Balancing the Books on Biodiversity

Humanity has induced pervasive negative impacts on the world's biodiversity (Smith et al. 1993; Morris 1995). Earth is accumulating an ecological deficit that, when the accounting is complete, will be written off by massive global extinctions (Tilman et al. 1994). But the evolutionary ledger has another side. Adversity for one group of species often represents opportunities for others. Despite repeated mass extinctions, the history of life reveals an impressive and consistent increase in biodiversity (Rosenzweig 1995). Each mass extinction is associated with adaptive radiation of the survivors. We believe that human activities have placed us in the midst of one such major reorganization of life. We have, accordingly, searched a variety of sources in an attempt to identify the kinds and numbers of species flourishing under the current onslaught of human impacts. Table 1 summarizes examples of taxa capitalizing on human-induced ecological opportunity and reveals several important lessons regarding the future of Earth's biodiversity: (1) human activities have provided new habitats and food sources to numerous species; (2) many of these species appear to be adapting to humanaltered conditions; (3) adaptation to human-altered environments can occur rapidly; (4) most of the species are pests or pathogens of humans or their domesticated prey and mutualists. Our list of taxa capitalizing on anthropogenic change represents a minimum estimate gleaned from a subset of taxa and geographical locales. We excluded introduced species so we could concentrate on those taxa that we know are exploiting anthropogenic environments without direct human assistance. Our errors of omission are