Threats Facing Mammalian Biodiversity

The terrestrial environment is now dominated by people—1/4 to 1/3 of the land area has been transformed for human use (Vitousek et al., 1997). Additionally, human population density tends to be higher in species-rich areas, probably because productivity shapes both (Luck, 2007). Only a few mammal species fare well in human-dominated environments; the vast majority are vulnerable to the widespread and rapid anthropogenic changes. The main direct human-induced drivers that impact biodiversity now are habitat loss and fragmentation (the most important present threat), alien invasive species, overutilization, disease, pollution, and climate change (Baillie et al., 2004). The International Union for Conservation of Nature (IUCN) assessed how these drivers are affecting mammals. Nearly all mammal species have been evaluated and, provided enough information was available, placed in one of the following extinction risk categories: least concern (LC), near-threatened (NT), vulnerable (VU), endangered (EN), critically endangered (CR), extinct in the wild (EW), and extinct (EX). The resulting IUCN Red List (International Union for Conservation of Nature, 2007) lists 74 mammal species as having gone extinct in their native range since A.D. 1500, 1,094 (22.5% of those evaluated) as being threatened (i.e., VU, EN, or CR), and only 2,652 (54.5%) as giving no cause for concern (i.e., as being LC).

270 / T. Jonathan Davies et al.

Does extinction risk show any patterns that might help us to understand the processes affecting biodiversity loss? The ''field of bullets'' scenario, in which extinction strikes completely at random, is a widely used null model for extinction (Raup et al., 1973; Van Valen, 1976). The metaphor comes from trench warfare, where soldiers' survival may have depended more on luck than skill. This scenario predicts that threatened species should constitute a stochastically constant fraction of any sample. Mammalian extinction risk is not a simple field of bullets but shows both geographic and phylogenetic patterns (Russell et al., 1998; Mace and Balmford, 2000; Baillie et al., 2004). The prevalence of risk is higher in the Old World than in the New World and higher on islands than on continents (Mace and Balmford, 2000; Baillie et al., 2004). It varies among clades too, being higher than average in primates and perissodactyls and lower than average in rodents (Baillie et al., 2004). Species with few close relatives are also more likely to be at risk (Russell et al., 1998; Purvis et al., 2000a).

These patterns reject the original field-of-bullets model, but the model lacks a geographic dimension because there may have been fields that were near to the battle but that nevertheless had no bullets. Likewise, human pressures have changed some places beyond recognition but left others almost untouched. Because closely related species often live in the same region, geographic heterogeneity in threat intensity could, by itself, cause taxonomic selectivity in extinction risk (Russell et al., 1998). Alternatively, the selectivity could arise because biological differences among clades affect species' abilities to withstand threats (McKinney, 1997). How important for species' survival is staying out of the firing line, and how important is being bulletproof?

Human population density predicts proportions of threatened mammal species among continental countries (McKinney, 2001), supporting the ''firing-line'' model. However, an analysis of extinction risk within terrestrial World Wildlife Fund (WWF) ecoregions shows that phyloge-netic nonrandomness is common within single ecoregions, where pressures may be more even than across the globe. Within each ecoregion, we generated phylogenetically independent contrasts (essentially, differences between sister clades) (Harvey and Pagel, 1991) in extinction risk (0 for LC species, 1 for species having a higher status; data-deficient and unevalu-ated species were excluded) on the phylogeny (with polytomies resolved arbitrarily and branch lengths set to unity) and compared the sum of the absolute values of standardized contrasts with the sums obtained from 1,000 randomizations of the risk data among the ecoregion's species. If high-risk species are strongly clumped in the phylogeny, the observed sum will be lower than in 95% of the simulations. Of 691 ecoregions with at least three higher-risk species and some variance in extinction risk, 386 (54%) showed significant clumping. Interestingly, the strength (rather

FIGURE 14.3 Strength and significance of clumping in extinction risk within WWF ecoregions. Scale bar below the map indicates clumping strength. A value of 1 indicates randomness, and clumping is stronger for lower values. Circle size indicates the p value [radius is proportional to -ln(p)]; circle size for P = 0.05 is shown at the lower left.

FIGURE 14.3 Strength and significance of clumping in extinction risk within WWF ecoregions. Scale bar below the map indicates clumping strength. A value of 1 indicates randomness, and clumping is stronger for lower values. Circle size indicates the p value [radius is proportional to -ln(p)]; circle size for P = 0.05 is shown at the lower left.

than the significance) of the clumping is high in most realms apart from the Nearctic (Fig. 14.3). It also appears to be stronger in ecoregions with high disparity (Spearman's p = 0.316) and with relatively old diversity (Spearman's p = 0.195). These correlations should not be taken as evidence of a functional syndrome unless confirmed at more local scales: Some of the signal probably derives from differences among, rather than within, major biogeographic realms. The prevalence of clumping of risk implies that, faced with approximately equal pressures, species differ in their ability to persist because of lineage-specific characteristics. This finding invites a search for biological correlates of extinction risk.

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