First coined in 1973 by Van Valen, the island rule is not a set rule that applies in all cases, but a general principle. The island rule is built upon a set of hypotheses (Benton, 2010) that aim to explain why animals may change size on islands. Populations present on islands often share the same phylogenetic history with many mainland populations. However, different selection pressures unique to each island may cause changes in these island populations. Often individuals are selected for size attributes; known as insular dwarfism and giantism (not to be confused with medical terms for abnormal growth in individuals) (Lomolino, 1985). They are in fact both a phyletic change in size. Among mammals, rodents and some marsupials are subject to giantism, while carnivores, lagomorphs, elephants and artiodactyls appear to undergo dwarfism. There is no accepted definition for either term as both can be presented in a range of different magnitudes and originate from a range of different selection pressures. The terms are often used to describe animals that are one third, one half or twice or three times the size of the equivalent mainland population.
As with all science the principle of uniformitarianism means that the same processes that operate in the present operated in the past. Many now extinct smaller species of well known mammals may in fact be insular dwarf populations. There are many examples of prehistoric dwarf and giant island populations. If these species are diagnosed correctly then it can provide information on the changes in sea level and the distribution of animal populations at different times in earth's history.
Island colonisation is a risky process, with miles of ocean to cross and perhaps limited food on arrival. The chances of surviving the journey and contributing to a new isolated population are extremely rare. The faster the journey and the less energy expended on getting to the new situation the more likely the individual is to survive to propagate an island population. Planta and animals reach islands by many different means:
- A land bridge would be the least energy expensive and the easiest form of island colonisation for large animals. Animal populations present on the mainland will migrate across the land bridge. As the sea level falls the mainland and isolated populations have their gene flow prevented by the prezygotic barrier of the ocean now in place of the land bridge. This is the optimum form of colonisation for mammal populations as it doesn’t even require the individuals to travel across oceans.
- The most common form of instant colonisation involves the animals travelling to the island via floatation either on rafts of storm debris or through swimming action. This method of colonisation is much more suited to reptilian populations reptilian populations, and the reason why many islands are colonised by reptiles before mammals. Being exotherms reptiles require much less food than mammals and so can survive longer on long ocean voyages. Reptiles are also more likely to become a parthenogetic species; where the females are born pregnant so can reproduce without the need for males. This allows one individual to colonise an entire island. This process has yet to be documented in mammals.
- Many small invertebrates arrive on islands from being dropped by wind currents which pick up thier minuscule juvenille forms and blow them from island to island.
- Obviously birds can fly from island to island gliding for long periods. Often if birds land on an island abundant with food and nest space but without the threat of ground predators, they will evolutionary lose the costly ability of flight as it is no longer needed to either search for food or escape predators. This effect is visible in the many endemic bird species of New Zealand, the flightless cormorant of the Galapagos and the extinct Dodo of Mauritius.
References and Resources
Benton, M.J., et al., 2010. Dinosaurs and the island rule: Dinosaurs from Hateg Island. Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2010.01.026
Lomolino, M.V. 1985. Body size on islands: The Island rule re-examined.The American Naturalist, 125, 310-316
Van Valen, L.M., 1973. Pattern and the balance of nature. Evolutionary Theory, 1, 31-49.
Main image of the Napali Coast, Molokai, above side menu sourced from BBC South Pacific, Episode 3.