Island Biogeography: Types of Species

Evidence from island biogeography has played an important and historic role in the development of evolutionary biology. For purposes of biogeography, islands are divided into two classes. Continental islands are islands like Great Britain, and Japan that have at one time or another been part of a continent. Oceanic islands, like the Hawaiian islands, the Galapagos islands and St. Helena, on the other hand are islands that have formed in the ocean and never been part of any continent. Oceanic islands have distributions of native plants and animals that are unbalanced in ways that make them distinct from the biotas found on continents or continental islands. Oceanic islands do not have native terrestrial mammals (they do sometimes have bats and seals), amphibians, or fresh water fish. In some cases they have terrestrial reptiles (such as the iguanas and giant tortoises of the Galapagos islands) but often (for example Hawaii) they do not. This despite the fact that when species such as rats, goats, pigs, cats, mice, and cane toads, are introduced to such islands by humans they often thrive. Starting with Charles Darwin, many scientists have conducted experiments and made observations that have shown that the types of animals and plants found, and not found, on such islands are consistent with the theory that these islands were colonized accidentally by plants and animals that were able to reach them. Such accidental colonization could occur by air, such as plant seeds carried by migratory birds, or bats and insects being blown out over the sea by the wind, or by floating from a continent or other island by sea, as for example by some kinds of plant seeds like coconuts that can survive immersion in salt water, and reptiles that can survive for extended periods on rafts of vegetation carried to sea by storms.

Many of the species found on remote islands are endemic to a particular island or group of islands, meaning they are found nowhere else on earth. Examples of species endemic to islands include many flightless birds of New Zealand, lemurs of Madagascar, the Komodo dragon of Komodo, the Dragon’s blood tree of Socotra, Tuatara of New Zealand, and others. However many such endemic species are related to species found on other nearby islands or continents; the relationship of the animals found on the Galapagos Islands to those found in South America is a well-known example. All of these facts, the types of plants and animals found on oceanic islands, the large number of endemic species found on oceanic islands, and the relationship of such species to those living on the nearest continents, are most easily explained if the islands were colonized by species from nearby continents that evolved into the endemic species now found there.]

Other types of endemism do not have to include, in the strict sense, islands. Islands can mean isolated lakes or remote and isolated areas. Examples of these would include the highlands of Ethiopia, Lake Baikal, Fynbos of South Africa, forests of New Caledonia, and others. Examples of endemic organisms living in isolated areas include the Kagu of New Caledonia, cloud rats of the Luzon tropical pine forests of the Philippines, the boojum tree (Fouquieria columnaris) of the Baja California peninsula, the Baikal Seal and the omul of Lake Baikal.

Oceanic islands are frequently inhabited by clusters of closely related species that fill a variety of ecological niches, often niches that are filled by very different species on continents. Such clusters, like the Finches of the Galapagos, Hawaiian honeycreepers, members of the sunflower family on the Juan Fernandez Archipelago and wood weevils on St. Helena are called adaptive radiations because they are best explained by a single species colonizing an island (or group of islands) and then diversifying to fill available ecological niches. Such radiations can be spectacular; 800 species of the fruit fly family Drosophila, nearly half the world’s total, are endemic to the Hawaiian islands. Another illustrative example from Hawaii is the Silversword alliance, which is a group of thirty species found only on those islands. Members range from the Silverswords that flower spectacularly on high volcanic slopes to trees, shrubs, vines and mats that occur at various elevations from mountain top to sea level, and in Hawaiian habitats that vary from deserts to rainforests. Their closest relatives outside Hawaii, based on molecular studies, are tarweeds found on the west coast of North America. These tarweeds have sticky seeds that facilitate distribution by migrant birds. Additionally, nearly all of the species on the island can be crossed and the hybrids are often fertile, and they have been hybridized experimentally with two of the west coast tarweed species as well. Continental islands have less distinct biota, but those that have been long separated from any continent also have endemic species and adaptive radiations, such as the 75 lemur species of Madagascar, and the eleven extinct moa species of New Zealand.