SREL Reprint #2175

Environmental pseudointeraction: the effects of ignoring the scale of environmental heterogeneity in competition studies

Karen A. Garrett and Philip M. Dixon

Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802

Abstract: Spatially explicit models of competition such as neighborhood and area-of-influence models are useful approaches for gaining insight into interactions between individual organisms. The environments in which these models are parameterized may be spatially heterogeneous. We demonstrate that, even in the absence of interaction between individuals, nonzero estimates of competitive or facilitative coefficients can result from environmental heterogeneity when densities are not experimentally imposed. We show this using a model of individual position and size in a simple environment composed of two patch types. Suppose two noninteracting species are intermixed in the study area and species 1 becomes established more readily in the patch type in which species 2 grows larger and has higher fecundity. Analyses that ignore environmental heterogeneity may indicate that species 1 facilitates species 2 because larger individuals of species 2 will tend to be associated with greater numbers of species 1. Similarly, environmental pseudocompetition can result if species 2 is more frequent among small individuals of species 1. The strength of the environmental pseudointeraction depends on (a) the magnitude of the size and fecundity differences between patch types, (b) the magnitude of the establishment differences between patch types, and (c) the size, relative to the environmental grain, of the neighborhood or quadrat used for sampling the number of competitor individuals.

SREL Reprint #2175

Garrett, K.A. and P.M. Dixon. 1997. Environmental pseudointeraction: the effects of ignoring the scale of environmental heterogeneity in competition studies. Theoretical Population Biology 51:37-48.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).