Evolution of the equilibrium concept: Seeking balance in nature
From the balance of Aristotle’s “Golden Mean” to the harmonious precision of Linnaeus’ “Oeconomia naturae,” observers of nature have long noted the beneficial stability that apparently results from the complementary interactions of opposing factors. This multi-component stability seemed to be confirmed by early research in limnology, as well as work on plant succession in grasslands and forests. The concept of the “climax community” as the final stage of succession has suffered from the confusing complexities of plant succession, but survives as the core generalization underlying the concept of “biomes.” With the development of intuitively attractive mathematical models of the interactions of multiple species by Lotka and Volterra, the combinations of properties that allowed this “balance” to occur could be dissected and quantified. The cycles of predators and prey populations that Lotka developed showed a balanced alternation of population abundances that could be either stable or unstable. In the case of competition models, balanced coexistence of multiple species required a specific combination of relative properties, without which the extinction of all but one of the species was inevitable.
The precise balance of competitive abilities required to allow species coexistence, and thus explain species diversity, is typically calculated by solving for parameter values under conditions of equilibrium, when the growth rates of all populations are zero. Coexistence under equilibrium (i.e., stable, unchanging) conditions is considered the primary requirement for the long-term maintenance of species diversity. Recent critiques of well-known models that predict responses of species diversity to variation in environmental conditions have asserted that the models lack the mechanisms necessary to allow long-term coexistence under equilibrium conditions, and thus cannot be valid. However, with some exceptions, these models have been remarkably successful at predicting variation in species diversity under natural conditions and in experimental studies. Resolution of the continuing controversies about explanations for observed patterns of species diversity will require more attention to the interaction of rates of competitive processes with the longevity of organisms, as well as the spatial dynamics and multi-scale averaging of meta-communities and the meta-populations within them. Ultimately, we must determine the relative importance of the internal dynamics of density-dependent population interactions that drive equilibrium patterns of species diversity, in relation to the external density-independent forcings of environmental variability. These external forcings can potentially destabilize either equilibrium coexistence or equilibrium competitive exclusion, and potentially stabilize the chaotic population fluctuations that can occur under equilibrium conditions.