habitat loss (Fisher and Owens, 2004). A small geographic range size may reflect narrow tolerances and increase the risk that the whole of the species’ range is in the firing line. Which of these features matter most for extinction risk, and are any associations consistent across all mammals?
Cardillo et al. (2005) carried out the most comprehensive investigation to date. Threatened species were included only if they were on the IUCN Red List because of observed decline, to avoid autocorrelation with predictor variables. Red List status, on a 0–5 scale, was used as the response variable. Many facets of geography (including human population density), ecology, and life history were tested as predictors of extinction risk, by using phylogenetically independent contrasts. A phylogenetic approach is needed because, although extinction risk and some of the possible predictors listed above (e.g., geographic range size) do not evolve along the phylogeny’s branches like, say, body size does, they nonetheless tend to show phylogenetic signal [i.e., they tend to take more similar values in close relatives than in species chosen at random; (McKinney, 1997; Purvis et al., 2000b; Fisher and Owens, 2004)]. Minimum adequate models were derived from a large initial set of predictors. This approach helps exclude variables that correlate only indirectly with extinction risk, for example, because another variable shapes both them and risk.
The predictors of risk were significantly different for smaller and larger species, with the importance of many predictors changing markedly at a body size of ≈3 kg. Species smaller than this fit the firing-line model: They are more likely to be threatened if they have small geographic ranges, live in temperate areas, face high human population densities, and live where a high proportion of the other mammal species are also threatened. Larger species, however, face multiple jeopardy: Biology matters as well as geography, with high-abundance, small neonates, and many litters per year all independently helping to bullet-proof species. High abundance is predicted to bullet-proof species if the field-of-bullets model operates at the level of individuals rather than species (Erwin, 2006a), but such a model also predicts that no other biological traits would independently predict risk.
For both large and small mammals, the most important single risk factor is small geographic range size. The firing-line model predicts that small-ranged species will be most at risk because a single localized threat can impact their entire distribution. However, range size itself varies systematically among clades [although it shows weaker phylogenetic signal than, e.g., body size (Gaston, 2003; KE Jones et al., 2005)], suggesting that it is shaped, at least in part, by organismal traits such as dispersal ability (Böhning-Gaese et al., 2006) or niche breadth as well as by circumstances of geography. For example, small-ranged species are more common at low latitudes and within climatically stable regions. Any traits, including