Extinction and biodiversity loss
Population declines lead to species declines, local extirpation, and eventually to global extinction of a species. Extinction is a natural process, and natural processes can be characterized by average rates. These rates have accelerated dramatically during past periods of major environmental change. But never in the history of the earth, as we are able to read that history, have global environmental changes resulted from the actions of a single species, nor have extinction rates approached the pace established in the last decades of the 20th century.
Historically, extinction rates for animals average 9% of existing species every million years, or one to two species per year. During the celebrated Permo-Triassic and Cretaceous–Tertiary mass extinctions that marked the ends of the Paleozoic and Mesozoic eras, respectively, extinction rates accelerated to perhaps 50–75% of the marine fauna over a period of 10,000 to 100,000 years (Raup 1988; Jablonski 1991). In stark contrast, extinction rates at the close of the 20th century have been estimated at upwards of 300 species per day or 100,000 per year, about 1000 to 10,000 times background levels and 10 to 100 times greater than the major extinction catastrophes of the past (Wilson 1988; Mann 1991). While the accuracy of such estimates is difficult to verify, there is little argument that extinction rates today exceed any in the past, recent millenia. This astounding loss of biodiversity, defined as the variety of life forms and processes, can be directly linked to the activities of an overgrown and overconsumptive human population (Groom et al. 2006).