Components of an ecosystem and stratification

Start with a pond

Look at a small pond. Green algae float in the water. A water lily spreads its leaves on the top. Small animals swim between them, and a fish eats the small animals. On the bottom, bacteria and fungi work on dead leaves. The water, the light and the warmth are there too.

Nothing in this pond lives alone. Each living thing needs the water and the light, and each needs the other living things. A place seen in this way is called an ecosystem.

An ecosystem is a functional unit of nature, where the living organisms interact among themselves and also with the physical environment around them. The word was first used by A. G. Tansley in 1935.

Big and small ecosystems

An ecosystem can be as small as a pond or as large as a forest or a sea. In the end the whole biosphere can be seen as one global ecosystem. NCERT groups the ecosystems like this.

• Terrestrial (on land): forest, grassland and desert.
• Aquatic (in water): pond, lake, wetland, river and estuary.
• Man-made (made by people): a crop field and an aquarium.

The non-living part

The non-living part is the abiotic component. It holds the water, the light, the temperature, the soil and the nutrients (the minerals that the plants take in). The Sun gives the light. The temperature, the length of the day and the climate decide how fast everything in the ecosystem works.

The living part

The living part is the biotic component. It has three kinds of members, and each does one job.

1. Producers make food from simple substances. Green plants, algae and some bacteria are producers. They are autotrophs. In a pond, the producers are the phytoplankton (the tiny floating algae), the other algae, and the floating, submerged and marginal plants.
2. Consumers cannot make food. They eat other living things. The primary consumers eat the producers. They are the herbivores. The secondary consumers eat the herbivores. They are the carnivores. In a pond, the consumers are the zooplankton, the free swimming animals and the animals that live on the bottom.
3. Decomposers live on the dead remains of plants and animals. They break them down into simple substances. They are mainly bacteria and fungi.

Notice that the first two jobs are about food, and the third is about the dead. Without the third job, the dead matter would pile up for ever.

Four jobs of the whole ecosystem

NCERT says that an ecosystem has four functions: productivity, decomposition, energy flow and nutrient cycling. The next two courses teach the first three. Nutrient cycling is the fourth. This part is not in the NEET syllabus, but A-Level and IB ask it, and the last course of this chapter teaches it.

Structure: species and layers

The living and non-living parts together give each ecosystem its own structure. Two features of it are easy to see.

• Species composition. If you list and count the plant and animal species of a place, you have its species composition.
• Stratification. The species do not all live at one height. They sit at different levels, one above the other. This vertical distribution is called stratification.

A forest is the best example. The trees form the top layer. The shrubs form the second layer. The herbs and grasses form the bottom layer. Each layer has its own animals too.

Why are the layers in this order? The tall trees catch most of the light, so the layers below get less. The plants of the bottom layer must manage with the light that is left. Slide the layers in the model below.

You should find that the herbs get only a small share of the light when the tree layer is thick. This is why many forest herbs can grow in shade.

Try it in Python

The program sorts the living things of a pond by their job and counts them. The abiotic parts are not on the list, because they are not living.

Practise with new numbers

Common mistakes

• Putting the Sun among the producers. The Sun gives light and is an abiotic part. The producers are the living things that use the light.
• Putting decomposers among the consumers. A consumer eats living things, or the bodies of animals it has caught. A decomposer lives on dead matter and wastes.
• Thinking that stratification means a food chain. It is about the height at which species live, not about who eats whom.
• Counting the soil as a producer. The soil is abiotic. The plants growing in it are the producers.

Remember these

• An ecosystem is a functional unit where the living organisms and the physical environment interact.
• Types: terrestrial (forest, grassland, desert), aquatic (pond, lake, wetland, river, estuary) and man-made (crop field, aquarium).
• Abiotic: water, light, temperature, soil, nutrients. Biotic: producers, consumers, decomposers.
• Functions: productivity, decomposition, energy flow and nutrient cycling.
• Structure: species composition and stratification (trees, shrubs, herbs).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three groups of living members of an ecosystem. Give one example of each from a pond.

2. Name four abiotic components of a pond.

3. What is stratification? Name the layers of a forest from the top.

4. Is a tadpole that eats algae a producer, a consumer or a decomposer?

5. Give an example of a man-made ecosystem.

6. Which one of the four functions of an ecosystem is outside the NEET syllabus?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on ecosystem structure and function.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on the ecology of ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy.