Geographic Distribution of Biodiversity

Among the many trends known about the geographic distributional patterns of biodiversity, the most evident is that of the increase in species diversity with latitude. This trend is consistent across many groups of organisms that have been well analyzed. Among insects this pattern is spectacular. For example, the number of species of ants in local regions increases from about 10 at latitude of 60° N, to some 2000 at latitudes near the equator. Figure 9 highlights the case of plants and a representative group of vertebrates, birds. In both cases, the upper bound limit of species richness shows a dramatic peak at lower latitudes. In the case of plants, different analyses (e.g., local or regional floras, species density comparisons, standardized a

No. of species 0 600 1400

No. of species

No. of species 0 600 1400

Figure 9 Gradients of latitudinal variation in species richness for birds (left) and plants (right). For both groups, species concentrates toward the Tropics peaking around the equator. The data for plants correspond to the number of species per 0.1 ha in different localities of the Western Hemisphere. The number of bird species is standardized per unit of sampling effort in different localities of the world.

Figure 9 Gradients of latitudinal variation in species richness for birds (left) and plants (right). For both groups, species concentrates toward the Tropics peaking around the equator. The data for plants correspond to the number of species per 0.1 ha in different localities of the Western Hemisphere. The number of bird species is standardized per unit of sampling effort in different localities of the world.

diversity samples, etc.) show a very consistent gradient of decrease in plant species diversity with latitude. In broad geographic terms, species densities range from over 5000 species/10 000 km2 in tropical regions, to less than 100 in the highest latitudes. In terms of local (a) species diversity, values range from an average of 270 species per 0.1 ha in Colombia to c. 15 near the USA-Canada border. Breeding birds increase from about 56 species in Greenland, to 105 in New York, to 1010 in Mexico, to 1395 in Colombia.

Another increasingly evident pattern of geographic variation in diversity has to do with intercontinental variation, specifically the patterns comparing the Neotropics (i.e., the tropics of the New World) with the Paleotropics. Data indicate that about 90 000 species of plants, approximately twice as many as in Africa, south of the Sahara, occur in the Neotropics and that the comparable area of Asia is roughly intermediate in this respect. Fogging sampling techniques using standardized protocols to compare data for canopy beetles (e.g., in terms of species per cubic meter) show the same tendency, although the values are even more contrasting than in the case of plants: 1.17 in Panama and 1.15 in Peru > 0.29 in New Guinea > 0.02 in Australia and Sulawesi. Similar trends have been observed in numerous other groups, including butterflies (Neotropics > Southeast Asia > Africa), frogs (Neotropics > Africa/Asia > Papua/ Australia), and birds (Neotropics > Africa > Asia/ Pacific > Australopapuan). In mammals, the number depends on the particular group; specious groups such as bats are considerably more diverse in the Neotropics than in the Old World, while some groups with relatively few species in general, such as primates, show the opposite trend: Old World > New World.

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