During Earth's most recent 3.5 billion junkets around the Sun, its inhabitants have busied themselves adapting to myriad physical surroundings and finding niches within niches to carry out their life functions. In doing so, they established a bewildering number of species, each depending on thousands of fellow species for survival. Five times over the past 440 million years, the number of species crashed in mass extinctions initiated by exogenous shocks to their environments from meteorites, ice ages, and volcanic eruptions. After each shock, the number of species rebounded, and after the most recent shock 66 million years ago, the number rebounded to the 10 million or more species currently inhabiting Earth.
Today, these inhabitants are again experiencing a mass extinction, although many argue this event is not due to an exogenous shock, but to the endogenous activities of a single species. There is evidence that species are disappearing worldwide at rates 10 to 1,000 times greater than natural rates of extinction (Jablonski 1991; May, Lawton, and Stork 1995; National Research Council 1995; Pimm, Russell, and Gittleman 1995). A casual look at data in the contiguous United States reveals a telling correlation between human populations and threatened and endangered species. Table 1.1 displays a state-by-state tally of species on the endangered species list (ESA) along with the percentage change in state population densities from 1970 to 1997, which encompasses the period the ESA has been in existence.