Latitudinal Gradient of Species Diversity

Latitudinal Gradient of Species Diversity

The latitudinal gradient in species diversity is one of the most striking patterns in the distribution of organisms on the planet. Simply put, the average number of species per unit area increases dramatically the closer the area is to the equator, almost entirely regardless of the type of organism being considered (Pianka, 1994). Researchers investigating the gradient have formulated a wide variety of hypothesis explaining the higher level of species diversity in the tropics. These include but are not limited to: a greater degree of evolution and radiation in tropical species due to the long and relatively stable geological history of the area, seasonal climatic stability and/or predictability, a higher level of productivity, an increased rate of competition and a higher predation intensity (Pianka, 1994). Another theory is that tropical soils somehow influence species diversity and thus cause the latitudinal gradient. This paper will further investigate this final theory by outlining the basic characteristics of tropical soils, summarizing the mechanisms invoked to explain species diversity with these soil characteristics, and evaluating how well this research agrees with what is known about tropical soils. For the sake of narrowing the topic somewhat, attention is limited to the soils of and research occurring in Latin America.

Tropical Soils

In the past, tropical soils have been over-simplified and misunderstood (Sanchez, 1976), and this situation plagued soil science at least until the late seventies (Van Wambeke and Dudal, 1978). Sanchez (1976) attributes this misunderstanding of tropical soils to the fact that when temperate region-trained soil scientists first went to the...

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