Species adapted to survive in areas with seasonal rainfall must be able to deal with these extremes. Variability of sunlight energy varies least in tropical areas, whereas seasonal extremes become greater with increasing distance from the equator. Ecological factors affecting species distributions Many ecologists believe that species ranges are shaped in large part by biotic interactions. Predation and competition may be important on the small scale to limit populations. One introduced cat belonging to a lighthouse owner on an island was responsible for the extinction of a bird species.
Large islands provide larger targets for dispersing animals and support more varied habitats.
They also can maintain a larger population size. Larger populations are less subject to random fluctuations that would reduce the population to zero. This model is important in determining the effective sizes of areas to preserve. With increasing development and deforestation, we are left with patches of habitat that are in effect islands. These rates are a few magnitudes higher than the background extinction rate, of 1-2 species per million species present per year.