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Latitudinal Gradients of Species Diversity

Is species diversity really higher in the tropics than at higher latitudes? Most landscapes familiar to European explorers were disturbed by human activity. The diversity in tropical regions might have appeared high to these explorers, simply because tropical forests were less disturbed by human activity than forests in their European homeland, where the goal of forest management frequently was to eliminate "non-economic" species from the forest. To answer the question, scientists began to conduct surveys in relatively undisturbed areas along latitudinal gradients. By the 1980s, it was possible to show that there is, in fact, a gradient of increasing diversity with decreasing latitude. Figure 2.1, based upon the number of plant species encountered in 0.1-ha

Fig. 2.1. Latitudinal gradient of number of plant species in 0.1-ha plots as a function of latitude for New World plants. (Adapted from Rosenzweig 1995, with permission from Cambridge University Press)

Fig. 2.1. Latitudinal gradient of number of plant species in 0.1-ha plots as a function of latitude for New World plants. (Adapted from Rosenzweig 1995, with permission from Cambridge University Press)

0 10 20 30 40 Latitude0

Fig. 2.2. Latitudinal gradient of number or termite species. (Adapted from Rosenzweig 1995, with permission from Cambridge University Press)

plots located at various latitudes, shows that tropical forests are in fact richer in species compared with forests of higher latitudes. Despite significant scatter, the trend toward higher diversity at low latitudes is clear.

Animals as well as plants show the trend of increasing diversity with decreasing latitude. Figure 2.2 illustrates the trend for termites. Similar patterns for bats, mammalian quadrupeds, snakes, frogs, lizards, fishes and fossil For-aminifera are shown in Rosenzweig (1995). Fischer (1960) presents evidence that ants, nesting birds, and a variety of invertebrates are more diverse at low than at high latitudes. Diversity of insect herbivores in the wet tropics is extremely high. Erwin (1982) estimated that 1 ha of forest in Panama may have in excess of 41,000 species of arthropods. In tropical forests, there is a high diversity of organisms that decompose leaves and wood.

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