110-學測試辦-英文-題組-35-38

$~~~~$Islands are different from a continent or mainland. The size and isolation of islands have a profound effect on island ecosystems and their inhabitants.

$~~~~$Scientists who specialize in species evolution have found that, among mammal species that settle on islands, big species tend to shrink while small ones are apt to enlarge. This phenomenon, discovered by J. Bristol Foster in 1964, has been called Foster’s rule, or the island effect. For instance, rodents (such as rats and squirrels) living on islands tend toward gigantism, while big mammals (like deer and elephants) are more likely to become dwarfed. Although there are a number of exceptions to this pattern, the trend generally holds true for both fossil species and living island mammals.

$~~~~$Foster’s rule shows that body size regulates everything. In a mainland environment, being large is often a safer and adaptive form of avoiding predators. On an island, with few natural predators and less competition, being really big is no longer an advantage; in fact, it can be a hindrance since a huge animal will need a lot more food in order to survive and reproduce.

$~~~~$A new fossil study of island-dwelling proboscideans (i.e. prehistoric hairy elephants) further shows that the physical attributes and ecological structure of an island may affect the degree of shrinking in big mammals. On relatively balanced and species-rich islands, competition with other species often result in a relatively less dwarfed body size. In contrast, on smaller islands where food sources are limited and competitors lacking, members of this group become smaller surprisingly quickly. On one of the Channel Islands 15 miles off the coast of France, the red deer dwarfed to one-sixth the size of deer on continental Europe in a mere 6,000 years after the island became isolated.