Remove the Decomposers and the Whole System Grinds to a Halt

Not the tigers. The bacteria and fungi.

Remove every large carnivore from a forest and the forest survives, badly balanced but alive. Remove the decomposers and everything stops within a few seasons. Dead leaves, dead animals and waste would pile up untouched, and every mineral locked inside them would stay locked. The soil would slowly run out of nutrients, the plants would fail, and with the producers gone so would everything above them.

Decomposers are the only group that sends material back to the start. This page covers the first part of the ICSE Class 8 Biology chapter on ecosystems: the vocabulary, the components, the kinds of ecosystem, and who does what inside one.

They describe the same place at four different levels of scale.

Habitat — the specific place where an organism lives, providing it with food, shelter and conditions for breeding. The habitat of an earthworm is soil; of a frog, a pond and its banks; of a squirrel, a tree.

Population — all the individuals of one species living in a given area at a given time. All the lotus plants in a pond are one population; all the frogs in it are another.

Community — all the populations of different species living together and interacting in an area. The lotus plants, algae, fish, frogs, insects, snails and birds of a pond, taken together, form the pond's community.

Ecosystem — the community together with its non-living surroundings, interacting as a self-sustaining unit. The pond's plants and animals plus its water, soil, sunlight, dissolved gases and temperature make up the pond ecosystem.

The hierarchy, from smallest to largest:

individual $\to$ population $\to$ community $\to$ ecosystem $\to$ biosphere

The biosphere is the whole of the Earth's surface where life exists, taken as one enormous ecosystem.

The distinction that gets tested. A population is one species; a community is many. And an ecosystem is the only one of the four that includes the non-living part. Saying all the plants and animals of a pond describes a community, not an ecosystem — the water and the sunlight have to be included.

What self-sustaining means. An ecosystem needs nothing from outside except energy from the Sun. Its materials are used, passed on and returned within the system, cycling indefinitely. That cycling is exactly what the decomposers of the last section make possible, and it is why an ecosystem can persist for a very long time without anything being added to it.

Biotic components are the living parts; abiotic components are the non-living parts. Every ecosystem has both, and neither works without the other.

Biotic components — all the organisms, in three functional groups:

- Producers — green plants and algae
- Consumers — all animals
- Decomposers — bacteria and fungi

These three are the subject of the last section of this page.

Abiotic components — everything non-living:

- Sunlight — the source of all the energy entering the system
- Air — supplying carbon dioxide for photosynthesis and oxygen for respiration
- Water — as a solvent, a habitat and a raw material
- Soil — anchorage and a store of minerals
- Temperature — determining which organisms can survive there
- Minerals — nitrogen, phosphorus, potassium and others, taken up by plants
- Humidity, wind and pH

How the two sides depend on each other. Plants need sunlight, carbon dioxide, water and soil minerals — four abiotic things — to make food. In doing so they release oxygen, which every animal needs. Animals release carbon dioxide, which plants need. And decomposers return minerals to the soil, which plants need again.

So the living part does not merely sit in the non-living part. It changes it, continuously, and that exchange is what ties an ecosystem together.

Why the abiotic factors decide the ecosystem's character. A desert and a rainforest may lie at the same latitude and receive the same sunlight, and they differ almost entirely because of water. Change one abiotic factor sharply and you get a different ecosystem with different organisms — which is why the same country holds deserts, forests, grasslands and wetlands.

A common misclassification. Dead leaves and dead animals are not biotic components in the living sense — they are organic matter being acted on by decomposers. And humus in soil, though it came from living things, counts with the abiotic soil. What makes a component biotic is being alive.

Ecosystems are classified two independent ways — by who made them and by where they are.

By origin:

Natural ecosystems form and run without human help, and maintain themselves. Examples: a forest, a pond, a lake, a river, an ocean, a grassland, a desert.

Artificial or man-made ecosystems are created and maintained by humans, and they collapse without that maintenance. Examples: an aquarium, a garden, a crop field, a park, an orchard, a reservoir behind a dam.

The real difference between them. A natural ecosystem is self-sustaining and usually has many species, so it is stable — if one species declines, others take up the slack. An artificial ecosystem has few species, often just one crop, and depends on humans for water, fertiliser, weeding and pest control. Stop the maintenance and weeds and pests move in within a season.

This is the same argument as the disadvantage of vegetative propagation: low diversity means low stability.

By location:

Terrestrial ecosystems are on land — forest, grassland, desert, and the soil beneath them.

Aquatic ecosystems are in water, and split further:

- Freshwater — pond, lake, river, stream, wetland
- Marine — sea and ocean, with much higher salt content

The two classifications combine. A pond is natural and aquatic; an aquarium is artificial and aquatic; a forest is natural and terrestrial; a crop field is artificial and terrestrial. A question asking you to classify an ecosystem usually wants both labels, and giving only one is half an answer.

An instructive borderline case. A village pond dug by people and then left alone gradually acquires its own plants, insects, fish and frogs, and becomes largely self-sustaining. It began artificial and is drifting towards natural — which shows the distinction is about whether it needs us, not about who first dug the hole.

By asking how it obtains its food.

Producers, also called autotrophs, make their own food by photosynthesis, using sunlight, carbon dioxide and water. All green plants and algae are producers.

Their role is the most fundamental in the ecosystem: they are the only entry point for energy. Sunlight cannot be eaten, and no animal can use it directly. Producers convert it into chemical energy in food, and everything else in the ecosystem lives on what they have made.

Consumers, also called heterotrophs, cannot make their own food and must obtain it from other organisms. All animals are consumers, and they are grouped by what they eat.

- Herbivores, the primary consumers, eat plants: cow, goat, deer, rabbit, grasshopper, elephant.
- Carnivores eat animals. Those that eat herbivores are secondary consumers — frog, small fish; those that eat other carnivores are tertiary consumers — snake, hawk, tiger, lion.
- Omnivores eat both plants and animals: human, crow, bear, dog, sparrow.

Two further kinds of consumer are worth naming:

- Scavengers feed on dead bodies: vulture, crow, hyena, jackal. They clear away carcasses that would otherwise rot in the open.
- Parasites live on or in another living organism and take food from it: tapeworm, roundworm, and the plant Cuscuta.

Decomposers, also called saprophytes or reducers, are bacteria and fungi. They feed on dead organic matter — dead plants, dead animals and waste — breaking it down into simple substances and returning minerals to the soil.

That return is their role, and nothing else performs it. Producers can only take up minerals in simple soluble form, so without decomposers the nutrients in every dead body would stay locked up and unavailable for ever.

Why this makes the ecosystem a cycle rather than a line. Energy flows one way — in from the Sun, through the producers and consumers, and out as heat, never to be reused. But materials go round: soil minerals to plants, plants to animals, animals to decomposers, and decomposers back to soil minerals. Decomposers are the step that closes that loop.

A classification trap. A fungus is not a producer despite looking plant-like — it has no chlorophyll and cannot photosynthesise, so it is a decomposer. And a scavenger such as a vulture is a consumer, not a decomposer, because it eats visible dead flesh rather than breaking matter down to minerals.