Interactions Between Environment and Competition: How Fluctuations Mediate Coexistence and Competitive Exclusion

Abstract

One of the major theoretical and empirical challenges in ecology today is elucidating the role of various kinds of heterogeneity, such as environmental fluctuations, in the dynamics of populations and the organization of communities. There is substantial evidence that stochastic environmental fluctuations have a strong role in population and community processes (Andrewartha and Birch 1954, 1984, Hutchinson 1961, Sale 1977, Connell and Sousa 1983, Sale and Douglas 1984, Grubb 1977, Wiens 1977, 1986, Murdoch 1979, Underwood and Denley 1984, Simberloff 1984, Murdoch et al 1985, Strong 1986, Victor 1986), and as a consequence, a variety of verbal theories of community structure in a stochastic environment have been developed (Hutchinson 1951, 1961, Paine and Vadas 1969, Sale 1977, Grubb 1977, Wiens 1977, Connell 1978). However, the mathematical theories of stochastic environments have not provided an adequate alternative to the classical theory based on deterministic models. There seem to be two reasons for this. First, the existing stochastic models have generally not provided the sort of simple quantitative results that often follow from deterministic models. Second, there is a widespread perception that models incorporating temporal variability may give unfathomable or inconsistent results (Hastings and Caswell 1979, Levin et al 1984). Indeed, an examination of models of communities of competitors reveals that a stochastic environment can have essentially any effect depending on the specific model involved and the assumptions that are made about it. Some models say that environmental variability promotes coexistence (Chesson and Warner 1981, Abrams 1984, Ellner 1984, Shmida and Ellner 1985), others say that environmental variability has little effect on coexistence (Turelli 1981, Chesson and Warner 1981), while others say that environmental variability promotes competitive exclusion (Chesson and Warner 1981).