Biology Reference
In-Depth Information
1 . Concepts and problems
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;
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