page_title: Insular biogeography
section_title: Theory
hierarchical_section_title: Insular biogeography / Theory
caption_attribution_description: English: This diagram shows the effect of an island’s size on the amount of species richness. The diagram shows two islands equidistant from the mainland. Island 1 receives less random dispersion of organisms. While island 2 receives more of the arrows and therefore more random dispersion of organisms.
context_page_description: Insular biogeography or island biogeography is a field within biogeography that examines the factors that affect the species richness and diversification of isolated natural communities. The theory was originally developed to explain the pattern of the species–area relationship occurring in oceanic islands. Under either name it is now used in reference to any ecosystem that is isolated due to being surrounded by unlike ecosystems, and has been extended to mountain peaks, seamounts, oases, fragmented forests, and even natural habitats isolated by human land development. The field was started in the 1960s by the ecologists Robert H. MacArthur and E. O. Wilson, who coined the term island biogeography in their inaugural contribution to Princeton's Monograph in Population Biology series, which attempted to predict the number of species that would exist on a newly created island.
context_section_description: The theory of insular biogeography proposes that the number of species found in an undisturbed insular environment ("island") is determined by immigration and extinction. And further, that the isolated populations may follow different evolutionary routes, as shown by Darwin's observation of finches in the Galapagos Islands. Immigration and emigration are affected by the distance of an island from a source of colonists (distance effect). Usually this source is the mainland, but it can also be other islands. Islands that are more isolated are less likely to receive immigrants than islands that are less isolated.
  The rate of extinction once a species manages to colonize an island is affected by island size; this is the species-area curve or effect. Larger islands contain larger habitat areas and opportunities for more different varieties of habitat. Larger habitat size reduces the probability of extinction due to chance events. Habitat heterogeneity increases the number of species that will be successful after immigration.
  Over time, the countervailing forces of extinction and immigration result in an equilibrium level of species richness.
caption: This diagram shows the effect of an island’s size on the amount of species richness. The diagram shows two islands equidistant from the mainland. Island 1 receives less random dispersion of organisms. While island 2 receives more of the arrows and therefore more random dispersion of organisms.
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