16.4: Where is most biodiversity?

If you were to stand at the equator in South America and identify the number of land mammals, you would count about 400 different species. If you then started walking southward, away from the equator, and assessed the diversity of land mammals when you got to 20° latitude, you would find about 350 different species. Continuing your long walk south, when you got to 30° latitude you would find only 200 different species. And the trend would continue for as far south as you could go, with 100 species at 40° latitude and fewer than 50 species at 50° latitude. On a similar walk northward from the equator, through North America, you would observe the same trend, with more than 300 species of land mammals at 20° latitude, reduced to only 100 species in northern Canada (FIGURE 16-6). Biodiversity is not evenly distributed throughout the world. And perhaps the strongest trend in terrestrial biodiversity distribution is this: as you move away from the equator in either direction, diversity is reduced with increasing latitude.

The strong biodiversity gradient from the tropics to the Poles occurs not just in land mammals but in nearly any group of plants or animals observed—recent surveys have documented the trend in woody plants, herbaceous plants, and mangrove trees. Interestingly, even the diversity of organisms in the oceans follows this trend. One survey of marine copepods—tiny crustacean animals, including plankton—found the same trend moving northward from the tropical region of the Pacific Ocean (80 species present) to the Arctic Ocean (fewer than 10 species). Similar patterns have been documented in fishes, mollusks, marine and terrestrial arthropods, corals, and marine protists.

Numerous factors that influence species richness— that is, the number of species in an area—are responsible for producing the tropical-to-temperate-to-polar gradient in biodiversity. Three, in particular, play strong roles (FIGURE 16-7).

1. Solar energy available. Perhaps the simplest predictor of species diversity is the amount of solar energy available in an area. Solar energy is, after all, the ultimate fuel for nearly all life. As we saw in Figure 15-5, the sun shines most directly on the equator, and, as we move away from the equator, the earth’s curvature causes the solar energy hitting the earth to be spread out over increasingly large areas. This leads to less solar energy at any one point as latitude increases. In a variety of species of plants and animals, researchers have documented strong relationships between energy availability and species richness.

2. Evolutionary history of an area. Communities diversify over time. Consequently, the more time that passes, generally speaking, the greater is the diversity in an area. A high level of biodiversity in an area, however, can be knocked back down by climatic disasters such as glaciations. Thus, for example, biodiversity declines due to glaciations may have disproportionately occurred in temperate and polar regions (and on the tops of some mountains), which have been affected more frequently than tropical regions as glaciers have advanced and retreated over hundreds of millions of years.

3. Rate of environmental disturbance. Over time, the best competitor for a resource is expected to outcompete other species, excluding them from the community and reducing species richness. However, communities may be kept from reaching this species-depleted state. Factors such as predation, or environmental disturbances such as floods, fires, and volcanic eruptions that may wipe out most species in an area, can prevent any one species from excluding others. For this reason, a habitat with an intermediate amount of environmental disturbance—that is, not so high that many or all species are wiped out, and not so low that organisms with a competitive advantage drive all of the less competitive species from the community—is expected to have the greatest species richness.

Conservation biologists are increasingly interested in biodiversity hotspots, those regions of the world having significant reservoirs of biodiversity that are under threat of destruction. Twenty-five biodiversity hotspots have been identified around the globe (FIGURE 16-8). These hotspots cover less than 1% of the world’s area but have 20% or more of the world’s species. Habitats included among the biodiversity hotspots are tropical rain forests, coral reefs, and islands. While not considered a hotspot, the deepest regions of the oceans have recently proved to hold great biodiversity.

Tropical Rain Forests These habitats are common near the equator in Asia, Australia, Africa, South America, and Central America, as well as on many Pacific islands. An important, but still poorly studied, region of tropical rain forests is the canopy, the uppermost region of a forest, where nearly all of the sunlight is intercepted. The canopy, which can be up to 30 feet (almost 10 m) thick, is the most diverse part of the tropical rain forests; estimates suggest that as many as half of all species on earth reside in the canopy of these rain forests. Due to its inaccessibility—the canopy is 65–135 feet (20–40 m) above the ground—the canopy is not easily studied, and the extent of biodiversity there is still not fully known.

Coral Reefs Built by corals, members of the animal kingdom, coral reefs occur in ocean areas with low levels of nutrients. The physical structures created by the corals provide a home and habitat for huge numbers of other species. The Great Barrier Reef of Australia (which is not listed among the 25 biodiversity hotspots because it currently is a focus of extensive conservation efforts) is an example of just how much diversity can be supported by coral reefs. It is the largest coral reef system in the world and covers such a large area (almost the size of California) off the northeast coast of Australia that it can be seen from space. It is home to numerous species of whales, dolphins, sea turtles, sharks, and stingrays, as well as 1,500 species of fishes and 5,000 species of mollusks. Five hundred species of algae are found there, too. Above the Great Barrier Reef, more than 200 species of birds are supported by the rich diversity of life below. Other coral reefs are not as well protected by laws and conservation efforts, and the world’s second largest coral reef, in New Caledonia in the southwestern Pacific, is one of the 25 biodiversity hotspots.

Islands Because of their generally smaller size, which limits the population size of organisms living on them, islands can be regions where biodiversity is at risk. Madagascar, located in the Indian Ocean off the southeastern coast of Africa, is the fourth largest island in the world (just a bit smaller than Texas). It is home to 5% of the world’s species, 80% of which are endemic (exclusively native to a place)—a higher percentage of endemic plants and animals than in any comparably sized area on earth. One of the 25 biodiversity hotspots, Madagascar is an area unusually rich in species diversity.

Deep Oceans The oceans, covering two-thirds of the earth’s surface, can be much more difficult to explore than terrestrial habitats. Deep ocean regions, in particular, are much less studied than the terrestrial parts of the world. Once thought to be completely devoid of life, the deepest regions of the oceans have recently been found to be teeming with biodiversity. One survey of the deepest parts of the Antarctic Ocean, sampling at a depth of more than 18,000 feet (6,000 m), discovered 585 new species of crustaceans alone.