Why the Tropics are Biodiversity Hotbeds
Accumulating research is gradually fitting together answers to the question of why, for millions of years, the tropics have housed so many more species than have temperate and polar regions. Since the dinosaurs were around, a gradient of increasing diversity of plants and animals, both terrestrial and aquatic, has stretched from the earth's poles to its equator.
While not all pieces of the biodiversity gradient puzzle have yet been discovered, a synthesis of the published literature finds consistent evidence for some hypotheses.
One line of thinking, that's become reasonably well substantiated, maintains that tropical habitats are biologically rich because they've had a longer time to accumulate species. Regions near the equator have not been repeatedly inundated by glaciers burying all forms of life, as have places like Canada, northern United States and northern Europe. One of several studies that supports this hypothesis shows that the earlier that frogs from the South American Hylidae family moved into an area, the more hylid species the area now holds.
Not only are the tropics a "museum" of species, they are also where new species are most likely to arise. The rate at which new types of organisms form increases as one moves closer to the equator. Support for this principle comes from both fossil evidence and from the extent of genetic diversity found within living species. Vertebrate animals in equatorial regions, for instance, have on average more subspecies than do those closer to the poles.
Why species originate more readily near the equator is far from being entirely clear. Notions about the roles of genetic drift, climatic change and dispersal have yet to be convincingly backed up by research findings. New ideas are still being tested with advances in scientific technology. One intriguing strand of enquiry suggests that higher temperatures may speed evolution. At a molecular level, mutations happen more frequently under warmer conditions.
Among the key puzzle pieces not yet adequately uncovered is whether the rate of species extinctions also differs between the poles and equator. Despite this gap in understanding, it is generally accepted that the tropics do have a higher rate of net species diversification, resulting from the difference between the formation and the loss of species.
Over time, species radiated from their tropical origins, so many temperate groups have tropical ancestors. But not all tropical lineages have expanded into temperate regions. For example, only half the families of tropical flowering plants contain representatives in cooler landscapes.
Altogether, the two driving factors of long accumulation coupled with rapid diversification means that the tropics sustain greater numbers of both young and old taxa than do temperate and polar regions.
Gary G. Mittelbach, Douglas W. Schemske, Howard V. Cornell, Andrew P. Allen, Jonathan M. Brown, Mark B. Bush, Susan P. Harrison, Allen H. Hurlbert, Nancy Knowlton, Harilaos A. Lessios, Christy M. McCain, Amy R. McCune, Lucinda A. McDade, Mark A. McPeek, Thomas J. Near, Trevor D. Price, Robert E. Ricklefs, Kaustuv Roy, Dov F. Sax, Dolph Schluter, James M. Sobel and Michael Turelli. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecology Letters. 10(4): 315-331.