First proposed by the researcher J. Bristol Foster in 1964, the island rule demonstrated that as mammals adapted to insular island ecosystems, they would dramatically increase or decrease in size relative to their closest mainland cousins. Moreover, based on Foster's survey of 116 island-dwelling mammals, a simple trend emerged: larger mammals grew smaller on the islands, while smaller mammals grew larger, as if both types of species were progressing toward some ideal statistical mean. This trend became known as the "island rule."
In 1967 the island rule graduated from a rudimentary observation to a legitimate field of study, thanks to the publication of The Theory of Island Biogeography by Robert H. MacArthur and Edward O. Wilson. This book set forth the basic principles of the newly defined subfield of island biogeography, which deals with the ecological development of living creatures within insular environments. Among the principal preoccupations of island biogeography are the twin phenomena of "insular dwarfism" and "insular gigantism," the technical terms for the respective decrease or increase in size of island-dwelling species as compared with their closest mainland relatives.
Put simply, island biogeography is directly concerned with turning the hypothetical island rule into a full-fledged science. In the 40 years since the publication of Foster's original two-page survey paper, myriad exceptions to, elaborations upon, and contradictions to the original island rule have come to light, replacing the basic concept of progressing toward the mean with a complex array of observations and guidelines that have yet to coalesce into a coherent whole. The process of evolution is never easy, not even on a tropical island.
The most glaring problems facing biogeographers are oppositional examples of insular dwarfism and gigantism, wherein one island offshoot of a mainland species grows large, and a laterally related cousin on another island grows small. These cases indicate that a species may not be necessarily obligated toward gigantism or dwarfism and that the attributes unique to each island environment can tip the balance either way.
Giant versions of a given species have advantages against predators. They can sustain themselves by preying on large animals and can give birth to more numerous groups of young, but they must pay for these advantages by consuming more food, which makes them more vulnerable to the effects of drought and famine. In contrast, dwarf species consume less food and can camouflage themselves or take refuge in a wider variety of terrains, but they give up the predatory and reproductive advantages of larger size. While the island rule is generally accurate in predicting which way many species will likely adapt to island environments, there is no hard-and-fast guarantee.
As the science of biogeography endures these growing pains, it has been presented with its most tantalizing research subject yet--Homo floresiensis, a cousin species to H. sapiens that may offer the first concrete example of insular dwarfism in protohumans. Discovered in late 2003, these so-called hobbits stood a mere 3 feet (0.9 meters) tall, had brains one-third the size of those of H. sapiens, and lived as recently as 18,000 years ago on the Indonesian island of Flores--meaning that they may have shared the environment with full-sized humans. Faced with such a complex and contradictory evolutionary puzzle, H. floresiensis may forever change the way biogeography looks at human beings, and vice versa.
