Ecosystem Roles and Nutrient Cycling

An ecosystem connects living things with their surroundings

An ecosystem includes living organisms and the non-living environment with which they interact. Plants, animals, fungi and bacteria are its biotic components. Soil, water, sunlight and temperature are abiotic components.

A small area of UK deciduous woodland provides a useful example. Its trees and other plants need sunlight, water and nutrients to grow. Animals obtain food from plants or other animals, while decomposers break down dead material. Feeding and nutrient cycling connect these parts into a system.

Producers bring energy into the food chain

Producers make their own food through photosynthesis, using energy from sunlight. In woodland, producers include oak and beech trees, grasses, ferns and bluebells. They provide the food on which the rest of the food chain depends.

Consumers obtain energy and nutrients by eating other organisms. Their names describe their feeding positions, or trophic levels:

• Primary consumers eat producers. Woodland examples include rabbits and deer eating plants.
• Secondary consumers eat primary consumers. A fox eating a rabbit acts as a secondary consumer.
• Tertiary consumers eat secondary consumers.

These positions depend on what an animal eats, rather than being permanent labels for every species. An animal that eats organisms from different trophic levels can occupy more than one position in a food web.

Food chains show one feeding route; food webs connect many

A food chain follows one route through an ecosystem. A simple woodland example is:

Woodland plants → rabbit → fox

The rabbit eats the plants, and the fox eats the rabbit. Each arrow points from the food towards its consumer, showing the direction in which energy is transferred. The chain begins with a producer, but the original energy source is sunlight.

Real feeding relationships are more complicated than one chain. Woodland plants may be eaten by rabbits, deer and insects. Foxes eat rabbits but can also eat other animals, including voles. These overlapping feeding routes form a food web.

To read a food web, follow an arrow towards an organism to identify one of its food sources. Follow an arrow away from it to identify something that eats it. Several arrows leading into one consumer show that it has more than one food source. A web therefore represents feeding relationships more fully than a single chain.

Decomposers return nutrients to the environment

Dead leaves, dead animals and waste still contain nutrients. Decomposers, especially fungi and bacteria, break down this organic material and release nutrients back into the environment. In woodland, this returns nutrients to the soil, where plants can take them up through their roots.

Decomposers do not act only at the end of a food chain. Material from producers and consumers at every trophic level can be decomposed. Their activity connects living organisms with the soil and makes nutrients available for further plant growth.

Nutrient cycling links plants, litter and soil

Nutrients move repeatedly between three important stores: living organisms, litter and soil. In deciduous woodland, trees shed their leaves, adding material to the litter store. As decomposers break down the litter, nutrients enter the soil. Roots absorb these nutrients, returning them to living plants. Feeding then transfers some nutrients from plants to animals.

For example, nutrients in an oak leaf can pass into the litter when the leaf falls, into the soil through decomposition, and back into a plant through root uptake. Dead animals and animal waste also provide material for decomposition.

Nutrients cycle, but energy does not. Energy enters mainly as sunlight and is transferred through feeding. Organisms use energy for their life processes, and energy is eventually transferred to the surroundings as heat. Plants therefore need a continuing supply of sunlight, even though nutrients can be reused.

Feeding roles

• Producers: make food by photosynthesis; examples include oak trees and grasses.
• Primary consumers: eat producers, for example rabbits.
• Secondary consumers: eat primary consumers, for example a fox eating a rabbit.
• Tertiary consumers: eat secondary consumers.
• Decomposers: fungi and bacteria break down dead material and waste, releasing nutrients.

Chains and webs

Woodland plants → rabbit → fox

Arrows show energy transfer from food to consumer. A food chain shows one feeding route; a food web links several routes. An animal’s trophic level depends on what it eats.

Nutrient cycling

Soil nutrients → plant uptake → living organisms → litter and waste → decomposition → soil nutrients

Feeding transfers nutrients between organisms. Deciduous leaf fall adds to woodland litter.

Nutrients are recycled; energy flows through the ecosystem and is eventually transferred to the surroundings.