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G.F. Gause 1934 If two species, with the same niche, coexist in the same ecosystem, then one will be excluded from the community due to intense competition.

The niche of a species consists of its role in the ecosystem (herbivore, carnivore, producer etc), its tolerance limits (e.g. soil pH, humidity) and requirements for shelter, nesting sites etc etc, all varying through time.

In mathematical terms this can be represented by an n-dimensional hypervolume!
To keep things simple the niche can be represented by a two-dimensional shape.

Example: Squirrels in Britain

The Red Squirrel (Sciurus vulgaris) is native to Britain but its population has declined due to competitive exclusion, disease and the disappearance of hazel coppices and mature conifer forests in lowland Britain.

The Grey Squirrel (Sciurus carolinensis) was introduced to Britain in about 30 sites between 1876 and 1929. It has easily adapted to parks and gardens replacing the red squirrel.

Today’s distribution is shown below.

The Grey Squirrel The Red Squirrel

© Maps prepared by the Biological Records Centre, CEH Monks Wood, from records collated by the Mammal Society and others mainly between 1965 and 1993, also including earlier, published records and a few additions up to 1997. The maps were drawn using Dr Alan Morton's DMAP software

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© Paul Billiet 2016