What factors limit the geographic range of a species?
The distributional limits of a species are set by its ecological attributes:
• Fundamental niches
If a species is able to tolerate a certain range of physical factors such as temperature, humidity, and so on and it has the capacity in theory live anywhere within these tolerance limits, this is its fundamental niche.
• Realized niches - Competing species
However, if there are competing species that occupy part of this range 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 is a good example. Each species' realized niche will be smaller than its physiology makes possible. In other words each species will occupy a smaller range than it otherwise would in the absence of competition.