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Concept
Island gigantism
Summary
Island gigantism, or insular gigantism, is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relatives. Island gigantism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies (insular dwarfism). This is itself one aspect of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. Following the arrival of humans and associated introduced predators (dogs, cats, rats, pigs), many giant as well as other island endemics have become extinct (e.g. the dodo (Raphus cucullatus) evolved from a Nicobar pigeon). A similar size increase, as well as increased woodiness, has been observed in some insular plants such as the Mapou tree (Cyphostemma mappia) in Mauritius which is also known as the "Mauritian baobab" although it is member of the grape family (Vitaceae).
Large mammalian carnivores are often absent on islands because of insufficient range or difficulties in over-water dispersal. In their absence, the ecological niches for large predators may be occupied by birds, reptiles or smaller carnivorans, which can then grow to larger-than-normal size. For example, on prehistoric Gargano Island in the Miocene-Pliocene Mediterranean, on islands in the Caribbean like Cuba, and on Madagascar and New Zealand, some or all apex predators were birds like eagles, falcons and owls, including some of the largest known examples of these groups. However, birds and reptiles generally make less efficient large predators than advanced carnivorans.
Since small size usually makes it easier for herbivores to escape or hide from predators, the decreased predation pressure on islands can allow them to grow larger. Small herbivores may also benefit from the absence of competition from missing types of large herbivores.
Official source
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