Fluctuation-Independent Mechanisms

Fluctuation-independent mechanisms are those that function independently of the presence of fluctuations over time in population densities and environmental factors. They may be affected by the presence of fluctuations, but do not require them for their operation, and in general still function in the presence of fluctuations. A classic example is resource partitioning (Chesson 1991). When the population dynamics with resource partitioning are modeled according the Lotka-Volterra competition model, then presence of fluctuations over time in terms of a disturbance to the population per capita growth rate, has no effect what so ever on species coexistence (Chesson 1994). Naturally, fluctuation-independent mechanisms are to be contrasted with fluctuation-dependent mechanisms.  This distinction was first presented in Chesson (1994), and discussed in more detail in Chesson (2000).  Note that both fluctuation-dependent and fluctuation-independent mechanisms may well operate together in a given ecological system (Chesson 1994, 2000).  The related concepts to do with spatial variation are termed variation-dependent, and variation-independent mechanisms.  The old distinction between equilibrium and nonequilibrium, as discussed in Chesson and Case (1986), has become the modern distinction between fluctuation-dependent and fluctuation-independent mechanisms.  The reason for the change in terminology is discussed in Chesson (2000): equilibrium means different things to different people. In Chesson and Case (1986), the equilibrium was a point equilibrium, but some people point out that limit cycles and stationary probability distributions are in a certain sense equilibria too. So the terms were changed to contain common words with less labile meaning. 

References

Chesson, P. 1990.  MacArthur's consumer-resource model.  Theoret. Pop. Biol., 37, 26-38.

Chesson, P.L., Case, T.J. 1986.  Nonequilibrium community theories: chance, variability, history, and coexistence. Pp 229-239 in J. Diamond and T. Case, eds, "Community Ecology," Harper and Row

Chesson, P. 2000. Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics 31, 343-66.