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The Island Rule: Made to be Broken?
25 Nov, 2008 05:51 pm
Think of island-dwelling animals and enormous Kodiak bear, giant tortoises, and huge man-eating lizards alongside miniature mammoths, and dwarf hippos spring to mind. In 1964, a scientist called John Bristol Foster quantified the size changes seen in island animals, relative to populations of the same species on the nearby mainland. Foster found that while rodents usually grow larger on islands ungulates, carnivores and lagomorphs (rabbits and their relatives) are smaller on islands than their direct relatives on the mainland. Although Foster found no island gigantism in the smallest mammals (shrews), his findings were interpreted as a general pattern for dwarfism on islands of large mammals, and gigantism in small mammals. This pattern was later named ?the island rule?.
Many more studies of body size evolution on islands were published in the last few years. Some of these described patterns consistent with the island rule in groups such as birds and snakes, whereas others showed that some groups of animals, such as lizards and mammalian carnivores, did not follow the rule. These disparate results highlighted the need for both better data and better methods to analyze these data, so that the true nature of size evolution on islands will be revealed.
In our study we assembled a large a data set of body sizes of island and mainland mammals, while maintaining strict control over the quality of these data.
Our dataset comprises 1184 island–mainland population pairs, representing 276 species in 45 families and 15 orders of mammals, from bats and shrews to rodents, carnivores and ungulates. We used three approaches to analyze these data. The first was similar to Foster’s: we examined whether different mammalian groups had an overall tendency to either grow larger or smaller on islands.
The second method was similar to Lomolino’s: we examined whether large species dwarfed more and small species grew larger. We also used a more sophisticated approach that take into account the evolutionary relationship between the different species in our database when examining their size evolution on islands.
The results of first method suggest that only one mammalian group regularly show gigantism on islands: murid rodents (rats and mice). Some groups were characterized by island dwarfism: small carnivores (mongooses and genets), even-toed ungulates, and Heteromyids: a group of small rodents. The great majority of mammalian groups, however, showed an overall tendency towards neither gigantism nor dwarfism on islands. Using the second method we found a result that is consistent with the island rule, but much much weaker than that reported before for mammals. Within different mammalian groups, however, there was no relationship between body size and the tendency to grow larger or smaller: small mice, for example, did not grow on islands to a greater degree than did somewhat larger rats.
Using the third, evolutionary method we did not find any pattern consistent with the island rule, either in mammals as a whole or in different mammalian subgroups - except in rabbits.
We think, therefore, that the island rule as usually perceived is an over-simplification of the way size evolves on islands. Whether an animal will grow larger or smaller on an island depends, we believe, on the ecological community of animals found on each particular island, and the way it interacts with the biology of the particular species that colonizes that island. Carnivores, for example, are likely to be influenced mostly by the abundance of their prey, and by prey size (after all carnivores need to be large enough to overcome their prey). Size evolution of herbivorous animals may be more likely to be influenced by the presence of competitors, or predators, or by the productivity of their plant foods.
Size evolution on islands is often striking and unpredictable, and is not controlled by any single factor such as the size of the colonizing species. Only be examining carefully different ecological communities on islands, and the interactions between members of these communities are we likely to have a good understanding of the forces that drive size evolution.
1. Meiri, S, Cooper, N, Purvis, A. The Island Rule: Made to be Broken? Proceedings of the Royal Society B: Biological Sciences. Online: 6 November 2007. Abstract available here.