Ecosystems » Ecosystems and a UK Pond

What is an ecosystem?

An ecosystem is a natural system made up of living things and the non-living world around them, all interacting with each other. Ecosystems exist at a range of scales. A rotting log, a garden pond, a woodland and the whole Amazon rainforest are all ecosystems.

Every ecosystem has two types of component. Biotic components are the living parts, such as plants, animals and bacteria. Abiotic components are the non-living parts, such as sunlight, temperature, water, soil and oxygen. Living things depend on the abiotic parts, and they also affect them. For example, pond plants release oxygen into the water.

Key terms:

• Ecosystem: a natural system of living things and their non-living surroundings, which interact with each other.
• Biotic: the living parts of an ecosystem.
• Abiotic: the non-living parts of an ecosystem.
• Interrelationship: a link between two parts of an ecosystem, where each one affects the other.

A small scale UK ecosystem: the freshwater pond

A pond is a good example of a small ecosystem. You can see most of its parts in one place, and it shows how everything is connected.

Producers, consumers and decomposers

The living parts of the pond fall into three groups, based on how they get their energy.

• Producers make their own food using sunlight. In a pond these are algae, pondweed and water lilies. All the energy in the ecosystem starts here.
• Consumers cannot make food, so they eat other living things. A primary consumer eats producers, for example a water snail or a tadpole eating algae. A secondary consumer eats primary consumers, for example a diving beetle eating a water snail. A tertiary consumer eats secondary consumers, for example a heron eating a diving beetle.
• Decomposers break down dead plants and animals and waste. In a pond these are bacteria and fungi.

Key terms:

• Producer: a living thing that makes its own food from sunlight.
• Consumer: a living thing that gets its energy by eating other living things.
• Decomposer: a living thing that breaks down dead material and waste.

Food chains and food webs

A food chain shows who eats whom, in a single line. The arrows show the direction the energy moves. A pond food chain could be:

algae → water snail → diving beetle → heron

Real life is not this simple. A heron eats frogs as well as fish, and a dragonfly nymph eats many different animals. A food web shows all the food chains in an ecosystem joined together. It is a better picture of the real pond, because it shows that most living things have more than one food source and more than one predator.

Key terms:

• Food chain: a single line showing what eats what, and how energy passes along it.
• Food web: many food chains linked together in one ecosystem.

Nutrient cycling

Plants need nutrients from the water and mud to grow. These nutrients are not used up. They go round and round in a cycle.

Without decomposers, dead material would pile up and the nutrients would stay locked inside it. The producers would run out of nutrients, and the whole pond would suffer.

Key term:

• Nutrient cycling: the movement of nutrients from the soil and water into living things, and back again when they die and decompose.

Balance between the components

In a healthy pond, the components are in balance. There are enough producers to feed the primary consumers, enough primary consumers to feed the secondary consumers, and so on. Abiotic conditions such as light and oxygen stay in a range that the living things can cope with. Because all the parts are linked, a change to one component affects the others. The effects spread through the food web and the abiotic parts.

The impact of changing one component

Both examples show the same idea. One small change can move through the whole ecosystem, because every part depends on other parts.

Exam-style question

Part of a UK pond food web is described. Algae are eaten by water snails and tadpoles. Tadpoles are eaten by dragonfly nymphs. Dragonfly nymphs are eaten by fish.

(a) State one biotic and one abiotic component of the pond ecosystem. [2 marks]

(b) Name the producer in the food web. [1 mark]

(c) Explain what would happen to the other living things in the pond if most of the tadpoles died. [3 marks]