Glossary of "Environmental Science: Earth as a Living Planet (2nd ed.)" by Botkin & Keller (1998)
- Used in discussions of endangered species to mean variation in the physical or biological environment, including variations in predator, prey, symbiotic, or competitor species that can threaten a species with extinction.
- Botkin, D. B. & Keller, E. A. 1998. Environmental Science: Earth as a Living Planet, 2nd ed. -xxxxii + 649 pp. John Wiley & Sons, Inc., New York.