There are many hypotheses which have been presented in an attempt to provide as succinct a general island law as possible. However with such a long standing area of study there are many theories that have been shown to be rather unlikely (Benton, 2010);
- The relict population viewpoint (Hinton 1926). Rodent island populations that appear to be a form of gigantism were proposed to be relicts of the original rodent mainland populations. Those giant rodent populations have since become extinct due to environmental stresses that are not present in the isolated island ecosystem. This theory was disproved as there was limited evidence to show giant mainland rodents becoming displaced by smaller mice.
- Reduced prey size. Further suggestion is that reduced prey size on islands could induce dwarfing among island predators. It was hypothesised that with fewer predators small scale prey is abundant and thus those predators that are isolated on islands will scale down in order to take advantage of this opportunity. This hypothesis however cannot explain the dwarfing of herbivores as they do not match their size to the size of the plants they eat.
- Sexual selection. It was also hypothesised that sexual selection on islands could be a contributing factor in increasing body size where selection pressures present on the mainland have been removed. It may have been a contributing factor for some mammals; where sexual selection favours large size, but clear examples have not yet been demonstrated.
- Optimum body size. It is thought that island populations may move towards an optimum body size when pressures present on the mainland are reduced. The optimum is described as the size at which energy capture from the environment is maximised. However related island species showed different optimum body sizes which depended on the local conditions; therefore being no tendency towards an optimum body size.
- Selection of immigrants for large size. Another suggested explanation was based on the fact that larger and stronger animals were more likely to swim farther or more successfully, based on size-related metabolic demand. The founder population would therefore be genetically composed of larger individuals. This theory may apply in some cases but not all. Methods of animal colonisation.
There are many hypotheses which are still upheld by the majority of the scientific community; they go some way in order to explain the apparent change in body size of island populations.
- Ecological release: Island species are released from pressures which constrain the size of the individual such as;
- Less competition for food and shelter, the increased abundance of resources allows individuals to grow larger as they take advantage of them.
- An absence of predators removes the defensive benefits of being large, so it makes sense to conserve energy by not undertaking this energy expensive process. As with birds an absence of predators results in the loss of the energy expensive process of flight (McNab 1994).
- Niche Expansion: (Heany 1978) the reduced biodiversity present on the islands results in many niches present to not be exploited to their fullest potential, giving those island populations the evolutionary incentive to take advantage of those niches. Animals constrained to small sizes in the mainland populations can expand and grow to exploit new situations.
- Resource Limitation: Large herbivores require enormous amounts of vegetation to provide them with energy for a day to day existence. Often these large herbivores will migrate enormous distances to exploit areas rich in new growth; a behaviour that is simply unfeasible on an island. Therefore animals must reduce in size so as to require a reduced amount of energy to exist.
- Optimization of life history traits: (Palombo 2007) Animals that reach sexual maturity more rapidly and therefore reproduce at a smaller size are likely to be better colonisers.
It has been suggested however that mammal populations can change their size in order to exploit the most appropriate empty niche and in response to the abundance of competitors or the presence of predators (Meiri et al. 2004, 2006) Therefore there may be no definite island rule. It is more likely to be an intricate interaction of many hypotheses that results in the final island adapted population. Both dwarfing and gigantism are results of intensified natural selection promoting directional shifts in mean body size.
References and Resources
Benton, M.J., et al., Dinosaurs and the island rule: Dinosaurs from Hateg Island, Palaeogeography, Palaeoclimatology, Palaeoecology (2010), doi:10.1016/j.palaeo.2010.01.026
Heaney, L., 1978. Island area and body size of insular mammals, evidence from tricolored
squirrel (Callosciurus prevosti) of southeast Asia. Evolution 32, 29–44.
Hinton, M.A.C., 1926. Monograph of the Voles and Lemmings (Microtinae). British Museum (Natural History), London.
McNab, B.K., 1994. Energy conservation and the evolution of flightlessness in birds.American Naturalist 144, 628–642.
Meiri, S., Dayan, T., Simberloff, D., 2004. Body size of insular carnivores: little support for the island rule. American Naturalist 163, 469–479.
Meiri, S., 2007. Size evolution in island lizards.Global Ecology and Biogeography 16,702–708.
Palombo, M.R., 2007, How can endemic proboscideans help us understand the “island rule”? A case study of Mediterranean islands. Quaternary International 169–170, 105–124.