(Bininda-Emonds et al., 2007). [Many node ages in Bininda-Emonds et al. (2007) were slightly affected by a software bug; all our analyses use corrected dates (Bininda-Emonds et al., 2008).] As always with a model system, advantages come at a price. Mammals are atypical (e.g., they are much larger than most other species), so results from them cannot necessarily be extrapolated more broadly. However, mammals are a charismatic group of special interest to many people, so results have value even if they cannot be generalized. We start with a snapshot of present mammalian diversity and the (overwhelmingly anthropogenic) pressures that species face, before going on to describe recent and ongoing attempts to understand the present and possible future consequences of those pressures.
Mammalian species are distributed very unevenly among genera, families, and orders (Purvis and Hector, 2000; Wilson and Reeder, 2005). Differences in age among taxa of a given rank (Avise, Chapter 15, this volume) confound evolutionary interpretation of the pattern, but the phylogeny permits a test of whether the chances of diversification have indeed varied among lineages. Under the equal-rates Markov model (ERM), in which chances are equal, phylogenies should have a weighted mean I [the degree to which species are partitioned unequally between sister clades (Purvis et al., 2002)] not significantly >0.5. The estimate of phylogeny (Bininda-Emonds et al., 2007) has a weighted mean I of 0.657 (SE = ±0.0131), well above 0.5 (weighted t test vs. 0.5: t848 = 11.98, p << 0.001), indicating that lineages have had different propensities to diversify. Such inequality is common throughout the Tree of Life (Mooers and Heard, 1997; Purvis, 1996) and prompts the search for traits that might be responsible. Phylogenetic analyses reveal that large litter size and high abundance are both linked with high richness in sister-clade comparisons pooled across four orders (primates, carnivores, marsupials, and bats), whereas small body size and short gestation period also predict high richness within carnivores (Isaac et al., 2005). In common with most comparable studies on other taxa (Coyne and Orr, 2004), however, the biological traits leave most of the variance in richness unexplained, suggesting a possible role for the environment.
The geographic distribution of mammalian species is also very uneven (Fig. 14.1). Mammals follow global trends for higher tropical diversity, with a strong latitudinal diversity gradient (Fig. 14.1a). Within the tropics, richness seems to correlate with productivity and water–energy dynamics, peaking in Amazonia at the base of the Andes, in the Great Rift Valley in Africa (where richness exceeds 250 species per 10,000 km2), and in an arc running from the Himalayas into southeastern Asia (Fig. 14.1b). These