Structure of Ecosystems

What is an ecosystem?

An ecosystem is a system in which organisms interact with each other and with their environment. There are two parts: the entire complex of organisms , or biome, living in harmony and the habitat in which the biome exists.

The nature of ecosystems

Ecosystems occur at a range of different scales. Micro-habits are ecosystems that are found in small, specific locations e.g. under a stone, beneath a leaf etc. Habitats are specific locations that have conditions which the community has adapted e.g. hedgerow, pond. Zones are area within a biome. An example of this would be one of the four layers of the rainforest. A biome is a large scale ecosystem which extends across continents. Each has its own flora and fauna. Examples include the tropical rainforest, hot desert and tundra.


The main driving force in terms of energy in all ecosystems is the sun. It provides the heat that warms up plants, animals and their abiotic environment. It also drives the water cycle and other important flows. The sun is also essential in providing energy for photosynthesis.

Food Chain

Once food has been used to build a plant (producer) it becomes available for other organisms to consume (consumers). These organisms become available to other organisms. Decomposers, such as bacteria and fungi, take the remains of dead plants and animals, along with secreted waste, and converts them back to CO2 and nutrients.  This flow of energy through plants and animals then back to the soil is known as a food chain.

Figure 1. below shows the flow of energy in a typical food chain. It is estimated that there is a 90% loss of energy between each transfer.

Figure 2. shows a simple food chain. Primary produces, also known as autotrophs, get energy from photosynthesis or chemosynthesis (chemical energy). Heterotrophs are consumers. These can be herbivores (consumers of plants) or carnivores (derives energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue). Both heterotrophs and autotrophs are eventually decomposed by bacteria and/or fungi.

Each stage in of the food chain is known as a trophic level. Figure 3. shows the trophic levels in a typical ecosystem. There are fewer living organisms the higher you move up the trophic levels due to the loss of energy from one level to the next due to to excretion, respiration and transfer of heat to the atmosphere. Also, living organisms become more complex as you move up through the trophic levels which means more lower level organisms are needed to support those above which creates a trophic pyramid as shown below.