peaks suggest that topographical heterogeneity has also shaped species-richness patterns (Kerr and Packer, 1997). At higher latitudes, the relationship between richness and productivity is much weaker, and ambient energy is a better predictor of richness (Kerr and Packer, 1997). The geographic pattern of mammalian richness is highly congruent with those of birds and amphibians (Grenyer et al., 2006), which are also well predicted by the same environmental variables (Hawkins et al., 2003; Buckley and Jetz, 2007). All these groups have many narrow-range species on the Neotropical mainland, but other “hotspots of endemicity” differ—continental shelf island systems for mammals, oceanic archipelagoes for birds, and mainland locations for amphibians (Grenyer et al., 2006). The latitudinal gradient in median geographic range size in mammals (Fig. 14.1c) correlates strongly with the land area available within the same latitudinal bands (r = 0.68, although spatial autocorrelation complicates significance testing). However, median range size remains high in the northernmost bands despite the rapid decline in land area toward the Pole. Bird range sizes show similar patterns (Orme et al., 2006).
The tropics have been described as an evolutionary powerhouse, acting as a “cradle” for diversity (Stebbins, 1974). Might Fig. 14.1, therefore, reflect longstanding geographic differences in diversification rates? Cradles of diversity could be of particular interest if conservation actions are to be targeted toward conserving the processes that generate diversity, a point to which we return below. The geographic pattern of recent diversification can be examined in terms of taxonomy or phylogeny, although both have their problems. Boundaries to higher taxa can be arbitrary (Avise,