Unit 9: Biodiversity Decline // Section 4: Biodiversity Hotspots
Some areas of the globe are richer in species than others. As discussed in Unit 4, "Ecosystems," a latitudinal biodiversity gradient exists for animals and plants, with more species found in tropical than in temperate or polar regions. Recent work suggests that microbial communities are more diverse in temperate zones (footnote 10).
Conservationists refer to areas especially rich in biodiversity as hotspots. Based on work by British ecologist Norman Myers, who first proposed the concept, the nonprofit group Conservation International defines hotspots as regions that have at least 1,500 species of vascular plants that are endemic (found only in that area) and that have lost at least 70 percent of their original habitat. Drawing the exact borders of hotspots can be difficult, but Myers and others define a hotspot as "a separate biota or community of species that fits together as a biogeographic unit"—in other words, a community of organisms that live in a geographically unified zone and interact with each other (footnote 11).
Conservation International identifies 34 such hotspots in tropical and temperate regions around the globe (Fig. 5). There are three hotspots on U.S. territory: the California Floristic Province, a Mediterranean climate zone that covers the state's west coast and much of its central region; the Caribbean islands, including the U.S. Virgin Islands and Puerto Rico; and small patches of the Madrean-Pine Oak Woodlands, mountainous forests extending from Mexico into southern New Mexico and Arizona (footnote 12).
Figure 5. Biodiversity hotspots
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Source: © 2005. Conservation International.
Hotspots are a tool for setting conservation priorities. Because endemic species are found only in one place, protecting them requires preserving the areas in which they live. The 34 hotspots identified by Conservation International represent 2.3 percent of Earth's land surface but are home to at least 150,000 endemic plant species (50 percent of the world's total number of plant species) and nearly 12,000 terrestrial vertebrates (42 percent of the world's total number of terrestrial vertebrates).
What makes hotspots such rich biodiversity nodes? Many are located in moist tropical forests, the most diverse of Earth's major biomes. A number are islands or are physically bounded by deserts or mountain ranges, which has facilitated the evolution of endemic species by keeping populations in relative isolation and minimizing hybridization with other species. Most hotspots have widely varied topography, from lowlands to mountains, which produces a broad range of climatic conditions.
Advocates contend that hotspots should be protected because their loss could greatly accelerate what many scientists believe is an ongoing mass extinction (see Section 6, "A Sixth Mass Extinction?," below). Conversely, protecting them could save many of Earth's most threatened species. By definition, world hotspots have already lost at least 70 percent of their land area; many scientists warn that fragmenting them further will accelerate species loss because populations of endemic species will become smaller and more extinction-prone. This view is based on the theory of island biogeography, which is discussed further below in Section 7 ("Habitat Loss: Causes and Consequences").
About 10 percent of the original area of world biodiversity hotspots is currently protected as parks or reserves. Conservation International calls many of these areas "paper parks," where protection requirements are not enforced or where the covered zones have low biodiversity value (footnote 13). A 2004 study of how well the global network of protected areas preserved biodiversity found that many species were not covered by any habitat protections. "Global conservation strategies based on the recommendation that 10% (or other similar targets) of each country or biome be protected will not be effective because they are blind to the fact that biodiversity is not evenly distributed across the planet; by the same token, neither should protected areas be," wrote the authors (footnote 14).