Literature review: The Island Rule
Literature review: The Island Rule
Previous research and hypotheses have provided explanations for the differences in size of island-dwelling herbivores. This was considered to be due to the effect of size upon the animal becoming prey (where larger size can reduce the chances of being eaten) and allowing more mammals to compete for the same amount of food. Where there was little or no chance of predation, herbivores tended to become smaller. However, many factors probably affect the size that certain species reach in an island environment over and above competition and predation. This research provides a new development of the Island Rule and shows the differences of factors affecting the size of herbivores compared with those for carnivores.
Most examples of changes in animal size are found in the Mediterranean fossil record. The data on fossil carnivores is not as extensive as that for herbivores, but is still sufficiently large, supplemented by data from various natural history museums, to allow some comparison between the two.
Dwarfism is more common and advanced in herbivores and is accompanied by an increase in the number of offspring produced over a lifetime. At the same time, larger numbers of young being produced increases the chances of infant mortality so the journal considers that there is no advantage of one over the other. Previous research discussed the suggestion that organ growth competes with reproduction so that more energy goes into producing offspring so that less is available for growth. It would also appear that mammals that actively defend their young maintain their size in the presence of carnivorous predators.
The journal article research used data on 30 large mammals from different Mediterranean islands. Small mammals were excluded due to inconsistent fossil records. This number of samples was considered suitable due to the range of species and islands covered. Ascertaining the size of the mammals was difficult due to the range of species. The third lower molar was chosen because of the close correlation with body size. If this tooth was unavailable, the third upper molar was used. In addition, a separate size comparison was made using the length of metatarsal or metacarpal bones. These teeth and bones were used because they were the most distinctive and easiest to recognise although others could be better calculating body sizes. This provides validation of the size calculations by using both the teeth and bones, and by providing alternatives for both, it was possible to calculate sizes for all the fossil data.
For extant carnivores the crania was measured using the condylo-basal length (CBL). This was chosen as it does not vary in adults, has low measurement error and does not change over time. It was felt by the authors that this method of measurement provided an accurate measurement of carnivore size to compare with the Pleistocene data.
The results from this research give strong support to two of the hypotheses: herbivore size decreases in the absence of competitors and predators, and decreases to a lesser extent when there is some competition and predation. Carnivore size patterns supported the assumptions that the nature of the resource base is important for its effects on body size but go against the prediction that competition, predators and diet are also important. The authors have discovered that competition appeared to be more important than predation, as some dwarfing occurred even when large predators were present.
The size of the islands was not found to be a predictor of species size, although this was not tested for explicitly. Elephants from Crete, Cyprus, Rhodes, Sicily and Tylos were of comparable size with each other although the area of the islands varied from 25,700 km2 to 61 km2. The authors' conclusion was that size evolution on islands was not directly dependent on influences such as area and isolation. However, they can have an indirect influence as they affect the number and type of insular species that occur. The size of mammals depends on the resources of a given island at a given time, and on the biological attributes of the resident species. The authors were also keen to emphasise that species are real, interacting entities and not just figures in tables. This is an important distinction to make as it makes the reader aware that the Island Rule involves many species living together and interacting over a large timescale.