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On a half-hour walk through a deciduous forest—for example, in the northeastern United States or southeastern Canada—we can expect to see maybe a dozen different kinds of tree: two or three kinds of oak perhaps, a couple of maple species, birch, beech, hornbeam, ironwood, one or two pine species, and possibly a sycamore, poplar, or spruce, depending on just where we are. Standing in front of a tree and turning around, we can almost always see several more trees of the same kind. In contrast, on a similar walk through a forest in the Amazon or in Borneo, we would pass 300 or 400 different tree species, with almost no near neighbors of the same kind. The great Victorian naturalist Alfred Russel Wallace took note of the low population density of trees in tropical forests and observed, “If the traveller … notices a particular species and wishes to find more like it, he may often turn his eyes in vain in every direction.”

This intuitive feel for biodiversity is reflected in the global distribution of plant species richness shown in Fig. 48.21. Tropical rain forests harbor the highest number of plant species, and in general the amount of plant diversity tails off toward the poles. Deserts have particularly low plant diversity, a pattern linked to their low primary productivity. Perhaps more surprising, Mediterranean climates—temperate regions with dry summers and wet winters—can support large numbers of plant species, for example along the southern tip of Africa, southwestern Australia, and Mediterranean Europe. These regions show that additional factors only indirectly linked to climate influence regional plant diversity, including habitat diversity (in Mediterranean regions, a mixture of woodland, savannahs, and grassland) and frequent fires (disturbance, discussed in Chapter 47).

It isn’t just trees. Mammals, birds, insects, and many other kinds of animal show similar patterns of species diversity. Fig. 48.22, for example, shows the global distribution of mammal species. To get a sense of animal diversity in tropical rain forests, consider that one survey found more than 700 species of leaf beetles in a single small set of trees in the Amazon, a number equal to the total diversity of leaf beetles in all of eastern North America. A single tree in this same forest harbored 43 species of ants, exceeding the total for all of Great Britain.