XXII.6.3 The likelihood of extinction is inversely correlated with the size of the geographic range of species; however, this correlation may disappear during periods of mass extinction
As catastrophes of local character are far more numerous that catastrophes of greater geographic extent, species with a small geographic range are more readily affected by extinction.This trend can actually be observed in the paleontological record.As species living in the tropics have smaller geographic ranges, tropical species generally have shorter lifetimes than species living close to the poles.However, this trend disappears at times of mass extinctions (Raup 1994).
Species occurring on islands have a very high tendency towards extinction.Even in recent times, most of the species that have become extinct in historical times lived on islands.However, other factors in addition to the small area of occurrence are also important for island species.Island species evolved in isolation from other flora and fauna.Thus, they very readily succumb to competition or predation on the part of “more experienced” species that penetrate into their environment from the continents.In addition, the risk of competition displacement is also increased by the fact that, for allopatrically evolved island species, differentiation of ecological niches compared to the parent species living on the continent need not have occurred or the species specialized in the use of a resource that was already better utilized on the continent by the members of some other taxon.For species living on small islands, it is also very important that these are relatively small populations, so that they can become extinct even under stable conditions as a consequence of random fluctuations, and they need not wait for the occurrence of a local catastrophe.
The differences between species with larger and smaller ranges apparently decrease or even disappear at times of mass extinction.It is probable that this is a consequence of the global character of the catastrophe causing this extinction event.If the relevant negative factor, for example a reduction in the intensity of solar radiation and temperature, acts over the entire surface of the planet, the geographic range of a species logically ceased to be of great importance in determining the probability of extinction.
Research performed on marine bivalves at the time of the mass extinction at the boundary between the Cretaceous and Tertiary has, however, indicated that the size of the geographic range could play a positive role at the level of whole genera (Jablonski & Raup 1995).This difference could be a consequence of the fact that species belonging to a genus with a large geographic range differ in their ecological requirements more than species of a genus with a small range.If at least some type of environment was not affected by a catastrophe of global extent, members of the genus as a whole could survive.