Classifying Species According to Their Demographic Strategy. I. Population Fluctuations and Environmental Heterogeneity

We model the effects of environmental fluctuation on population growth as acting through an interaction between microhabitat heterogeneity and fluctuations in a regional environmental variable. The results are richer in biologically important detail than are the results of introducing a simple noise term to modify a variable in a model, such as the logistic. In particular, we find that the value of a parameter describing the mean value of a random environmental variable affects both the variance and the skew of the frequency distribution of regional carrying capacity for the species, which in turn will contribute to determining the species' manner of population fluctuation. We broadly classify the patterns of fluctuating carrying capacity according to the skew. Three basic patterns emerge, and we name these and relate them to the selective circumstances that they will engender. They involve, respectively, survival in a predominantly unfavorable environment (adversity selection), utilization of an unpredictable and intermittently favorable environment (exploitation selection), and competition in a predominantly favorable and fully occupied environment (saturation selection). Using these patterns of fluctuation in conjunction with two models of population growth, we obtain population simulations which suggest demographic strategies appropriate in each context. The regimes are not readily categorizable as to the intensity of r-selection versus K-selection, which suggests that the r-K classification is oversimplified. The nature of the fluctuations in growth parameters experienced by a given species are sufficiently sensitive to properties of the species itself that many different adaptive demographies may be represented in a community occupying a single environment.