Latitude N

figure 8.1 The number of species (species richness) of breeding birds in Central and North America declines as you go from the equator towards the North Pole (redrawn from Begon et al., 1996, who give the source of the original data).

for identifying extreme environments? We could look for patterns in the species richness of a series of environments with an increasing challenge imposed by some physical factor. Average temperatures decline as you travel from the tropics to the poles; there is also a decline in species richness (Figure 8.1). This pattern of decline in species richness with increasing latitude is seen in many groups of terrestrial and marine organisms. It is difficult, however, to attribute it solely to decreasing average temperatures. Many other things change with latitude, including the range of temperatures experienced in any one place, the extent to which conditions change with the season, the availability of water in terrestrial habitats, the length of the growing season and the

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