Ecosystem interdependence

An ecosystem is a natural system in which a community of living organisms interacts with its non-living environment. The living parts are called biotic components. The non-living parts, or abiotic components, include water, light, temperature, air, soil and mineral nutrients.

These components are interdependent: organisms depend on environmental conditions and on one another, while their activities can also change those conditions. A change in one part of an ecosystem can therefore affect other parts.

Ecosystem scale and boundaries

Ecosystems can be studied at different scales, from a small pond to an extensive forest. A boundary identifies the system being studied, but it does not completely close that system off. Water, organisms, nutrients and energy can cross ecosystem boundaries.

Interdependence in a freshwater pond

In a small UK freshwater pond, aquatic plants depend on light, water and dissolved nutrients to grow. Animals depend on suitable water conditions and on other organisms for food. Plants, in turn, provide habitats and release oxygen during photosynthesis, influencing conditions for other pond organisms.

Dynamic balance and change

Ecosystem balance is dynamic, not an unchanging state. Populations and environmental conditions fluctuate, but their interactions can maintain a broadly functioning system.

For example, if pond water becomes more turbid, or cloudy, less light reaches submerged plants. Their growth may decline, reducing the food and shelter available to animals. Changes to those animal populations can then produce further changes across the pond community. The initial change in water conditions spreads through the ecosystem because its components depend on one another.

Food chains and food webs

Organisms are connected by feeding relationships: when one organism eats another, food and energy are transferred between them.

Producers, usually green plants or algae, use sunlight to make organic material by photosynthesis. Consumers obtain food by eating other organisms. A primary consumer eats a producer; a secondary consumer eats a primary consumer. Higher-level consumers eat other consumers.

Food chains: one feeding pathway

A food chain shows one pathway along which food and energy are transferred. Each arrow points from the organism being eaten towards the organism that eats it.

In a small UK freshwater pond, algae can be eaten by water fleas, which can be eaten by small fish:

The algae are the producer, the water fleas are primary consumers, and the small fish are secondary consumers in this chain.

Food webs: connected feeding pathways

A food web links several food chains to show the feeding relationships within a community. It gives a fuller picture than a single chain because an organism may eat several species and may itself be eaten by more than one species.

In the pond, consumers other than water fleas can feed on algae, and small fish can eat prey other than water fleas. These connections mean that a change in one population can affect both the organisms it eats and the predators that eat it.

For example, if the water flea population falls, less algae may be eaten and small fish may have less food. The effects can extend beyond the algae–water flea–fish chain because other consumers and prey are connected to it in the food web.