Ecosystem: Class 12 Biology · Chapter 12

Leave a bucket of rainwater in a sunny corner of the school ground during the monsoon. Within a fortnight the water turns green, mosquito larvae wriggle in it and a slimy film coats the bottom. Nobody put any of this there on purpose, yet the bucket now works as a small system: algae trap sunlight, tiny animals graze on the algae, and microbes dismantle whatever dies. Ecologists call such a system an ecosystem (पारितंत्र): living organisms interacting with one another and with their non-living surroundings so closely that the whole behaves as one functional unit.

Ecosystems come in every size, from a bucket or a village pond to the Sundarbans mangroves or the Arabian Sea. For convenience we group them into a few broad kinds (see the comparison). At the other extreme, all of them together, the whole biosphere, can be thought of as a single global ecosystem.

One way to read this chapter is as an energy budget. Producers earn the income (productivity), food chains pass it along, every organism spends some of it as heat, and decomposers recycle the materials left behind. We look first at how an ecosystem is built, then follow the budget step by step, and finally draw it as pyramids.

12.1 Ecosystem – Structure and Function

Ask what makes a mangrove different from a grassland and you get two kinds of answer: the living organisms (biotic components) and the physical conditions (abiotic components: light, temperature, water, air, soil and minerals). Neither explains the difference alone. Salt water and tides decide which trees can grow in a mangrove, and the trees in turn trap mud and shade the water. Out of this two-way interaction comes the characteristic layout of each ecosystem, its structure.

Structure: species composition and stratification

Two features describe that structure. The first is species composition: the list of species present and how common each one is, which ecologists build up by naming and counting organisms in sample plots. The second is how those species are arranged vertically, which ecologists call stratification (स्तरविन्यास).

Walk into such a forest and the layers are easy to pick out (Fig. 12.1). Each layer gets a different share of the light, so each suits different plants, and different animals feed or shelter among them. Water bodies are layered too: floating plants, open-water algae and weeds rooted in the bed sit at different depths.

Function: four processes

Structure tells us what an ecosystem contains; function tells us what it does. Four processes tie its parts into a working unit, and the rest of the chapter takes them one at a time:

• Productivity: producers build organic matter using sunlight (Section 12.2).
• Decomposition: decomposers break dead matter back into simple substances (Section 12.3).
• Energy flow: energy passes from producers to consumers and leaves as heat (Section 12.4).
• Nutrient cycling: elements such as carbon, nitrogen and phosphorus are used again and again (box below).

A pond as a model ecosystem

For a working model, ecologists like a pond: you can see across it, yet nothing essential is missing. Step through Fig. 12.2 and sort its parts by the job each one does.

Abiotic parts. Compare the same pond in a foggy January and a bright April: in April everything grows, feeds and rots faster. Light, temperature and day length act like a throttle on the whole pond. The rest of the non-living part is material: water with minerals, gases and organic compounds dissolved in it, and a bed of nutrient-rich mud.

Producers. Look from the bank inwards: reeds and sedges rooted at the edge (marginal plants), lilies and duckweed on the surface (floating plants), hydrilla waving below it (submerged plants), and, invisible without a microscope, phytoplankton and other algae drifting in the sunlit water.

Decomposers and consumers. The mud on the bed is the busiest place for decomposers: bacteria, fungi and flagellate protists work through everything that sinks. Above them live the consumers: snails and worms crawling over the bottom, fish, frogs and insect larvae swimming in open water, and zooplankton such as water fleas and rotifers grazing on the phytoplankton.

Seen as a whole, the pond treats matter and energy differently. Matter is recycled: one phosphorus atom may pass from mud to hydrilla to snail and back to mud many times over. Energy is not. It arrives as sunlight, is fixed by producers, moves one way to the consumers and finally escapes as heat. Matter cycles, energy flows: this contrast is the most useful single idea in the chapter.

12.2 Productivity

Whatever an animal eats, its food began as plant matter built with the energy of sunlight. So the first question to ask of any ecosystem is how much new plant matter it makes, and how fast: that sets the budget for every other organism in it.

Suppose you want to compare a paddy field with a coral reef. The plant matter made by photosynthesis on each square metre over some period is a start: that amount, as a mass in $\mathrm{g}{\mathrm{m}}^{-2}$ or as the energy it holds in $\mathrm{k}\mathrm{J}{\mathrm{m}}^{-2}$ (older books use kcal), is the primary production. But a reef grows all year while a paddy crop grows for a few months, so a fair comparison needs a rate. Primary productivity is production per unit time, in $\mathrm{g}{\mathrm{m}}^{-2}\mathrm{y}{\mathrm{r}}^{-1}$ or $\mathrm{k}\mathrm{J}{\mathrm{m}}^{-2}\mathrm{y}{\mathrm{r}}^{-1}$.

Gross and net primary productivity

Plants do not keep everything they make. Like all living things they respire, burning part of their own sugar to power growth, repair and transport. So we separate the total from what is left over.

Respiration usually claims a large share of GPP, often between a third and two-thirds depending on the plants and the climate. Follow one year's production through Fig. 12.3.

NPP is what the rest of the ecosystem lives on. Some of it is eaten by herbivores, a larger part on land dies uneaten and feeds decomposers, and some remains as extra plant body. When consumers turn the food they assimilate into their own new tissue, that is secondary productivity (द्वितीयक उत्पादकता).

Why productivity differs

Productivity is far from uniform. It depends on several things at once:

• The producers themselves. Species differ in leaf area, growth rate and photosynthetic capacity; C${}_4$ plants such as sugarcane and maize fix carbon especially fast in hot, bright weather.
• Light and temperature. Long, warm growing seasons allow more photosynthesis than short, cold ones.
• Water. The Thar is unproductive because it is dry, not because it lacks sunlight.
• Nutrients. Nitrogen and phosphorus, and in parts of the sea iron, are often in short supply.

Put numbers on it and the oceans look surprisingly poor. On a widely quoted estimate, plants on land make about 115 billion tonnes of dry organic matter a year and the oceans about 55 billion: roughly a third of the world total of 170 billion, from a sea that covers about 70% of the Earth (Fig. 12.4).

Why is so much water so unproductive? Light fades quickly with depth, so producers can photosynthesise only in a thin surface layer. Nutrients, meanwhile, sink with dead plankton and droppings into the dark water below, where no producer can use them. Over most of the open ocean the light and the nutrients end up in different places. Only where currents bring deep water back up, as off some coasts, or where rivers pour in nutrients, does the sea become highly productive.

12.3 Decomposition

Each spring the peepal and neem trees around a school ground drop their old leaves, and for a few weeks the ground crunches underfoot. By the end of the monsoon most of that litter has gone without anyone sweeping it away. It has been decomposed: taken apart by living organisms into carbon dioxide, water and mineral nutrients.

Earthworms get most of the credit, and farmers value them for the loose, crumbly soil they leave behind. Most of the chemistry, though, is done by bacteria and fungi too small to see. Without them, dead leaves, branches and bodies would pile up, and the nutrients locked inside would never return to the soil.

Processes in decomposition

Decomposition is not one reaction but several processes acting on the same detritus at once. It helps to name them separately (Fig. 12.5), as long as you remember that in real soil they overlap.

• Fragmentation. Detritivores (अपरदाहारी) such as earthworms, millipedes and woodlice chew detritus into small pieces, hugely increasing the surface that microbes can attack.
• Leaching. Rain washes the soluble minerals out of the detritus and down through the soil layers. Some end up deep in the soil as insoluble salts that roots cannot take up.
• Catabolism. Bacteria and fungi pour out enzymes that dismantle the large organic molecules of detritus, step by step, into small inorganic ones.
• Humification. Some of the half-digested matter turns into humus, a dark colloidal material with no plant structure left in it, which soaks up water and nutrients like a sponge. Few microbes can digest it, so it can persist in soil for years.
• Mineralisation. Slowly, some microbes do break humus down, and as they do, inorganic nutrients such as ammonium and phosphate are set free for roots to absorb.

What controls the rate?

A banana peel left on damp soil vanishes within weeks; a fallen sal log may take decades. Two kinds of factor decide the pace.

What the detritus is made of. Microbes digest sugars, starch and nitrogen-rich proteins quickly. Lignin, the polymer that stiffens wood, and chitin, found in insect exoskeletons and fungal walls, are tough and slow. So under the same conditions soft, nitrogen-rich leaves rot faster than woody twigs.

The climate of the soil. Compare a forest floor in Kerala with a bog high in the Himalaya. The warm, damp Kerala soil keeps its microbes busy all year, and fallen leaves vanish within months. The bog is cold and waterlogged; with little oxygen in the soaked soil (anaerobiosis), the mostly aerobic decomposers barely work, and undecomposed plant matter piles up as peat. Of the climatic factors, soil temperature and moisture matter most, because they control how active the microbes are.

12.4 Energy Flow

Here is a sobering number: of all the sunlight that falls on a field, only about one part in a hundred ends up stored in new plant matter. Fig. 12.7 shows where the rest goes. Less than half of sunlight lies in the band of wavelengths that chlorophyll can use, called photosynthetically active radiation (PAR). Plants trap roughly 2–10% of that PAR; the remainder is reflected, passes through the leaves or turns into heat. Every meal eaten by every animal on Earth traces back to that thin slice.

Producers, consumers and food chains

Every food chain starts with an organism that makes its own food. On a hillside these producers are herbs, shrubs and trees; in a lake they are phytoplankton, other algae and rooted water plants. Everything else, from a grasshopper to a tiger to you, has to eat to get its energy and is a consumer (a heterotroph); some eat plants, others eat animals that once ate plants.

Each consumer can be named in two ways, by its feeding step and by its diet, and the two sets of names are offset by one. One step from the plants is a primary consumer, a herbivore: a grasshopper, a parakeet or a chital, or a snail or zooplankton in water. Two steps away is a secondary consumer, a carnivore that eats herbivores and so is called a primary carnivore. Three steps away is a tertiary consumer, a secondary carnivore.

A sequence of who eats whom is a food chain. In a Haryana field one grazing food chain (GFC) runs grass → grasshopper → frog → snake. Energy enters through living plants and moves up the chain each time one organism is eaten by another.

Not everything gets eaten. A tree drops its leaves, a deer leaves dung, and sooner or later every plant and animal dies. All this dead matter feeds a second kind of chain, the detritus food chain (DFC). Bacteria and fungi dominate it, and they feed in an unusual way: instead of swallowing food, they release digestive enzymes onto dead matter, let digestion happen outside their bodies, and absorb the small molecules that result. Organisms that feed like this are saprotrophs (Greek sapros, rotten). Detritivores such as earthworms also feed in this chain.

Food webs and trophic levels

Real feeding links are never as tidy as one chain. A myna eats earthworms from the detritus chain as well as grasshoppers from the grazing chain, so the two chains are joined. Omnivores such as rats, jackals and wild boar eat plant food, animal food and scraps alike. Linked up in this way, the chains form a food web (खाद्य जाल), as in Fig. 12.8.

Each organism's position in a chain or web, fixed by how it gets its food, is its trophic level (पोषण स्तर). Producers form the first trophic level (T1), herbivores the second (T2), the carnivores that eat herbivores the third (T3) and their predators the fourth (T4). Where would you place yourself? We come back to that question in Section 12.5.

Which chain carries more energy depends on the ecosystem. In water, much of the phytoplankton is eaten alive, so the grazing chain is the main route for energy. On land, herbivores eat only a small part of the plants; most leaves, wood and roots die uneaten, so most of the energy ends up in the detritus chain rather than the grazing chain.

Standing crop

Freeze an ecosystem for a moment and count, or weigh, everything alive at one trophic level. What you get is that level's standing crop (खड़ी फसल): a snapshot, not a rate. Ecologists report it either as a count per unit area or as biomass, the mass of living matter per unit area, and they almost always weigh biomass dry. The activity below shows why.

The 10 per cent law

When a chital grazes, most of the energy in the grass never becomes deer. Roots and tough stems are left uneaten. Part of what is eaten passes out undigested in the dung. Most of what is digested is burned in respiration to keep the deer alive, moving and warm, and leaves as heat. Only a small remainder becomes new deer tissue that a tiger could eat. Fig. 12.9 follows these losses up a chain.

Multiply by one-tenth at every step and the energy shrinks fast: of 20 000 kJ of producer NPP, about 2 000 kJ reaches the herbivores, 200 kJ the carnivores and just 20 kJ the top carnivores. A fifth level would receive 2 kJ. Somewhere along this fall too little energy is left to support a population that can find enough food, so grazing food chains rarely have more than four or five links.

12.5 Ecological Pyramids

Stack the trophic levels one above another, producers at the bottom and top consumers at the apex, and draw each as a bar whose width shows how much that level holds. Usually the stack narrows upwards like a pyramid. Such a diagram is an ecological pyramid (पारिस्थितिक पिरामिड). The idea goes back to the ecologist Charles Elton, who pointed out in the 1920s that small animals far outnumber the larger ones that eat them.

Three different quantities can be stacked:

• Pyramid of numbers: how many individuals live at each level in a given area.
• Pyramid of biomass: the dry mass of living organisms at each level at one moment (the standing crop).
• Pyramid of energy: the energy flowing through each level per unit area over a fixed time, usually a year.

Step through Fig. 12.11 to compare them. Whichever you draw, build it from a census of the whole level: weighing three grasshoppers and calling that the herbivore level would give a meaningless bar, because every herbivore species on the plot counts.

Upright and inverted pyramids

A grassland gives the textbook shape. Grasses outnumber the grasshoppers and outweigh them many times over; grasshoppers in turn outnumber and outweigh the frogs and snakes; and each level passes on only a fraction of its energy. All three pyramids come out upright, as they do in most ecosystems.

Pyramids of numbers turn over easily, because individuals differ so much in size. One big peepal tree can feed thousands of caterpillars, aphids and beetles, which feed a dozen insect-eating birds, which may be hunted by a single hawk. The base of that pyramid is one individual, so it is inverted at the base: narrow at the bottom and widest among the herbivores. Add parasites, such as lice living on the birds, and it widens again near the top.

Biomass pyramids can flip as well. Sample the open sea at any moment and the animals, zooplankton and fish together, outweigh the microscopic phytoplankton they live on: an inverted pyramid of biomass.

Energy behaves differently. No level can receive more energy than the level feeding it, because every transfer bleeds some away as heat, so a pyramid of energy can only ever be upright. Its bars show the energy passing through each level on a given area in a fixed period, typically a year. Try building pyramids of each kind below.

A trophic level is a role, not a species

Trophic levels describe what an organism is doing, not which species it belongs to. A myna pecking at ripe figs is feeding at the second trophic level; the same bird snapping up a grasshopper is feeding at the third. You are the clearest case of all: one meal can place you at several levels at once (Fig. 12.13).

Limitations of ecological pyramids

Pyramids are useful precisely because they are simple, and that simplicity has a price:

• Mixed feeders. A myna, a bear or a person feeding at two levels has to be split up or forced into one bar.
• Webs. A pyramid is a stack of single links, but real ecosystems are tangled webs that no stack of bars can draw.
• Decomposers. Bacteria and fungi (saprophytes), which process most of the dead matter, appear nowhere in the stack.