Figure 22.21 Patterns in North American diversity. When a researcher examined the number of species living in the United States and Canada, he found strong patterns in species richness. The contour lines on the map indicate the numbers of species found in blocks of land that were 241 km on a side. (a) Mammals increase in species richness from north to south and from east to west. (b) Reptiles increase in species richness from north to south. (c, d) For both trees and amphibians, the highest number of species occurs in the southeast where there is a combination of high precipitation and warm temperatures.

Data from D. J. Currie, Energy and large-scale spatial patterns of animal- and plant-species richness, American Naturalist 137 (1991): 27–49.