User:Brookehinkle/Island Ecology

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Island Biogeography

Nonnative species introduced onto islands can have profound effects on an island's ecosystem, more so than nonnative species introduced to continental land (Platenberg). The higher impact of introduced and nonnative species on islands is largely a result of lower biodiversity levels (Platenburg et al). Biodiversity on islands is especially threatened by logging, hunting, fishing and plant gathering. However, islands’ vulnerability to introduced species is also due to agricultural, economic, and health differences as compared to continental land (Russel et al).

The smaller land area and population sizes of islands compared to continents create greater vulnerability to the impacts of introduced species (Russel et al). Introduced species negatively impact ecosystems through altered predator-prey interactions that can cause harm or even local extinction to native species populations (Towns et al). There are many examples of animal species such as birds, reptiles, and aquatic insects being harmed by the introduction of predators such as rats, cats, and ants. For example, seabirds on islands in Hawaii are impacted by non-native predators like barn owls and ants. At a colony in Hawaii, at least 20% of wedge-tailed shearwater eggs were taken by mynas, dark-plumaged birds of the starling family (Towns et al). Also in Hawaii, more than forty four introduced ant species, especially tropical fire ants, caused detriments to the growth of shearwater chicks, such as loss of tissue in their feet (Towns et al).

Other examples of high-impact species to island ecosystems are the Cuban treefrog and cane toad, which were introduced to the US Virgin Islands under various circumstances (Platenberg). The Cuban treefrog has been present in the USVI since arriving unintentionally on a cargo ship in the 1970s (Platenberg). The Cuban tree frog is known for its ability to survive under harsh conditions, and it is highly adaptable, as it will eat a wide variety of organisms (Platenberg). Native frog and anole lizard populations have declined, likely due to Cuban tree frog interference (Platenberg). In contrast to the accidental introduction of the Cuban treefrog, the cane toad was deliberately introduced to control agricultural pests (Platenberg). Similar to the Cuban treefrog, the cane toad is also omnivorous. Cane toad tadpoles compete for limited freshwater resources with the white-lipped frog, a native species, thus limiting their resources (Platenberg). In general, when the introduction of a nonnative species results in extinction, the ecosystem experiences losses in some trophic levels (Platenberg). For example, vertebrate herbivores that are prone to extinction change the ecosystem function in plant communities. This phenomenon is seen in New Zealand, where the loss of bird species may have changed dynamics in avian-induced vegetation communities and impacted abundances of forest plants (Platenberg).

Also in New Zealand, small mammalian predators such as rodents have little direct effect on vegetation but have greater effect on island faunas through extinction and displacement (Wood et al). Pacific rats, for example, were thought to be causing the local extinction of large, nocturnal ground-dwelling lizards, and they were assumed to have minimal effect on diurnal species, those that sleep at night, such as shore skinks (Wood et al). In places where Pacific rats were removed, however, shore skink populations rapidly changed, indicating that the rats’ impact has been underestimated (Wood et al).

The loss of a keystone species, a species that all other species in the ecosystem depend on such as seabirds, can also have significant effects (Towns). Seabirds are essential to the structure of the ecosystem because they transport large amounts of nutrients into ecosystems and burrow into soils, helping vegetation (Towns). However, since human settlement, seabird communities have been severely impacted on islands across the globe (Towns). While some exotic species may perform ecological roles similar to those of extinct species, there are many roles that cannot be fulfilled by other species. Therefore, similar exotic species do not offer complete replacement for extinct species in the community (Towns). Even if the number of species introduced to an island is roughly the same as the number of extinct ones, the ecological traits are not comparable enough to make up for losses (Towns).