Direct exploitation of wildlife species by human beings takes a variety of forms, from subsistence hunting (Brashares et al., 2004) to the harvesting of wild plants and animals for conversion into luxury goods and pets (Root et al., 2006; O’Brien et al., 2003). Large mammals and fish suffer disproportionately from direct human predation. Many of these vertebrates (e.g., apex carnivores, large ungulates, etc.) are strongly interacting species in their native ecosystems (Terborgh et al., 2001; Pringle et al., 2007; Ripple and Beschta, 2007a; Palmer et al., 2008), and overharvesting them may have destabilizing effects on biodiversity and ecological processes such as seed dispersal, nutrient cycling, and even primary production. In oceans, top piscivores suffer disproportionately as fleets fish down the food web (Pauly et al., 1998). Industrialized fisheries have often devastated community biomass of predatory fish within a few decades (Myers and Worm, 2003), with even sharper declines common among the apex predators (Baum and Myers, 2004).
Nonnative species introduced by people into naive ecosystems have occasionally wrought havoc on local biodiversity via predation, competition, and the disruption of co-evolved interactions. Biotic interchange is likely to increase with increasing mobility in an increasingly globalized world; under business as usual, biogeography will be increasingly homogeneous.
A cryptic yet critical threat to biodiversity is the loss of future evolutionary potential. Extinction of genetically distinct populations, decreases in effective population sizes, and homogenization of habitat types are all likely to have negative effects on future biodiversity (Myers and Knoll, 2001; Woodruff, 2001). The positive relationship between speciation rate and habitat area (Losos and Schluter, 2000) indicates that decreases in species geographic ranges will diminish future speciation rates, which in turn will impoverish future diversity (Rosenzweig, 2001). Speciation of large vertebrates, which are highly mobile and require large habitats, may cease entirely (Woodruff, 2001), and biodisparity—the range of morphological and physiological variety on Earth—will decrease as phylogenetically distinct, species-poor branches are pruned from the tree of life (Jablonski, 1995).
Loss of microevolutionary potential will also limit the capacity of populations to adapt to changing environmental conditions, highlighting another important point: The drivers of biodiversity loss will often act synergistically in imperiling populations and species. Habitat loss and fragmentation compound the effects of climate change, as species are unable to track their thermal niches spatially (Travis, 2003). The interactions among logging, fire, and climate change threaten to transform the Amazon rainforest into savanna (Oyama and Nobre, 2003; Terborgh, 2007). Such positive feedbacks seem to be a rule, rather than an excep-