Island syndrome describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. These differences evolve due to the different ecological pressures affecting insular species, including a paucity of large predators and herbivores as well as a consistently mild climate.
Ecological driving factors
Reduced predation. Island ecosystems cannot support a sufficient biomass of prey in order to accommodate large predators. This largely relieves prey species of the risk of predation, which mostly removes the selection pressure for morphologies, ecologies and behaviours that help to evade large predators. Reduced biodiversity. Insular ecosystems tend to comprise large populations of a limited number of species (a state termed density compensation), therefore, they exhibit low biodiversity. This results in reduced interspecific competition and increased intraspecific competition. Reduced sexual selection. There is also reduced sexual selection in insular species, which is especially prominent in birds which lose their sexually dimorphic plumage used in sexual displays. Reduced parasite diversity. Finally, there is reduced parasite diversity in insular ecosystems which reduces the level of selection acting on immune-related genes.
Features of island syndrome in animals
Body size
Interspecific competition between continental species drives divergence of body size so that species may avoid high levels of competition by occupying distinct niches. Reduced interspecific competition between insular species reduces this selection pressure for species to occupy distinct niches. As a result, there is less diversity in the body size of insular species. Typically small mammals increase in size (for example fossa are a larger insular relative of the mongoose) while typically large mammals decrease in size (for example the Malagasy hippopotamuses are smaller insular relatives of continental hippopotamuses). These are examples of insular gigantism and insular dwarfism respectively. This observed effect is called Foster's rule. Conversely, birds and reptiles tend to exhibit insular gigantism, exemplified by the moa, cassowary and Komodo dragon.
Although the giant tortoises of the Galápagos Islands and the Seychelles (the Galápagos tortoise and Aldabrachelys respectively) are sometimes given as examples of insular gigantism, they are now thought to represent the last remaining populations of historically widespread giant tortoises. The remains of tortoises of similar or larger size have been found in Australia (Meiolania), southern Asia (Megalochelys), Madagascar (Aldabrachelys), North America (Hesperotestudo) and South America (Chelonoidis). The extant giant tortoises are thought to persist only in a few remote archipelagos because humans arrived there relatively late and have not heavily predated them, suggesting that these tortoise populations have been less subjected to overexploitation.
Locomotion
Since insular prey species experience a reduced risk of predation, they often lose or reduce morphologies utilised in predator evasion. For example, the wings of weevils, rails and pigeons have become so reduced in insular species that many have lost the ability to fly. This has occurred in several ratites including the kiwi and the cassowary as well as in the dodo and the kakapo after invading island habitats. The extinct moa of New Zealand exhibit the most extreme known example of insular wing reduction; there is no osseous evidence of even vestigial wings and the pectoral girdle is reduced to a scapulocoracoideum which would be unable to bear a forelimb as it lacks a glenoid fossa. Therefore, it is the only bird known to have completely lost its wings after a shift to insularity. Loss of flight allows birds to eliminate the costs of maintaining large flight-enabling muscles like the pectoral muscles and allows the skeleton to become heavier and stronger. Insular populations of barn owl have shorter wings, representing a transitional stage in which their capacity for flight is being reduced.
Adaptive coloration
Due to the reduced sexual selection of insular species, they tend to exhibit reduced sexual coloration so as to conserve the energy that this demands. Additionally, the low biodiversity of insular ecosystems makes species recognition less important so species-specific coloration is under less selection. As a result, insular bird species often exhibit duller, sexually monomorphic plumage.