AQA GCSE Biology: Communities and Competition

An ecosystem is a web of living things interacting with each other and their surroundings. This guide explains how an ecosystem is organised, what organisms compete for and why, how species depend on one another, and what makes a community stable. The idea of a balance between many species runs through the whole topic.

Levels of organisation

An ecosystem is the interaction of a community of living (biotic) organisms with the non-living (abiotic) parts of their environment. Within an ecosystem there are several levels:

• an individual organism;
• a population, a group of organisms of one species in an area;
• a community, the many populations of different species in an area.

The three nest inside one another, and questions test whether you can keep them apart. One grey squirrel is an organism. Every grey squirrel in the wood is a population. Every grey squirrel, oak, earthworm, fungus and robin in that wood together is the community. Add the soil, the rainfall and the light reaching the floor and you have the ecosystem.

Two more words get swapped by mistake. A habitat is the place an organism lives, so the wood is a habitat. An ecosystem is that habitat plus everything living in it plus the way the two interact. A habitat is a location; an ecosystem is a system.

Competition

To survive and reproduce, organisms need materials from their surroundings and from other organisms. When they need the same resource and it is in limited supply, they compete.

• Plants compete for light, space, water and mineral ions from the soil.
• Animals compete for food, mates and territory.

The two lists differ because the two kinds of organism get what they need in different ways. A plant cannot move and makes its own food, so everything it competes for is something that has to arrive where it is already standing: light falling on its leaves, water and mineral ions reaching its roots, and room to spread into. An animal can move and has to find food that something else has already made, so it competes for the food itself, and for mates, and for the territory that holds both.

There are two kinds of competition, and the harsher one is the less obvious. Interspecific competition is between different species, and it is usually partial, because two species rarely want exactly the same things. Intraspecific competition is between members of the same species, and it is fiercer, because two individuals of one species need precisely the same food, the same kind of nesting site and the same mates. Competition is most intense where the needs overlap most completely, and that is within a species.

Competition is important because it enables species to evolve by natural selection. That follows directly from the paragraph above: within a species, the individuals slightly better at getting the limited resource survive and breed more often, so the alleles that made them better become more common.

Interdependence and stable communities

Species in a community rely on each other, which is called interdependence. Each species depends on others for things like food, shelter, pollination and seed dispersal, so if one species is removed it can affect the whole community. This interdependence is important because it lets an ecosystem resist change.

Interdependence is easiest to see by taking something away. Remove the bees from a meadow and it is not only the bees that have gone. The flowers they pollinated set fewer seeds, so there are fewer plants the next year, so the animals that ate those plants have less food, and the predators of those animals go hungry in turn. One removal travels outwards through the community, and that travelling is what "affects the whole community" means in a mark scheme.

A stable community is one where all the species and environmental factors are in balance, so the population sizes stay fairly constant.

Note what that does not mean. It does not mean nothing changes. Population sizes rise and fall constantly. What makes a community stable is that they move around a level rather than running away or collapsing, because the things pushing them up and the things pushing them down are in balance. A wood with an unchecked new predator in it is not a stable community, even though every original species is still present in it.

How this fits together

Communities are shaped by the conditions around them and by the other living things present. Those two kinds of factor, abiotic and biotic, are the subject of the abiotic and biotic factors guide.