Biogeography is the study of species distribution across geographic space and the processes that shape these distributions. This discipline is based on the assumption that each species within a location must have immigrated from another geographic area or evolved from a local species. Within each habitat, a variety of biotic and abiotic factors act on individual species to shape birth, death, immigration, and emigration rates. To avoid local extinction, the combined birth and immigration rates of a species must equal or surpass the death and emigration rates. These factors and others act in a dynamic fashion to determine the location and persistence of species across the globe.
In 1967, Edward O. Wilson and Robert MacArthur studied and characterized species distribution on isolated oceanic islands to better understand these phenomena. Such islands provided a simple framework to carve out early theories of biogeography, which continue to inform current understanding of population dynamics today. Furthermore, these theories have been applied not only to island ecosystems, but also to more complex isolated or semi-isolated landscapes, including inland lakes and forests fragmented by agriculture. The original theories established by Wilson and MacArthur were based on assumptions about island habitats; namely, that the number of species on each island is determined by the overall colonization and extinction rates and that island size and proximity to the mainland will affect these factors.
One tenet of island biogeography states that an island closer to the mainland will have a greater colonization rate than sites further away. This is predicted based on the assumption that the mainland (or larger, populated areas) will serve as a source of emigrating individuals that may populate nearby uncolonized locations. The process of colonization is more likely to occur over shorter distances, as it depends on the dispersal of organisms. For example, birds will fly from one location to another or seeds can drift passively on the wind. The biological limitations of most emigrating organisms, however, do not allow them to disperse to sites at great distance.
Early biogeographers also found that larger islands tend to support a greater number of species than smaller islands. This is because of the greater amount of resources and niches, as well as the larger population sizes that bigger areas can sustain. With more individuals, the risk of extinction for each species decreases. Generally, larger populations are less likely to be irreversibly affected by threats like natural disasters, disease, or genetic drift. However, as the number of species within a habitat increases, so do the overall extinction rates. This effect is largely due to the increased potential for antagonistic interactions, like competition or predation, with increasing diversity of organisms. Furthermore, species abundance generally results in a decrease in available resources and niches for new colonizers. Accordingly, each geographic area will support a limited number of individuals and species.
These findings predict the establishment of a dynamic equilibrium at which the number of species on an island stabilizes due to balanced immigration and extinction rates. This balance is influenced by the individual factors of each location, including the size, resources, and species present. While the overall number of species may remain relatively constant due to this equilibrium, the specific species composition of a habitat is expected to change as new immigrants arrive and old resident species become extinct. Since large islands near the mainland have the highest immigration rate and the lowest extinction rate, they are predicted to support the greatest number of species. The opposite is observed for smaller, more isolated islands. To reach equilibrium, extinction risk increases and colonization rate decreases as the number of species on an island increases. The point at which dynamic equilibrium is reached can be plotted as the intersection between the colonization rate and extinction rate. These factors together determine the diversity of species that can be supported by each habitat.
Though these theories were established based on island habitats, they have been tested and supported across a wide range of ecosystems and organisms1-2. For example, “sky islands” are individual peaks of high-altitude mountains that contain a distinct species composition. Many species in these locations are unable to survive in low-altitude climates, making dispersal between mountain peaks difficult. For organisms like amphibians, plants, and terrestrial insects with limited mobility, these “sky islands” are as isolated as true oceanic islands and behave according to the theories of island biogeography. Importantly, many of the species found in oceanic islands, sky islands, and other segregated habitats are endemic, meaning they are found only in that location and nowhere else. The specific evolutionary pressures of these local environments coupled with their isolation from outside populations often leads to speciation, or the evolution of new species. For instance, komodo dragons and Galapagos tortoises speciated from mainland ancestors to form distinct endemic populations. However, these endemic species are at a greater risk of extinction than their mainland counterparts due to smaller population sizes and more limited habitat areas. Conservation of such species requires an understanding of the tenets of biogeography, habitat dynamics, and the requirements of individual populations3-4.
Often, human activity reduces the amount of suitable habitat or causes fragmentation of habitats into smaller and further apart sections. In an effort to prevent species extinction, conservation biologists may attempt to artificially promote balanced immigration and extinction rates. To achieve this, man-made connections between habitats, called corridors, can be built to increase the movement of species between fragmented sites and increase overall habitat area. Species may also be preserved and protected in man-made nature reserves. These corrective measures require a deep understanding of biogeography, which continues to shape species survival, location, and conservation to this day.
