Ecosystem

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 12.

Why are there so few lions?

Walk through a grassland and count. Millions of grass plants, hundreds of thousands of insects and grazers, a handful of carnivores, and perhaps three animals at the very top. It looks like a numbers game, but it is really an energy game. Each time one organism eats another, most of the energy never makes it to the next mouth. This chapter follows that energy from the Sun to the soil, and it answers the question every CUET paper keeps asking in different words: how much is left at each step, and why?

What counts as an ecosystem

An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment around them. It can be as small as a pond or as large as a forest or sea. The whole biosphere is the global ecosystem.

Every ecosystem has a physical structure that comes from the interaction of its living and non-living parts. Two features describe it. Species composition is the list of species present. Stratification is their vertical layering: in a forest, trees on top, shrubs in the middle and herbs and grasses at the bottom.

A pond is the easiest model. The abiotic part is water with dissolved substances and the soil at the bottom. The producers are phytoplankton, algae and floating, submerged and marginal plants. The consumers are zooplankton and free-swimming and bottom-dwelling animals. The decomposers are fungi, bacteria and flagellates, most abundant at the bottom.

An ecosystem does four jobs, and the rest of the chapter takes them one at a time: productivity, decomposition, energy flow and nutrient cycling.

Productivity: how much food does the system make?

Plants capture light and build organic matter. Primary production is the amount of biomass, or organic matter, produced per unit area over a time period by plants during photosynthesis. It is expressed as weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is productivity, in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Keep the two straight: production is an amount, productivity is a rate.

The balance in one line: GPP − R = NPP

Think of a salary. GPP is what you earn, R is what you spend on living, and NPP is what is left over for others to feed on.

Pause and predict: in a field, plants produce 1500 units of organic matter by photosynthesis and burn 600 units in respiration. How much is available to the grazers and decomposers?

Check your answer: NPP = 1500 − 600 = 900. The grazers never see the 600 that the plants already used.

The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter. The oceans cover about 70 per cent of the Earth's surface but contribute only about 55 billion tons of it.

Beyond the textbook. For the exam: total NPP is about 170 billion tons and the oceans contribute only 55 billion tons in spite of their size. In reality: the usual explanation is that sunlight reaches only the surface layers of the sea and nutrients there are often scarce, so most of the open ocean is much less productive per square metre than most land. The oceans still matter hugely because their area is so large. Why it matters: it shows that "bigger" does not mean "more productive", the sort of reversal NTA likes.

Decomposition: the recycling of everything dead

Dead leaves, bark, flowers, animal remains and faeces together make detritus, the raw material of decomposition. Decomposers break complex organic matter into inorganic substances such as carbon dioxide, water and nutrients. Without them, dead matter would pile up and nutrients would stay locked away.

Five steps run simultaneously, not one after another.

Notice the difference that NTA exploits: detritivores do the physical work of fragmentation, while fungi and bacteria do the chemical work of catabolism, humification and mineralisation.

Decomposition needs oxygen. Its speed depends on what the detritus is made of and on the climate.

Why it works this way. Microbes need nitrogen to build their own proteins and an easy sugar supply for fuel. A leaf full of both is a feast. Lignin, the tough material in wood, and chitin, found in fungal walls and insect shells, are hard for enzymes to break, so they last. Cold slows the enzymes, and without oxygen the oxygen-requiring microbes cannot work.

Beyond the textbook. For the exam: low temperature and absence of oxygen inhibit decomposition. In reality: this is exactly why peat bogs exist. In cold, waterlogged and low-oxygen soil, dead plant matter decomposes so slowly that it piles up as peat instead of being recycled. Why it matters: it turns two factors from your table into a landscape you can picture.

Energy flow: a one-way street

The Sun is the only source of energy for nearly all ecosystems. The exception is the deep-sea hydrothermal vents. Here the energy comes from chemicals instead.

Of the solar radiation that reaches Earth, photosynthetically active radiation (PAR) is the part plants can use for photosynthesis, and it is less than 50 per cent of the incident radiation. Of that PAR, plants capture only 2 to 10 per cent. This small fraction supports the whole living world.

Energy flow follows the laws of thermodynamics and is unidirectional: Sun → producers → consumers → decomposers. It never returns to the Sun, and at each step part of it is lost as heat. This is why ecosystems need a continuous supply of sunlight, whereas nutrients are used again and again.

Food chains and webs. Organisms are linked by who eats whom. Each feeding position is a trophic level: producers occupy the first, herbivores the second, carnivores the third, and top carnivores the fourth.

The natural interconnection of food chains, with the GFC and DFC linked, is a food web. A single organism can sit at more than one trophic level. In the chain grass → goat → man, man is a secondary consumer, but a man eating a vegetable is a primary consumer.

Standing crop is the mass of living material (or the number of individuals) per unit area at each trophic level at a given time. Biomass is measured as fresh or dry weight, and dry weight is more accurate because the water content of fresh weight varies.

The 10 per cent law. Lindemann's law states that only about 10 per cent of the energy passes from one trophic level to the next. The rest is spent in respiration, lost as heat or left in dead bodies. Therefore the energy available shrinks very fast and food chains cannot have many links.

Worked example. A producer level has 20,000 J. Primary consumers receive 10 per cent of that: 2,000 J. Secondary consumers receive 10 per cent of that: 200 J. A common error is to stop one step early.

Beyond the textbook. For the exam: 10 per cent is transferred and 90 per cent is lost. In reality: 10 per cent is an average rule of thumb. The actual efficiency varies widely between ecosystems and between feeding groups, and it is often somewhere between about 5 and 20 per cent. Why it matters: it explains why you can still answer 10 per cent in the exam while knowing why real values scatter around it.

Ecological pyramids: drawing the energy game

An ecological pyramid shows the producers as the base and the top consumer as the apex. It can be drawn for numbers, for biomass or for energy.

Why the sea is inverted. Phytoplankton are tiny and short-lived, and they are eaten almost as fast as they grow. At any single moment their standing crop is small, but it is replaced so quickly that it supports a much larger standing crop of zooplankton and fishes. Biomass is a photograph of one instant. Energy is the film.

Why energy can never be inverted. At every transfer some energy is lost as heat. So each level always receives less energy than the one below. There is no way around this, even in the sea.

Pause and predict: the sea has an inverted biomass pyramid. Is its energy pyramid inverted too?

Check your answer: no. Biomass is a snapshot, and fast-growing phytoplankton produce far more energy per year than their small standing crop suggests. Energy pyramids are upright in every ecosystem.

Producers convert only about 1 per cent of the sunlight reaching them into NPP. Do not confuse these four numbers:

The limits of pyramids. They ignore the same species being at two or more trophic levels, they assume a simple food chain instead of a web, and they give no place to saprophytes, though these are vital to the ecosystem.

Nutrient cycling and ecosystem services

Energy flows through; nutrients go round. Nutrient cycling moves elements between the living and non-living parts. It is of two types, depending on where the reserve is.

The products of ecosystem processes are called ecosystem services. Forests, for example, purify air and water. Others include pollination and the recycling of dead matter into soil nutrients, which is the decomposition described above.

Exam traps

60-second revision

Where this chapter connects

• Previous: Organisms and Populations gives the species-level interactions (predation, parasitism, competition) that make up each link in these food chains.
• Next: Biodiversity and Conservation asks what happens to these services when species are lost.
• Class XI link: Photosynthesis in Higher Plants explains the light-capturing step behind PAR and GPP.
• Class XI link: Respiration in Plants explains the R that is subtracted from GPP.