Concepts of equilibrium (balance of nature) and nonequilibrium

The concepts of equilibrium/nonequilibrium have been used differently by different authors, as will be shown in the following selected examples.

Pianka (1974), in the second edition of his widely used Evolutionary Ecology, makes the case succinctly for equilibrium in ecological systems (modified somewhat in later editions). The main points listed by him are that:

1. ecological systems and their components have been shown to be in dynamic equilibrium near steady states in many studies;

2. in communities, production and respiration “must ultimately balance”;

3. even nonclimax communities, which have not reached a steady state, are probably “in some kind of equilibrium,” determined by the frequency of disturbances and destruction of other successional stages and the rate of successional change;

4. in most communities, rates of energy influx and outflow in each trophic level balance out exactly;

5. on islands, immigration and extinction of species are balanced;

6. in populations, over long periods, birth rates equal death rates; and

7. prey–predator and similar pairs must be “in some sort of ecological and evolutionary balance to coexist with one another over any period of time.”

The assumption that competition plays a central role in ecology is implicit in Pianka's discussion. This notion has since been explicated by Chesson and Case (1986), who define the assumptions of “classical competition theory” as follows:

1. life history characteristics of species are adequately summarized by the per capita growth rate of species;

2. deterministic equations are sufficient to model population growth, and environmental fluctuations need not be considered;

3. the environment is spatially homogeneous and migration is unimportant;

4. […]