Understanding Factors That Allow Biodiversity to Evolve and to Persist
Mark D. Hunter1 and Erin S. Lindquist2
1Institute of Ecology, University of Georgia and 2Organization for Tropical Studies, Costa Rica
Patterns of biodiversity on earth are dynamic, like an improvised play that changes every time that you watch it. The actors (species) and the sets (the environment) are constantly changing and responding to the changes that they, themselves, generate. Are there general rules that describe and predict the changing scenes in the theater of biodiversity?
Fundamentally, biodiversity depends upon rates of mutation, recombination, speciation, and extinction. Global patterns of biodiversity can be used to explore the generation and maintenance of biodiversity. Generally, biodiversity decreases from the equator to the poles, and considerable effort has gone in to explaining this pattern. As many as 120 different mechanisms have been proposed, although most attention has been focused on a small subset. Carbon fixed from the atmosphere by plants (productivity) provides the energy for life in most systems. Productivity is highest at the equator, leading many to suggest that biodiversity increases with available energy. Simultaneously, temperate ecosystems are relatively new and may suffer more frequent disturbance from climatic extremes. Consequently, the evolutionary time available for speciation may be lower in temperate regions. Because of high energy availability and fewer seasonal extremes, tropical regions may promote faster generation times. More rapid reproduction should increase realized rates of mutation and recombination, perhaps leading to greater rates of speciation.
The maintenance of biodiversity depends primarily upon rates of extinction. Extinction results from interactions among organisms (competition, predation, disease), changes in the environment, and stochastic (random) processes. Many theories of biodiversity maintenance rely on subtle differences among species or tradeoffs between competing demands (e.g. competition and colonization) although null models of coexistence suggest that such tradeoffs are not essential. Currently, human-induced environmental change is the greatest driver of species extinctions. Habitat loss and habitat fragmentation are dominant mechanisms, although effects on climate and resources are also important. Three examples are provided where predicted changes in a) rainfall patterns, b) atmospheric carbon dioxide, and c) nutrient loading are shown to change rates of extinction and biodiversity.