Our EnvironmentEco-System — What are its Components?

All organisms, such as plants, animals, microorganisms and human beings, interact with each other and with their physical surroundings, and together they maintain a balance in nature. All the interacting organisms in an area, together with the non-living constituents of the environment, form an ecosystem.

So every ecosystem has two kinds of components:

• the biotic components: all the living organisms;
• the abiotic components: the physical factors, such as temperature, rainfall, wind, soil and minerals.

Riya's class walks round the school garden with a notebook. They find grass and a big neem tree, flowering plants like rose, jasmine and sunflower, and animals like frogs, insects and birds. All these living things interact with each other, and how they grow and reproduce depends on the non-living part of the garden: how hot it is, how much it rains, what the soil is like.

Sort what Riya's class finds in the garden.

So the school garden is an ecosystem. Forests, ponds and lakes are ecosystems too. These are natural ecosystems, while gardens and crop fields are human-made (artificial) ecosystems: people plan them, plant them and look after them. A paddy field is a human-madenatural ecosystem, and a lake in the hills is a naturalhuman-made one.

Activity 13.1: An Aquarium That Runs Itself

Aman wants to keep fish at home, and an aquarium is a human-made ecosystem you can build on a table. What do the fish need? Space to swim (a large jar will do), water, oxygen and food. We can supply the oxygen with an air pump (an aerator) and buy fish food in the market. But if we add a few water plants and the right small organisms, the aquarium can become a self-sustaining system that needs nothing from us.

Choose what goes into the tank, then run it for 30 days. Complete all three missions.

In the tank that ran itself, every group had a job. The water plants used the energy of sunlight to make food and oxygen. The fish ate and breathed. And the bacteria and fungi in the gravel broke down the fish's waste into minerals, which the plants took up again.

Now you can answer the questions of Activity 13.1. A real aquarium has to be cleaned once in a while: it holds a lot of fish in a little water, and we feed them from outside, so waste piles up faster than its few decomposers can break it down. Nobody cleans a pond or a lake. Its large volume of water and its huge numbers of decomposers break waste down about as fast as it is made. A pond cleans itself because its decomposersfishwater plants recycle the waste.

Producers, Consumers and Decomposers

In earlier classes you grouped organisms by the way they get their food from the environment. Let's recall the three groups using the tank you just built.

• Producers make organic compounds such as sugar and starch from inorganic substances, using the radiant energy of the Sun in the presence of chlorophyll. All green plants, and certain bacteria that can make food by photosynthesis, are producers.
• Consumers eat the food made by producers, either directly or indirectly by feeding on other consumers. They can be grouped as herbivores (plant-eaters), carnivores (flesh-eaters), omnivores (eat both) and parasites (live on or inside a host and feed on it).
• Decomposers are microorganisms, bacteria and fungi, that break down dead remains and waste products of organisms. They turn complex organic substances into simple inorganic substances, which go into the soil and are used once more by plants.

Join each organism to its group.

Now imagine an ecosystem with no decomposers at all. Dead plants, dead animals and garbage would pile upvanish on their own, and the minerals locked inside them would never return to the soilreturn to the soil faster. Without decomposers, the natural replenishment of the soil stops.

Food Chains and Webs

Activity 13.2 asks you to write some organisms in order of who eats whom. A series of organisms feeding on one another like this is called a food chain.

Riya spots these five in a grassland. Drag them into a food chain: the producer at the top, then each organism followed by the one that eats it.

Each step or level of a food chain is called a trophic level. The autotrophs, or producers, are at the first trophic level: they fix solar energy and make it available to the heterotrophs, the consumers. The herbivores, or primary consumers, come at the second level; small carnivores, or secondary consumers, at the third; and larger carnivores, or tertiary consumers, at the fourth.

In Riya's chain, grass → grasshopper → frog → snake → eagle, the grasshopper is a primary consumera producera secondary consumer, the frog is at the thirdsecondfourth trophic level, and the snake is a tertiary consumera primary consumera producer. The eagle sits at the fifthfourththird level, at the top of this chain.

The 10 Per Cent Law

The food we eat is the fuel that gives us energy to do work. So when organisms eat one another, energy flows from one component of the ecosystem to the next. It starts with the autotrophs: they capture the energy of sunlight and convert it into chemical energy, the energy stored in food. This energy supports all the activities of the living world. From the autotrophs it passes to the heterotrophs and the decomposers.

But every time energy changes from one form into another, some of it is lost to the surroundings in forms that cannot be used again. Careful measurements of the flow of energy in ecosystems show that:

• the green plants of a land ecosystem capture only about 1% of the energy of the sunlight that falls on their leaves and turn it into food energy;
• when a primary consumer eats a plant, a great deal of the energy is lost as heat, some goes into digesting the food and doing work, and only the rest goes into growth and reproduction. On average only 10% of the food eaten becomes the animal's own body and is available to the next level of consumers.

So 10% is the average amount of organic matter (and energy) that passes from each step to the next. This is the 10 per cent law.

So little energy is available to each next level that food chains generally consist of only three or four steps: the loss at each step is so great that very little usable energy remains after four trophic levels. If the grass fixes 1000 J, the snake at the fourth level gets only 1 J10 J100 J.

The same rule explains why there are generally a greater number of individuals at the lower trophic levels of an ecosystem, and the greatest number are the producers. A field can feed thousands of grasshoppers, but only a handful of snakes.

Energy Flows One Way

Follow the energy through the pyramid once more and two things become clear.

A cow eats grass. Its dung falls on the field, decomposers break it down, and the grass grows greener. Does the grass get back the energy the cow took from it?

First, the flow of energy is unidirectional. The energy captured by the autotrophs does not go back to the Sun, and the energy that passes to the herbivores does not come back to the autotrophs. As it moves on through the trophic levels, it is no longer available to the previous level. Second, the energy available at each trophic level gets less and less, because some is lost at every level. Minerals cycle through an ecosystem; energy flows through it in one direction only.

From Chains to a Web

Real food chains vary greatly in length and complexity. Each organism is generally eaten by two or more other kinds of organisms, which in turn are eaten by several others. So instead of a straight-line food chain, the feeding relationships form a series of branching lines called a food web.

Draw the food web of a grassland: tap the food first, then the animal that eats it. Use all four clues.

Start at the grass and follow the arrows up to the hawk. Count every different path: there are 4352 different food chains ending at the hawk, all tangled into one web.

A web makes an ecosystem sturdier than a single chain. If a disease killed all the frogs, the snakes would still have mice to eatstarve at once.

Biological Magnification

Food chains carry more than energy. Some harmful chemicals travel along them too, and end up in our bodies without our knowing. You read in Class IX how water gets polluted; one reason is the use of pesticides and other chemicals to protect crops from diseases and pests. These chemicals are washed down into the soil or into water bodies. From the soil they are absorbed by plants along with water and minerals, and from water bodies they are taken up by aquatic plants and animals. This is one of the ways in which they enter the food chain.

Scientists studying a lake whose shores had been sprayed with the pesticide DDT measured the amount of DDT at every level of the lake's food chain. They measured it in ppm (parts per million): milligrams of DDT in every kilogram of water or body.

Predict first: where will the DDT be most concentrated? Then test each level, starting with the water.

These chemicals are not degradable, so they accumulate progressively at each trophic level: every animal eats many organisms from the level below and keeps the DDT from all of them. This build-up of harmful chemicals at higher trophic levels is called biological magnification. Every step up the chain, the concentration increasesdecreasesstays the same.

Human beings occupy the top level in many food chains, so the highest concentration of such chemicals can build up in our bodiesthe soilcrop plants. This is why our food grains such as wheat and rice, our vegetables and fruits, and even meat contain varying amounts of pesticide residues, which cannot always be removed by washing or other means.

Activity 13.3: The Pesticide Debate

Newspapers often report pesticide levels in ready-made food items, and some states have banned such products. The pesticides in them come from the crops used as ingredients, so the same pesticides can reach us through other foods as well: grains, vegetables, fruits and milk.

Which step would do the most to cut down the pesticides in our food?

The surest way is to stop the chemicals entering the food chain in the first place. Less spraying, safer pesticides, natural pest control and organic farming all reduce what reaches the soil and water, and so what reaches our plates.

Speed Round: Climb the Food Chain

Name the trophic level of the organism shown in blue, or use the 10 per cent law to find the energy that reaches a level.

Get five right to move on.