(a) Producers (also called autotrophs). The green plants in an ecosystem are called producers, because they fix the Sun's radiant energy and carbon dioxide to make food from simple inorganic materials. They occupy the first trophic level.

(b) Inverted. One big tree supports thousands of insects, which support a smaller number of small birds, which support fewer large birds. Counting individuals therefore gives a narrow base and a broad middle — the opposite of the upright grassland pyramid in which nearly 6 million plants support only three top carnivores.

(c) Light (along with nutrients). Light penetrates only a thin surface layer of water, so photosynthesis is confined to the lit zone; below it, no producer can fix carbon however abundant the nutrients. This is a principal reason why the oceans, despite occupying about 70 per cent of the surface, contribute only 55 of the biosphere's 170 billion tons of annual net primary production.

(d) Earthworms (also millipedes, woodlice, termites, beetle larvae and the like). Detritivores break down detritus into smaller particles, a process called fragmentation. The earthworm is called the farmer's ‘friend’ because it helps in the breakdown of complex organic matter as well as in the loosening of the soil.

(e) The oceans. Carbon follows a gaseous type of nutrient cycle, for which the atmosphere or hydrosphere is the reservoir, and the great bulk of the Earth's mobile carbon is dissolved in the oceans, which therefore regulate the amount of carbon dioxide in the atmosphere.

The correct answer is (d) Decomposers.

Why. Decomposers are microscopic — mainly fungi and bacteria — and they act on the dead remains of organisms at every trophic level, not just one. Because they draw on the whole ecosystem and because each individual is minute, their numbers exceed those of any other group by an enormous margin. A single gram of fertile soil contains millions of bacteria.

Why not (a). Producers have the largest population among the levels of the grazing food chain, and in a grassland they do outnumber everything above them — nearly 6 million plants supporting three top carnivores. But the question asks about the food chain as a whole, and decomposers form the detritus food chain, which they far outnumber the producers in.

Why not (b) or (c). The amount of energy decreases at successive trophic levels, following the 10 per cent law, so primary consumers are fewer than producers and secondary consumers fewer still.

A relevant footnote. This is exactly the group that ecological pyramids leave out: saprophytes are given no place in ecological pyramids even though they play a vital role in the ecosystem.

The correct answer is (b) Zooplankton.

Why. Producers belong to the first trophic level, herbivores or primary consumers to the second, and carnivores or secondary consumers to the third. In a lake the producers are the phytoplankton, and the organisms that graze on them are the zooplankton — so the zooplankton are the primary consumers and occupy the second trophic level.

Why not the others. (a) Phytoplankton are the producers, and therefore the first trophic level. (c) Benthos are the bottom-dwelling organisms, a mixed group defined by where they live rather than by what they eat; they may be primary consumers, carnivores or detritus feeders. (d) Fishes generally occupy the third trophic level or higher, feeding on zooplankton or on other fish.

A caution. Remember that a trophic level represents a functional level, not a species, and a given organism may occupy more than one trophic level simultaneously. Some zooplankton are themselves carnivorous and eat other zooplankton, which places them at the third level in that feeding relationship.

The correct answer is (d) None of the above.

Why the term is empty. Production, in the sense used in this chapter, means the creation of organic matter from inorganic materials using an external energy source. Only autotrophs do that, and they are simply called producers — there is no second tier of them. There is therefore no such category as a 'secondary producer'.

The term that does exist. Secondary productivity — which is the rate of formation of new organic matter by consumers, or the rate of assimilation of food energy by them. Note carefully that secondary productivity is a real quantity while a secondary producer is not: consumers form new organic matter, but they do not produce it in the ecological sense, because they add no new energy to the ecosystem. They only repackage energy the producers already fixed, losing most of it as heat.

Why (a) is the tempting wrong answer. Herbivores are the organisms whose activity constitutes secondary productivity, so they are often mislabelled 'secondary producers'. But herbivores are primary consumers, occupying the second trophic level.

The correct answer is (b) 50% — more precisely, of the incident solar radiation less than 50 per cent is photosynthetically active radiation.

Why the other figures are wrong, and why (d) is the trap. The figure 2–10 per cent in option (d) is a real figure from the chapter, but it is the proportion of the PAR that plants actually capture — not the proportion of solar radiation that is PAR. The two numbers are often confused.

Keep the sequence straight.

• Total incident solar radiation arrives at the surface.
• Less than 50 per cent of it is photosynthetically active radiation — the wavelengths plants can use.
• Plants capture only 2–10 per cent of that PAR.
• And this small amount of energy sustains the entire living world.

Option (c), 1–5 per cent, is not a figure given in the chapter at all.

(a) Grazing food chain and detritus food chain

(b) Production and decomposition

(c) Upright and inverted pyramid

(d) Food chain and food web

(e) Litter and detritus. Detritus is the whole category of dead organic matter that serves as raw material for decomposition — dead plant remains such as leaves, bark and flowers, and the dead remains of animals including fecal matter, whether above ground or within the soil, and at any stage of breakdown. Litter is the layer of largely plant debris lying on the soil surface, still recognisable and not yet appreciably decomposed. All litter is detritus; not all detritus is litter.

(f) Primary and secondary productivity. Primary productivity is the rate of biomass production by plants during photosynthesis — the rate of capture of solar energy — divided into GPP and NPP, where GPP − R = NPP. Secondary productivity is the rate of formation of new organic matter by consumers, or the rate of assimilation of food energy by them. Primary productivity creates organic matter from inorganic materials; secondary productivity only converts organic matter that already exists, and is therefore always far smaller.

An ecosystem is a structural and functional unit of nature comprising abiotic and biotic components.

Abiotic components. The inorganic materials — air, water and soil — together with the dissolved substances in them. In a pond, the abiotic component is the water with all its dissolved inorganic and organic substances and the rich soil deposit at the bottom. Solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire ecosystem.

Biotic components. These are of three kinds.

Producers. The green plants, which fix the Sun's radiant energy to make food from simple inorganic materials. In a terrestrial ecosystem the major producers are herbaceous and woody plants; in an aquatic ecosystem, phytoplankton, algae and higher plants. They occupy the first trophic level.

Consumers. All animals depend on plants directly or indirectly for their food needs, and are hence called consumers and also heterotrophs. Primary consumers or herbivores feed on the producers; secondary consumers or primary carnivores feed on the herbivores; tertiary consumers or secondary carnivores feed on those.

Decomposers. Heterotrophic organisms, mainly fungi and bacteria, also called saprotrophs. They meet their energy and nutrient requirements by degrading dead organic matter, secreting digestive enzymes that break dead and waste materials into simple inorganic materials which they then absorb.

The structure that results. Interaction of biotic and abiotic components results in a physical structure characteristic for each type of ecosystem, described by two features: species composition, obtained by identification and enumeration of the plant and animal species, and stratification, the vertical distribution of different species occupying different levels — trees in the top layer of a forest, shrubs in the second, herbs and grasses at the bottom.

How the components function as a unit. Through four aspects: productivity, decomposition, energy flow and nutrient cycling.

Definition. An ecological pyramid is a graphical representation of the food or energy relationship between organisms at different trophic levels, expressed in terms of number, biomass or energy. As in any pyramid, the base is broad and it narrows towards the apex: the base of each pyramid represents the producers or the first trophic level, while the apex represents the tertiary or top-level consumer. The three types usually studied are the pyramid of number, the pyramid of biomass and the pyramid of energy.

Pyramid of number. Each bar represents the number of individuals present at that trophic level.

Upright — a grassland. Only three top carnivores are supported in an ecosystem based on the production of nearly 6 million plants. The base is enormously broad and the apex extremely narrow.

Inverted — a single big tree. One tree supports thousands of insects feeding on it; those support a smaller number of small birds; and those support fewer large birds. The base is a single individual, so the pyramid is inverted.

What this reveals. A count of individuals ignores size, and size determines how much living material and energy a level actually holds. One tree may outweigh every animal in the ecosystem.

Pyramid of biomass. Each bar represents the standing crop — the mass of living material at that trophic level at a particular time, measured as biomass in fresh or dry weight, dry weight being more accurate.

Upright — most terrestrial ecosystems. The pyramid of biomass shows a sharp decrease in biomass at higher trophic levels, since producers greatly exceed herbivores and herbivores greatly exceed carnivores.

Inverted — the sea. The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton: a small standing crop of phytoplankton supports a large standing crop of zooplankton. The paradox is resolved by turnover — phytoplankton live only a few days, so a tiny standing crop yields a very large annual production.

The contrast with the pyramid of energy. Pyramids of number and biomass are snapshots and can therefore invert. A pyramid of energy is always upright and can never be inverted, because when energy flows from one trophic level to the next, some energy is always lost as heat at each step.

Limitations of both. They do not take into account the same species belonging to two or more trophic levels; they assume a simple food chain and cannot accommodate a food web; and saprophytes are given no place in them even though they play a vital role in the ecosystem.

Definition. Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed in weight (g m⁻²) or energy (kcal m⁻²). The rate of that production is primary productivity, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ so that different ecosystems can be compared. It is the rate of capture of solar energy by the producers, and is divided into gross primary productivity, the rate of production of organic matter during photosynthesis, and net primary productivity, which is GPP minus respiration losses and is the biomass available for consumption by heterotrophs.

Factors that affect it.

(i) The plant species inhabiting a particular area. Species differ in leaf area, growth form, life span and photosynthetic pathway, so the same site supports very different productivities under different vegetation.

(ii) Availability of nutrients. Nitrogen, phosphorus and other minerals limit growth wherever they are scarce. This is a main reason for the low productivity of the open ocean, where nutrients sink out of the lit surface layer.

(iii) Photosynthetic capacity of the plants. The maximum rate at which the plant's own machinery can fix carbon sets a ceiling that no extra light or nutrient can exceed.

(iv) Environmental factors. Solar radiation — in aquatic systems, light is the limiting factor because it penetrates only a thin surface layer — along with temperature, water availability, day-length and carbon dioxide concentration.

The consequence. Because all of these vary geographically, primary productivity varies greatly in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons of dry organic matter, of which the oceans contribute only 55 billion tons despite occupying about 70 per cent of the surface.

Definition. Decomposition is the process in which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

The raw material. Dead plant remains such as leaves, bark and flowers, and the dead remains of animals including fecal matter, constitute detritus, which is the raw material for decomposition.

The processes.

• Fragmentation — detritivores such as the earthworm break down detritus into smaller particles.
• Leaching — water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
• Catabolism — bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
• Humification — leads to the accumulation of humus, a dark coloured amorphous substance highly resistant to microbial action which decomposes extremely slowly and, being colloidal, serves as a reservoir of nutrients.
• Mineralisation — the humus is further degraded by some microbes and inorganic nutrients are released.

All these steps operate simultaneously on the detritus, and humification and mineralisation occur during decomposition in the soil.

The products. Carbon dioxide, water, inorganic nutrients, and humus.

What controls the rate. Decomposition is largely an oxygen-requiring process, and its rate is controlled by the chemical composition of the detritus and by climatic factors. It is slower if detritus is rich in lignin and chitin, and quicker if rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most important climatic factors, acting through their effects on soil microbes: warm and moist environments favour decomposition, while low temperature and anaerobiosis inhibit it, resulting in a build-up of organic materials.

The source and the fraction used. Except for the deep-sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation, and plants capture only 2 to 10 per cent of that PAR — and this small amount of energy sustains the entire living world.

Entry and direction. Plants and photosynthetic bacteria, the autotrophs, fix the Sun's radiant energy to make food from simple inorganic materials. All organisms depend for their food on producers, either directly or indirectly, so there is a unidirectional flow of energy from the sun to producers and then to consumers. This is consistent with the first law of thermodynamics, since energy is neither created nor destroyed; and ecosystems are not exempt from the second law either, since they need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency towards increasing disorderliness.

Through the trophic levels. Based on the source of their nutrition, organisms occupy a trophic level: producers the first, herbivores or primary consumers the second, carnivores or secondary consumers the third. A simple grazing food chain runs grass → goat → man. In parallel, the detritus food chain begins with dead organic matter and is made up of decomposers, mainly fungi and bacteria, also called saprotrophs, which secrete digestive enzymes to break down dead and waste materials and then absorb them. In aquatic ecosystems the GFC is the major conduit for energy flow; in terrestrial ecosystems a much larger fraction flows through the DFC. The interconnection of these chains makes a food web, since some DFC organisms are prey to GFC animals and some animals, such as cockroaches and crows, are omnivores.

Loss at each step. The amount of energy decreases at successive trophic levels. Transfer follows the 10 per cent law — only 10 per cent of the energy is transferred to each trophic level from the one below — and the rest is lost as heat or remains in material not consumed. No energy trapped in an organism remains in it forever: it is either passed to a consumer, or the organism dies, and the death of an organism is the beginning of the detritus food chain.

The consequences. The number of trophic levels in the grazing food chain is restricted, typically to producer, herbivore, primary carnivore and secondary carnivore. And the pyramid of energy is always upright and can never be inverted, since some energy is always lost as heat at each step. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time, or annually per unit area.