Biodiversity is an ecological notion that refers to the richness of biological types at a range of hierarchical levels, including: (1) genetic diversity within species, (2) the richness of species within communities, and (3) the richness of communities on landscapes. In the context of environmental studies, however, biodiversity usually refers to the richness of species in some geographic area, and how that richness may be endangered by human activities, especially through local or global extinction.

Extinction represents an irrevocable and highly regrettable loss of a portion of the biodiversity of Earth. Extinction can be a natural process, caused by: 1) random catastrophic events; 2) biological interactions such as competition, disease, and predation; 3) chronic stresses; or 4) frequent disturbance. However, with the recent ascendance of human activities as a dominant force behind environmental changes, there has been a dramatic increase in rates of extinction at local, regional, and even global levels.

The recent wave of...