a web represents the interconnections between several sequences.

What do trophic levels, standing crop and standing state describe?

An organism's trophic level depends on its source of food. Producers occupy the first level, herbivores the second, and secondary consumers the third. Tertiary consumers can occupy a fourth level. Each level depends on lower levels for its energy requirements.

Four boxes rise from producer to primary, secondary and tertiary consumer, joined by upward arrows. Alongside them are the first to fourth trophic levels. Examples include phytoplankton, grass and trees for producers, and zooplankton, grasshopper and cow for primary consumers.

See Fig. 12.2 in your NCERT textbook

Is a trophic level a permanent label for a species?

A trophic level is a functional position, not a species name. A sparrow acts as a primary consumer when eating seeds, fruits and peas, but as a secondary consumer when eating insects and worms. The same species may occupy more than one level simultaneously.

Any calculation of numbers, biomass or energy at a trophic level must include all the organisms at that level. Selecting only a few individuals cannot establish the relationship between whole trophic levels represented by an ecological pyramid.

How do the two standing measurements differ?

Standing crop is the mass of living material present at a trophic level at a particular time. It can be measured as biomass or as the number of organisms per unit area. Biomass may be expressed as fresh weight or dry weight.

Dry weight is mass after removal of water; fresh weight includes water. Measurement of biomass as dry weight is more accurate. A standing crop describes material present at a given time, whereas productivity describes its rate of production.

Standing state is the amount of inorganic nutrients present in an ecosystem at a particular time. It concerns the nutrient pool, rather than the living biomass or number of organisms measured as standing crop. Neither term, by itself, means a rate of production.

How do pyramids of numbers and biomass represent ecosystems?

An ecological pyramid represents relationships between trophic levels in terms of numbers, biomass or energy. Producers form the base, and the highest consumers form the apex. “Apex” means the top of the representation.

A pyramid of numbers compares the number of organisms at each trophic level. A pyramid of biomass compares their living mass. The same feeding relationships can therefore be represented using different measurements, which need not produce the same shape.

What do upright and inverted shapes mean?

An upright pyramid has a broad producer base and becomes narrower towards higher consumers. An inverted pyramid has a producer bar narrower than a consumer bar above it. In most ecosystems, pyramids of numbers, biomass and energy are upright.

Two four-level pyramids narrow upwards. The first compares numbers; the second compares dry weight. A third, two-level biomass diagram has a narrow producer bar labelled 4 below a wider primary-consumer bar labelled 21.

See Figs. 12.4a, 12.4b and 12.4c in your NCERT textbook

The four-level pyramids of numbers and biomass show the following values. kg m⁻² means kilograms per square metre, the dry-weight unit used in the biomass diagram. The two columns describe different measurements and should not be treated as conversions of one another.

The grassland numbers example shows very few top consumers supported by nearly six million plants. The biomass example shows a sharp decrease in dry weight at higher levels. Neither example requires equal percentage decreases between successive bars.

The sea's biomass pyramid is generally inverted, with fish biomass far exceeding phytoplankton biomass. The two-level diagram illustrates a small standing crop of phytoplankton supporting a larger standing crop of zooplankton. The biomass values are 4 for phytoplankton and 21 for zooplankton.

Why is the energy pyramid upright, and what are the limits of pyramids?

The 10 per cent law states that only 10 per cent of the energy at a lower trophic level is transferred to the next trophic level. The declining energy supply restricts the number of trophic levels in a grazing food chain.

A pyramid of energy shows energy at each trophic level for a given time, or annually, per unit area. It is always upright and can never be inverted: some energy is always lost as heat when energy passes from one trophic level to the next.

How is an energy calculation performed?

The next level's energy equals the lower level's energy multiplied by 10/100. Here 10/100 means ten per cent, or one tenth. J stands for joule, a unit of energy.

The sunlight input is labelled 1,000,000 J. Above it, the producer, primary-consumer, secondary-consumer and tertiary-consumer bars are labelled 10,000 J, 1000 J, 100 J and 10 J respectively. The bars narrow upwards.

See Fig. 12.4d in your NCERT textbook

Starting with the given producer energy of 10,000 J, the next level receives 10,000 × 10/100 = 1000 J. Repeating the transfer gives 100 J and then 10 J. These calculations illustrate the transfer between successive trophic levels.

The same diagram shows primary producers converting only 1 per cent of the available sunlight energy into net primary productivity. This refers to that ideal diagram. It is distinct from the percentage of PAR captured by plants and from transfer between trophic levels.

What does a pyramid leave out?

• Multiple feeding positions: pyramids do not account for the same species belonging to two or more trophic levels.
• Food-web complexity: they assume a simple food chain, something that almost never exists in nature, and do not accommodate food webs.
• Decomposer activity: saprotrophic decomposers are not given a place, although they play a vital role in ecosystem functioning.

Pyramids are useful comparisons, but their limitations prevent them from representing every ecological connection. An inverted biomass pyramid does not imply an inverted energy pyramid: living material present at one time and energy supplied over time are different quantities.

Glossary

• Ecosystem — A functional unit in which organisms interact with one another and their physical surroundings.
• Stratification — The vertical distribution of different species among different layers of an ecosystem.
• Primary productivity — The rate at which producers form biomass or organic matter through photosynthesis.
• Gross primary productivity — The total rate of organic matter production during photosynthesis before plant respiration losses are subtracted.
• Net primary productivity — Gross primary productivity remaining after respiration losses, available for consumption by heterotrophs.
• Secondary productivity — The rate at which consumers form new organic matter from their food.
• Detritus — Dead plant and animal remains, including faecal matter, that provide material for decomposition.
• Fragmentation — The breakdown of detritus into smaller particles by detritivores such as earthworms.
• Humification — The accumulation of dark-coloured, amorphous humus during the decomposition of organic matter in soil.
• Mineralisation — The release of inorganic nutrients when microbes further degrade organic matter such as humus.
• Trophic level — The feeding position occupied by an organism according to its source of nutrition.
• Standing crop — Living material present at a trophic level at a particular time, measured as biomass or numbers per unit area.
• Standing state — The amount of inorganic nutrients present within an ecosystem at a particular time.
• Food web — A network of feeding relationships formed through natural interconnections among food chains.
• Ecological pyramid — A representation of numbers, biomass or energy at successive trophic levels in an ecosystem.

Common errors and misconceptions

• Misconception: Gross and net primary productivity are identical. Correct: Net primary productivity remains after plant respiration losses have been subtracted from gross primary productivity.
• Misconception: Decomposition consists of five strictly successive stages. Correct: Its processes operate simultaneously on detritus; listing their names does not establish an obligatory chronological order.
• Misconception: Humus cannot be broken down by microbes. Correct: It is highly resistant and decomposes at an extremely slow rate, but some microbes further degrade it and release inorganic nutrients.
• Misconception: Plants capture 2 to 10 per cent of all incident sunlight. Correct: They capture 2 to 10 per cent of photosynthetically active radiation, which forms less than 50 per cent of incident radiation.
• Misconception: A species permanently occupies one trophic level. Correct: Its feeding role determines its level; a sparrow occupies different levels when eating plant food and animal food.
• Misconception: Every ecological pyramid is upright. Correct: Most ecosystems have upright pyramids, but exceptions occur for numbers and biomass. The energy pyramid is always upright.
• Misconception: Standing crop, standing state and productivity measure the same thing. Correct: They describe living material, inorganic nutrients and a rate of organic matter production, respectively.
• Misconception: Energy returns to producers in the same way as nutrients. Correct: Nutrients are reused, whereas energy flows in one direction and is dissipated as heat.

Exam-style questions with model answers

Q1. Define an ecosystem and distinguish its biotic and abiotic components. [2 marks]

1. An ecosystem is a functional unit of nature in which organisms interact with one another and with their surrounding physical environment.
2. Biotic components are living organisms, including producers, consumers and decomposers; abiotic components are non-living surroundings, including air, water and soil.

Q2. Define gross primary productivity, net primary productivity and secondary productivity. Include the relationship between gross and net primary productivity, defining its symbols. [3 marks]

1. Gross primary productivity, abbreviated GPP, is the rate of production of organic matter during photosynthesis in an ecosystem, before subtracting the amount used in plant respiration.
2. Net primary productivity, abbreviated NPP, is the remaining production after plant respiration losses, represented by R. Thus, GPP − R = NPP; this remaining biomass is available to heterotrophs.
3. Secondary productivity is the rate of formation of new organic matter by consumers, rather than the rate of photosynthetic production by plants.

Q3. Explain the roles of the abiotic environment, producers, consumers and decomposers in a pond ecosystem. [4 marks]

1. The abiotic environment includes water, dissolved substances and bottom soil deposits. Solar input, temperature cycles and day-length help regulate the pond's functioning.
2. Producers, including phytoplankton, algae and aquatic plants, use sunlight to convert inorganic substances into organic material that supplies food.
3. Consumers include zooplankton and free-swimming or bottom-dwelling forms. Their feeding transfers organic matter and its energy through the pond community.
4. Decomposers include fungi, bacteria and flagellates, especially abundant at the bottom. Their breakdown of dead material releases nutrients for reuse by producers.

Q4. Explain fragmentation, leaching, catabolism, humification and mineralisation during decomposition. Make clear whether these processes form a compulsory sequence. [5 marks]

1. Fragmentation is the breakdown of detritus into smaller particles by detritivores such as earthworms. The named decomposition processes operate simultaneously, rather than forming a compulsory sequence of completed stages.
2. Leaching carries water-soluble inorganic nutrients down into soil layers, where they become precipitated as unavailable salts. It involves movement of dissolved substances rather than physical fragmentation.
3. Catabolism involves bacterial and fungal enzymes degrading detritus into simpler inorganic substances. This chemical breakdown differs from detritivores breaking material into smaller particles.
4. Humification produces an accumulation of dark-coloured, amorphous humus. It is highly resistant to microbial action, decomposes extremely slowly and serves as a nutrient reservoir.
5. Mineralisation releases inorganic nutrients when some microbes further degrade humus. It therefore explains why resistance to decomposition does not make humus permanently unavailable for further breakdown.

Q5. Under the same climatic conditions, one sample of detritus is rich in lignin and chitin, while another is rich in nitrogen and water-soluble sugars. Predict their relative decomposition rates, explain the effect of warm and moist conditions, and state what low temperature and absence of oxygen do. [4 marks]

1. The lignin-rich and chitin-rich sample decomposes more slowly under the stated common climatic conditions because decomposition rate depends on detritus composition.
2. The sample rich in nitrogen and water-soluble sugars decomposes more quickly under those same conditions.
3. Warm and moist conditions favour decomposition through their effects on the activities of soil microbes.
4. Low temperature and absence of oxygen inhibit decomposition, resulting in the accumulation of organic material; decomposition is largely an oxygen-requiring process.

Q6. In an ideal energy pyramid, producers contain 10,000 J of energy, where J means joule. Apply the 10 per cent transfer law to calculate the energy of primary, secondary and tertiary consumers, then explain the pyramid's shape. [4 marks]

1. Primary consumers receive 10 per cent of producer energy: 10,000 × 10/100 = 1000 J. The starting quantity is the stated producer energy.
2. Secondary consumers receive 10 per cent of primary-consumer energy: 1000 × 10/100 = 100 J.
3. Tertiary consumers receive 10 per cent of secondary-consumer energy: 100 × 10/100 = 10 J.
4. The energy pyramid is upright because energy decreases at successive trophic levels, with some energy always lost as heat during each transfer.

Q7. Distinguish a pyramid of biomass from a pyramid of energy, explain why a sea biomass pyramid may be inverted, and state the three limitations of ecological pyramids. [5 marks]

1. A biomass pyramid compares the mass of living material at successive trophic levels. The sea's biomass pyramid is generally inverted because fish biomass far exceeds phytoplankton biomass.
2. An energy pyramid compares energy at successive trophic levels over a given time per unit area. It is always upright because some energy is always lost as heat during transfer.
3. Ecological pyramids do not account for a species occupying two or more trophic levels. An organism's feeding role can change with the food it consumes.
4. They assume a simple food chain, something that almost never exists in nature. Consequently, they do not accommodate the interconnected feeding relationships of a food web.
5. They give no place to saprotrophic decomposers, even though these organisms play a vital role in ecosystem functioning by breaking down dead organic matter.

Q8. Distinguish standing crop from standing state. [2 marks]

1. Standing crop is living material present at a trophic level at a particular time, measured as biomass or number per unit area.
2. Standing state is the amount of inorganic nutrients present in an ecosystem at a particular time, rather than its living material.

Key takeaways

• An ecosystem combines living organisms and non-living surroundings through production, decomposition, energy flow and nutrient cycling.
• A pond contains interacting producers, consumers and decomposers, with sunlight and physical conditions regulating its functioning.
• Net primary productivity is gross primary productivity minus plant respiration losses; secondary productivity describes new organic matter formed by consumers.
• Fragmentation, leaching, catabolism, humification and mineralisation describe distinct but simultaneously operating processes of decomposition.
• Decomposition is largely oxygen-requiring; its rate depends on detritus composition and climatic effects on soil microbes.
• Food webs connect grazing and detritus pathways, while an organism's food source determines its trophic position.
• Standing crop measures living material at a particular time, whereas standing state measures inorganic nutrients present.
• Energy pyramids are always upright, but biomass and number pyramids have exceptions to the usual upright form.

Test yourself

What is the difference between species composition and stratification?

Species composition identifies and counts the species present. Stratification describes the vertical distribution of different species among different layers.

Why is net primary productivity lower than gross primary productivity?

Plants use a considerable amount of gross production in respiration. Net primary productivity is the production remaining after these respiration losses.

Which decomposition process is carried out when earthworms break detritus into smaller particles?

This is fragmentation, the physical breakdown of detritus into smaller particles by detritivores.

Does humus resist all further microbial breakdown?

No. Humus is highly resistant and decomposes extremely slowly, but some microbes further degrade it, releasing inorganic nutrients through mineralisation.

What fraction of incident solar radiation is photosynthetically active radiation?

Less than 50 per cent is photosynthetically active radiation; plants capture only 2 to 10 per cent of that radiation.

Which food-chain pathway carries the larger energy fraction in terrestrial ecosystems?

A much larger fraction of energy flows through the detritus food chain than through the grazing food chain.

A sparrow eats seeds and also insects. Why can it occupy different trophic levels?

It is a primary consumer when eating seeds and a secondary consumer when eating insects. Trophic position reflects feeding function rather than a fixed species label.

Why can the energy pyramid never be inverted?

Some energy is always lost as heat between trophic levels, so a higher trophic level has less energy than the level below it.