This obser vation has led to Rapoport's rule, namely that the latitudinal extent of a species' range
increases towards the poles (Rapoport 1982).Thus, peripheral pop ulations not only provide a refuge for endangered species but also represent geneticalAnalyses
of range contractions in a wide variety of animals and plants suggest that populations
often collapse first in the center, however, leaving isolated fragments on the periphery
(Lomolino and Channell 1995; Channell and Lomolino 2000), with these collapses
being due to the variety of causes outlined by Caughley (1994).