Island ecology is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth's total land area,Paulay, G. 1994. Biodiversity on Oceanic Islands: Its Origin and Extinction. American Zoology 34: 134-144. yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niches lead to increased speciation. As a result, island ecosystems comprise 30% of the world's biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species.Gerlach, Justin. 2008. Island biodiversity - issues and opportunities . World Conservation Congress Many species still remain unknown.
The diversity of species on islands is highly impacted by human activities such as deforestation and the introduction of exotic species. In response, ecologists and managers are directing attention towards conservation and restoration of island species. Because they are simple systems, islands provide an opportunity to study processes of extinction that can be extrapolated to larger ecosystems.
Ecological processes on islands
Islands are attractive sites for ecological research because they provide clear examples of evolution in action. They show interesting patterns of colonization, adaptation, and speciation.
Colonization and establishment
Islands are surrounded by water, and may or may not exist as part of a continental land mass. Oceanic islands arise due to volcanic activity or reef growth, and usually subside over time due to erosion and changing sea levels. When islands emerge, they undergo the process of ecological succession as species colonize the island (see theory of island biogeography). New species cannot immigrate via land, and instead must arrive via air, water, or wind. As a result, organisms with high dispersal capabilities, such as plants and birds, are much more common on islands than are poorly dispersing taxa like mammals. However, some mammals are present on islands, presumably from swimming or riding on natural "rafts" that are washed away from the mainland.
Of the species that arrive, only some will be able to survive and establish populations. As a result, islands have fewer species than mainland habitats. Island populations are small and exhibit low genetic variability (see founder effect), but are isolated from the predators and competitors that they initially evolved with. This can lead to a process called ecological release, where a species is released from its ancestral community interactions and then colonizes new niches.
Adaptation
In response to these changing ecological pressures, island species can become much more docile than their mainland counterparts, and may grow larger (see island gigantism) or smaller (see island dwarfism). Some of these unique adaptations are reflected in charismatic island species such as the Malagasy hippopotamus, Komodo dragon, or pygmy mammoths. 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 i.e. gigantism is an ancestral trait that occurred in the absence of insular selection pressures. The collection of differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts is termed Island syndrome. After immigration, birds and some reptiles or mammals tend to become larger and predatory,Alcover, J. A. and M. McMinn. 1994. Predators of Vertebrates on Islands. BioScience 44(1): 12-18. showing relaxed intraspecific competition.Stamps, J.A. and M. Buechner. 1985. The Territorial Defense Hypothesis and the Ecology of Insular Vertebrates. The Quarterly Review of Biology 60(2): 155-181. For mammals, small species will increase in size and large species will decrease in size.Van Valen, L.M. 1973. Pattern and the Balance of Nature. Evolutionary Theory 1: 31−49. This is referred to as the "island rule," and is suggested to minimize energy expenditure.McNab, B.K. 2002. Minimizing energy expenditure facilitates vertebrate persistence on oceanic islands. Ecology Letters 5(5): 693-704.
Other adaptations to life on islands include increased poikilothermy, relaxed anti-predator behavior,Blumstein, D.T. 2002. Moving to suburbia: ontogenetic and evolutionary consequences of life on predator-free islands. Journal of Biogeography 29(5): 685-692. and reduced sexual selectionGriffith, S.C. 2000. High fidelity on islands: a comparative study of extrapair paternity in passerine birds. Behavioral Ecology 11(3): 265-273. in animals, and loss of herbivore defensesVourc'h, G., J.L. Martin, P. Duncan, J. Escarre, and T.P. Clausen. 2001. Defensive adaptations of Thuja plicata to ungulate browsing: a comparative study between mainland and island populations. Oecologia 126(1): 84-93. and reduced dispersalCody, M.L. and J.M. Overton. 1996. Short-term evolution of reduced dispersal in island plant populations. Journal of Ecology 84(1): 53-61. in plants.
Speciation
The formation of new islands and their isolation from the mainland provides many unoccupied niches for species to adapt to. Since immigration of predators and competitors is limited, many organisms are able to persist in these new niches. This results in a high occurrence of endemism, where species are unique to a localized area. For example, 50% of endemic bird areas are found on islands.
Endemism is often the result of adaptive radiation. Adaptive radiation is when a single species colonizes an area and rapidly diversifies to fill all of the available niches. A common example is the assemblage of finch species documented by Charles Darwin in the Galapagos Islands. Darwin's finches exhibited adaptive radiation by evolving different beak sizes to exploit the diversity of seeds present on the different islands.
Because the distributions of these populations are limited by their island habitats, they tend to have fewer individuals than their mainland counterparts and lower genetic variation. This, along with the behavioral and ecological factors mentioned above, makes island species more vulnerable to extinction.
Survival
The continued survival of species on islands depends on factors such as natural selection, genetic variation, natural disturbances (hurricanes, volcanic eruptions) and human-caused disturbances (introduced species, habitat loss). Human-caused disturbances tend to be the greatest cause of mortality, and understanding the causes of extinction facilitates conservation efforts.
Human impacts on island ecosystems
The movement of humans to islands has led to rapid extinction of native island species either from hunting, habitat destruction, or introduced species.
Hunting
Many large animals on islands have been hunted to extinction by humans. A well-known example is the dodo, once found on the island of Mauritius. It evolved to become large, flightless and docile, and was subsequently driven to extinction by humans and introduced predators.
Habitat destruction The depletion of natural resources can have dramatic effects on island ecology. On Easter Island, the depletion of the forest by humans not only resulted in widespread loss of species, but also the collapse of the island civilization.Bahn, P and J. Flenley. 1992. Easter Island, Earth Island. Thames and Hudson, London. Today there are over 500 million people on islands, all dependent on local resources either directly (traditional use) or indirectly (ecotourism revenue). Population growth and development result in heavy deforestation, pollution, and over-exploitation. Overharvesting of ocean fauna is particularly troubling as yields of coral reef fish species are an important food source for island populations.
Introduced species
Humans have contributed to globalization and decreased effective isolation of island communities, allowing for invasion of exotic species. This can have a profound effect on the native species. In Guam, the introduced brown tree snake ate nearly all of the native vertebrate species to extinction. Feral cats and dogs have also greatly diminished native vertebrate populations on islands, through both predation and disease. Introduced ungulates are another major threat, as they graze on native vegetation and can destroy entire forests.Towns, D.R. and W.J. Ballantine. 1993. Conservation and Restoration of New Zealand Island Ecosystems. Trends in Evolution and Ecology 8(12): 452-457. Exotic grasses can out-compete native understory species and increase the risk of fire.D'Antonio, C.M. and P.M. Vitousek. 1992. Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecological Systematics 23: 63-87. Lastly, social insects such as ants also cause major problems.Cole, F. R., A.C. Medeiros, L.L. Loope, and W.W. Zuehlke. 1992. Effects of the Argentine ant on arthropod fauna of Hawaiian high-elevation shrubland. Ecology 73: 1313-1322.
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.).