Macalester Reviews in Biogeography


The Island Rule is the observed tendency for island isolated animals to either grow or shrink in size compared to their mainland counterparts. Though the phenomenon was initially observed to only occur in a handful of taxa (carnivores, artiodactyls, rodents, and lagomorphs), it was expanded to include 2 major trends: 1) Large animals from the mainland tend to shrink on islands, and 2) Small animals from the mainland tend to grow. The mechanisms attributed to those two trends generally involved factors that include resource availability, ecological release, niche expansion, predation, competition, and life history traits. Other theories were also proposed, but each had their own caveats that did not apply as a general rule. The study of the island rule, and island biogeography in general, allows a simplified view of dynamics that may possibly be reflected on mainlands. An example of this includes ecological release and niche expansion in the case of mammals following the Cretaceous/Tertiary extinction event. Following the collapse of dinosaurian prevalence, the relatively small mammals were given the opportunity to grow and speciate accordingly.

However, upon further observation, the island rule in its generality did not encompass all fauna, and exceptions were found for the insular trend. Bergmann’s rule of latitudinal differentiation for body sizes, as well as general climate change, have been found to potentially influence body size shifts as well. As a result, some have chosen to strip the Island Rule of its status as a virtual law, and instead explain the trend as being a phenomenon greatly affected by both biotic and abiotic components to determine insular body size. Regardless of the specific definition, it is maintained that a strong understanding of island processes may lend a better understanding of mainland developmental ecology and evolution.