Insular biogeography is a field within biogeography that establishes and explains the factors that affect the species richness of isolated natural communities. In this context an island can be any area of ​​habitat surrounded by areas unsuitable for the species; They may not be true islands surrounded by ocean, but also mountains surrounded by deserts, lakes surrounded by land, fragments of forests surrounded by human-altered landscapes. This field of study was created in the 1960s by ecologists Robert MacArthur and E. O. Wilson who coined the term island biogeography theory to refer to predictions about the number of species that could exist on a newly created island.

A few years after the theory was published, its application began to be put into practice in the field of conservation biology, giving rise to numerous debates among ecologists.[5]​ The understanding that reserves and national parks form islands within landscapes altered by humans (habitat fragmentation) caused great concern because it would mean that these islands could lose species when they approach equilibrium in the number of species. This problem is especially serious when it comes to species of larger physical size, which generally require larger areas of land. A work by William Newmark, published in Nature and summarized in The New York Times, showed a strong correlation between the size of protected national parks in the United States and the number of mammal species present.

This led to a debate known as "single large or several small" (SLOSS), described by writer David Quammen in Vanity of the Dodo as "the gentle ecological version of trench warfare." In the years since the publication of Wilson and Simberloff's work, ecologists have found more examples of the species-area relationship. That is why conservation plans have adopted the perspective that a large reserve can maintain a greater number of species than several small ones and that large reserves should be the norm in ecological reserve design. This view has been espoused especially by Jared Diamond. In turn, some ecologists, including Daniel Simberloff, considered that this is an unjustified simplification that could harm conservation efforts. Habitat diversity may be more important than the size of an area in determining the number of protected species.*.

The theory of island biogeography has led to the development of ecological corridors as a conservation tool to increase connectivity between isolated habitats. Ecological corridors or biological corridors can increase the movement of species between parks and reserves and, in this way, increase the number of species they can support.