Ecological factors

The distributional limits of a species are set by its ecological attributes:

• Fundamental niches

A species will be able to tolerate a certain range of physical factors - temperature, humidity, and so on - and could in theory live anywhere these tolerance limits were satisfied. This is its fundamental niche.

• Realized niches

However, competing species will often occupy part of this range and the competition may be too strong to permit both species to exist. The near extinction of the red squirrel in Britain due to competition from the grey squirrel (pictured opposite) is a good example. Each species' realized niche will then be smaller than its physiology would make possible: each will occupy a smaller range than it could in the absence of competition.

Much ecological research has been carried out to discover the factors - whether physical or biological - that act to limit particular species' distributions in any particular place.