|The Island Rule: controversies|
In 1964, Foster developed the theory that was later termed the ‘Island Rule’ by Van Valen in 1973. Foster observed ‘that there is a clear tendency for gigantism in insular rodents, while dwarfism is characteristic of insular lagomorphs (rabbits and allies), carnivores and artiodactyls (cloven-hoofed mammals)’ (Foster, 1964). Van Valen (1973) then generalised that dwarfism is seen in large species and gigantism is seen in small species when migrating to an island. Although Mark Lomolino supports the island rule, Shai Meiri and colleagues have opposed the island rule and provided the evidence for their alternative views.
In 1985, Lomolino ‘compiled information on 365 insular populations of terrestrial mammals from North and South America, Europe and Malaysia, thus expressing insular body size in terms of degree of giantism or dwarfism and not just whether insular populations were larger and smaller then their mainland counterparts’ which Foster formerly focussed on. When looking at the frequency of gigantism or dwarfism of insular populations, Lomolino agreed with Foster’s analysis: ‘rodents frequently exhibit gigantism on islands while carnivores and artiodactyls exhibit dwarfism’ (Lomolino, 1985). However when focussing on insular body size expressed as degree of gigantism or dwarfism, then ‘only the artiodactyls have a mean insular body size that differs significantly from their mainland counterparts’ (Lomolino, 1985). Nevertheless, this discrepancy in the island rule can be explained by examining the relationship between insular body size and body size of the species on the mainland. Lomolino believes that a modification of the island rule is needed: ‘rather than describing characteristic differences among mammalian orders, the island rule should account for the graded trend from gigantism in the smaller species of insular mammals to dwarfism in larger species.’ Although most insular populations follow the island rule, there is much unexplained variation from the main trend. This variation or ‘noise’ may provide information allowing the identification of ‘factors and processes influencing body size evolution’ (Lomolino, 2005).
The major factors affecting the insular body size of mammals are competitive release and immigration selection, leading to gigantism, and resource limitation, resulting in dwarfism. The importance of competitive release and immigrant selection should decrease with body size whereas resource limitation should increase with body size. Immigrant selection should select for the larger individuals particularly in smaller species because they expend proportionately less energy stores per kilometre travelled. Larger mammals require a greater amount of resources and so the effect of resource limitation is greater, hence causing dwarfism. ‘Consequently, insular body size of mammals should exhibit a trend grading from gigantism in the smaller species to dwarfism in the larger species (solid line in fig. 1) that is, the island rule’ (Lomolino, 1985).
Meiri et al. (2008) believe, ‘that there is no evidence for the island rule when phylogenetic comparative methods are applied to a large, high-quality dataset.’ The dataset concerned eliminates the errors encountered in datasets previously collected and analyzed. These errors include using very large islands, poor size indices, or mainland populations that are only distantly related to the insular ones. When investigating clade tendency to gigantism or dwarfism, they found that there was ‘no tendency for island populations to be larger or smaller than mainland ones’ (Meiri et al., 2008). However, ‘artiodactyls and carnivores (in particular, herpestids and viverrids) tend to become smaller on islands, whereas insular rodents (especially murids) tend to be larger than their mainland counterparts’ (Meiri et al., 2008). In turn, when focussing on phylogenetic comparisons, the analyses provided ‘no support for the predictions of the island rule either for all mammals or within clades’ (Meiri et al, 2008). This suggests that datasets that have shown ‘a graded trend from insular gigantism to insular dwarfism may have stemmed from pseudoreplication, with many small rodents (mostly mice) showing gigantism and many large artiodactyls (mostly deer) showing dwarfism’ (Meiri et al., 2008). In 2004, when investigating the body size of insular carnivores, Meiri et al. found that island carnivores often differed from those on the mainland but there was not a predictable and apparent pattern in body size. In addition, body size evolution is thought to be determined by factors such as island area, isolation and the presence or absence of carnivores. However, Meiri et al (2008) found no evidence of this. Hence, ‘there is little evidence to suggest that the island rule is a general pattern shared by all mammalian clades’ (Meiri et al., 2008).
Both Lomolino and Meiri have problems with the viability of each other’s datasets. When investigating the island rule, Lomolino used body mass whereas Meiri used skull and teeth measurements. Meiri found that carnivores do not tend to show dwarfism on islands, whilst Lomolino found that carnivores do show dwarfism. However it is thought that the difference in results is not down to body mass versus cranial and dental measurements but down to where the data was collected and methods of collection. Nevertheless, they are both agreed that more detailed studies need to be constructed to completely understand the reasons why body size changes on islands and why it does not occur in all orders.
In summary, Lomolino (2005) believes that ‘the island rule remains a general pattern’, as proposed by Van Valen in 1973. In contrast, Meiri et al. (2008) believes that the island rule cannot be generalised among mammalian clades because dwarfism and gigantism is only seen in specific orders. Thus, the detection of generalities will only be obtained when factors such as history, community composition and the biology of the colonizing species are scrutinised.