4.7.2.1a - Ecosystem Levels and Food Chains

Levels in an ecosystem

An ecosystem is easier to understand if you keep the levels of organisation separate. Each level is bigger than the one before it.

• An organism is one individual living thing.
• A population is all the organisms of one species living in the same place.
• A community is all the populations of different species living together in a habitat.
• An ecosystem is the community plus the non-living parts of the environment, such as water, soil, light, temperature and mineral ions.

A stable community is not one where nothing changes. It is one where the species and environmental factors are in balance, so population sizes stay fairly constant over time. They may rise and fall, but they do not usually crash to zero or grow without limit.

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Producers and food chains

Photosynthetic organisms are the producers of biomass for life on Earth. They make glucose by photosynthesis, then use glucose to make other biological molecules. Those molecules become biomass: the material that living organisms are made from.

All food chains begin with a producer. This is usually a green plant or alga. A food chain represents feeding relationships within a community, and each arrow points from the organism being eaten to the organism that eats it.

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The names in a food chain tell you what each organism is feeding on.

• A producer synthesises molecules and makes biomass.
• A primary consumer eats a producer.
• A secondary consumer eats a primary consumer.
• A tertiary consumer eats a secondary consumer.

Do not reverse the arrow. In grass -> grasshopper -> frog, the grasshopper eats the grass, and the frog eats the grasshopper. The arrow points in the direction that biomass is transferred by feeding.

Sampling abundance data

Ecologists often need evidence about organisms in a habitat. Two important ideas are distribution and abundance.

Distribution means where a species is found in a habitat. Abundance means how many organisms of a species are found there. Ecologists use sampling because it is usually not practical to count every individual organism in a whole ecosystem.

A quadrat is a square frame placed on the ground to sample a known area. It is most useful for plants and slow-moving animals. A transect is a line, often a tape measure, placed across a habitat. Quadrats can be placed along the transect to see how distribution and abundance change with distance or with a changing factor such as light intensity.