Human activities already threaten the globe's physical and biological systems. Worldwide, species extinction rates are estimated to be one thousand times what they would be in the absence of human activity (Wilson 1988). Raven (1988) estimates that 25% of the world's plant and animal species existing in 1985 may be extinct by 2015, with most extinctions occurring in tropical regions. While these extinction rates are staggering, global warming would greatly accelerate extinction rates that some scientists believe may already exceed those accompanying the decline of the dinosaurs 65 million years ago (Wolf 1987).
Lilieholm, R. J., "Preserves at Risk: An Investigation of Resource Management Strategies, Implications and Opportunities" (1993). Elusive Documents. Paper 106.