The Geography of Biodiversity

Geography is critical to understanding biodiversity. First, biodiversity is unevenly distributed over the surface of the Earth. In general, species diversity increases toward the equator, with the world's most species rich habitats occurring in the belt of tropical moist forests that falls between the Tropics of Capricorn and Cancer. Tropical moist forests are the most species rich terrestrial habitats, covering only 7% of the Earth's surface, but estimated to contain 50–90% of all species. Coral reefs, occurring only between 30° north and south of the equator, comprise about 0.25% of the area of the world's marine environment, but contain 25% of its total number of species. Furthermore, the geographic distribution of many species, particularly in the tropics, is extremely limited. Such species, often referred to as endemic or restricted range species, are found in a relatively small area and nowhere else on earth.

Second, our knowledge of biodiversity is greatly influenced by its geographic distribution in the biosphere. In general, the more inhospitable and inaccessible to humans a habitat is, the less we know about its biodiversity. For example, while oceans and seas by far make up the greatest area of the Earth's surface, they are also the least understood ecosystems and the biodiversity they contain is poorly documented. Indicative of this lack of knowledge is the fact that the most widely used maps of global biodiversity typically include only terrestrial ecosystems. Some marine habitats, such as those surrounding the hydrothermal vents in areas of seafloor spreading, are accessible only with the most advanced technology, yet they support a diverse community of species. Unique life forms, such as bacteria that use hydrogen sulfide as an energy source, have only recently been discovered near seafloor vents. Similar problems of accessibility exist in the terrestrial portion of the biosphere. While tropical moist forests are species rich, the most extensive are relatively inaccessible and difficult to traverse. Hence, scientists and conservation biologists have assessed only a small fraction of the tropical forests where most of the world's biodiversity is estimated to exist. In addition, most of the world's moist tropical forests are found in developing countries, where pervasive poverty, lack of infrastructure, and political and social instability may discourage sys tematic biodiversity assessments.

Third, certain physical geographic features, such as islands, mountaintops, and coral reefs, play key roles in global biodiversity. Islands are important to global biodiversity because many have been genetically isolated for millions of years, allowing for speciation and genetic variation to occur separately from that on continents. Many species are thus endemic to a single island or archipelago. In addition, the genetic isolation of islands creates evolutionary dynamics that produce unusual phenomenon such as dwarfism and flightlessness. Mountaintops are similarly, though not identically, isolated geographic features. The steep terrain, altitudinal climate zones, and fragmented landscapes of mountain regions create conditions for high rates of species endemism. Limestone ridges formed by living corals in shallow tropical seas create the conditions for concentrations of marine biodiversity. A wide variety of organisms, including, oysters, clams, crabs, sponges, turtles, and fish find shelter and food in the underwater architecture of the reefs.

Finally, biodiversity loss is strongly influenced by geography. For example, the three geographic features described above together account for a disproportionate amount of recent species extinctions and the world's most threatened species. The great majority of extinctions since AD 1500 have occurred on oceanic islands, where many species, having evolved in isolation, are relatively vulnerable to habitat disturbance and predation. However, this pattern has begun to change with the number of recent extinctions on continents equaling those of islands. Most of the species of birds, mammals, and amphibians currently threatened with extinction are located in the tropical latitudes. Tropical islands particularly stand out as having a disproportionate amount of the world's threatened species. Tropical moist forests also stand out in global assessments of threatened species, because they contain high concentrations of biodiversity, but are rapidly being converted to intensive agriculture, such as large scale corporate plantations. There is also a great deal of geographical variation in the extinctions and threats of extinctions among different taxonomic groups. For example, global mollusk extinctions are concentrated in North American rivers, whereas Australia has the largest concentration of mammal extinctions.

Given that both the levels of biodiversity and numbers of species threatened with extinction are often geographically concentrated, some conservation biologists base protection strategies on biodiversity hotspots. Norman Myers coined the term 'hotspots', in 1988, to identify areas in the biosphere that have high levels of species diversity, endemism, and habitat loss. This exercise was driven by practical conservationist concerns, the idea being to identify areas where biodiversity protection is most urgent and where the payoff, measured as numbers of species protected, would be greatest. Since the term was introduced, conservationists have identified 34 hotspots around the world in both terrestrial and marine environments. Biodiversity hotspots have become a major focus for conservation programs and funding, with Conservation International building its program agenda around the concept and the World Bank and MacArthur Foundation providing hundreds of millions of dollars of financing. Subsequent research, however, suggests that focusing on hotspots identified under this system may not be an appropriate strategy. For example, in the case of the world's terrestrial mammals, little geographic overlap was found between species richness on the one hand, and endemism and threats of extinction on the other. Furthermore, it is becoming increasingly clear that global climate change is causing shifts in the geographic distribution of species across many taxa, decreasing the effectiveness of the hotspots approach to biodiversity loss.