White-clawed crayfish

The game of life

Competing for survival

Most animals interact with other animals or plants directly because they depend on them for food. They also compete with animals of their own species, or with other animal populations, for the same food sources, especially when these are scarce.

Some species interact for mutual benefit – like the birds that feed on the mites that pester buffalo – and some, like viruses and parasites, only interact to help themselves.

Human populations are involved in more interactions with other species than it seems possible to count – from our domestication of animals for food and companionship, to the destruction of natural ecosystems, to the communities of bacteria in our guts that help us digest our food. Of course, human populations also interact with one another. Like all animals, we fight over territory and resources, even as we try to cooperate for the greater good.

Within all ecosystems there is competition for precious resources, such as light, water or space. When competition is between members of the same species, it is intraspecific; when it is between different species, it is interspecific.

Scientists have spent many years studying interspecific competition between crayfish species across Europe and North America. In many areas of the UK native white-clawed crayfish are being replaced by invasive North American signal crayfish, which originally escaped from British crayfish farms. The larger invaders oust the native crayfish from the limited refuges under rocks in streams and rivers and compete for food. They also carry a fungal plague, to which the UK species has no defence.

Their battle over niche is an example of the competitive exclusion principle, which states that it is impossible for two species in the same ecosystem to coexist if they are competing for the same resources.

Lead image:

White-clawed crayfish.

University of Leeds, Faculty of Biological Sciences

Further reading

About this resource

This resource was first published in ‘Populations’ in June 2014.

Ecology and environment
Education levels:
16–19, Continuing professional development