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 (present or past) 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. Definitions For biogeographical purposes, an insular environment or "island" is any area of habitat suitable for a specific ecosystem, surrounded by an expanse of unsuitable habitat. While this may be a traditional island—a mass of land surrounded by water—the term may also be applied to many nontraditional "islands", such as the peaks of mountains, isolated springs or lakes, and non-contiguous woodlands. The concept is often applied to natural habitats surrounded by human-altered landscapes, such as expanses of grassland surrounded by highways or housing tracts, and national parks. Additionally, what is an insular for one organism may not be so for others, some organisms located on mountaintops may also be found in the valleys, while others may be restricted to the peaks. Theory 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