Island gigantism

species in two ecosystems. The size and population of weta are affected by predation. Rats introduced on the mainland began to prey on weta, reducing their population; weta shrank in response. On an island isolated from predation, such as Little Barrier Island, weta have a dense population and have grown to a massive size. Insular species of giant weta are the only ones not facing extinction. As weta grow over time, bird predation declines. 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. 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. Benefits of large size that have been suggested for island tortoises include decreased vulnerability to scarcity of food and/or water, through ability to survive for longer intervals without them, or ability to travel longer distances to obtain them. Periods of such scarcity may be a greater threat on oceanic islands than on the mainland. Thus, island gigantism is usually an evolutionary trend resulting from the removal of constraints on the size of small animals related to predation and/or competition. Such constraints can operate differently depending on the size of the animal, however; for example, while small herbivores may escape predation by hiding, large herbivores may deter predators by intimidation. As a result, the complementary phenomenon of island dwarfism can also result from the removal of constraints related to predation and/or competition on the size of large herbivores. In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands. A further means of establishing island gigantism may be a founder effect operative when larger members of a mainland population are superior in their ability to colonize islands. Island size plays a role in determining the extent of gigantism. Smaller islands generally accelerate the rate of evolution of changes in organism size, and organisms there evolve greater extremes in size. == Examples ==

Mammals Many rodents grow larger on islands, whereas carnivorans, proboscideans and artiodactyls usually become smaller. ==== Eulipotyphlans ==== ==== Rodents ==== ==== Lagomorphs ==== ==== Primates ==== ==== Carnivorans ==== ==== Gondwanatherians ==== Birds Stem birds ==== Ratites ==== ==== Waterfowl ==== ==== Pangalliformes ==== ==== Gruiformes ==== ==== Pigeons ==== ==== Birds of prey ==== ==== Parrots ==== ==== Owls ==== ==== Caprimulgiformes ==== ==== Passeriforms ==== Reptiles ==== Iguanids ==== ==== Geckos ==== ==== Skinks ==== ==== Wall lizards ==== ==== Snakes ==== Dubious examples (Lesser Sundas)|thumb|150px |thumb|150px • The Komodo dragon of Flores and nearby islands, the largest extant lizard, and a similar (extinct) giant monitor lizard from Timor have been regarded as examples of giant insular carnivores. Since islands tend to offer limited food and territory, their mammalian carnivores (if present) are usually smaller than continental ones. These cases involve ectothermic carnivores on islands too small to support much mammalian competition. However, these lizards are not as large as their extinct Australian relative Megalania, and it has been proposed based on fossil evidence that the ancestors of these varanids first evolved their large size in Australia and then dispersed to Indonesia. If this is true, rather than being insular giants they would be viewed as examples of phyletic gigantism. Supporting this interpretation is evidence for a lizard in Pliocene India, Varanus sivalensis, comparable in size to V. komodoensis. (Titanochelon), Madagascar (Aldabrachelys), North America (Hesperotestudo) and South America (Chelonoidis, the same genus now found in the Galápagos), and on a number of other, more accessible islands of Oceania and the Caribbean.). The present situation of large tortoises being found only on remote islands appears to reflect that these islands were discovered by humans recently and have not been heavily settled, making their tortoises less subject to overexploitation. • Hatzegopteryx has features of island gigantism such as a more robust bodyplan and occupying niches taken by megafauna elsewhere (in this case, theropod dinosaurs). However, similar sized giant pterosaurs occurred elsewhere, though nowhere near as robust. ===Amphibians=== ===Arthropods=== ===Gastropods=== Flora In addition to size increase, island plants may also exhibit "insular woodiness". The most notable examples are the megaherbs of New Zealand's subantarctic islands. Increased leaf and seed size was also reported in some island species regardless of growth form (herbaceous, bush, or tree). == See also ==