Metapopulation dynamics: Indirect effects and multiple distinct outcomes in ecological risk assessment

Abstract

Abstract A toxicant-dosed metapopulation model was used to explore the range of possible dynamics of populations in contaminated field sites. A single species metapopulation model was developed that is discrete, deterministic, and incorporates doseresponse curves and biotic growth rates to describe the effects of contamination on a metapopulation. The distribution of the chemical contamination is assumed limited to one patch and contagious within that patch. Both persistent and degradable toxicants were modeled. Five principal conclusions resulted from our simulation studies. (1) Mortality in one subpopulation has ecologically significant effects on nondosed subpopulations. This hypothesis we term “action at a distance” because no direct contact with a toxicant has occurred except in the dosed patch. (2) Because uncontaminated sites connected to the contaminated site by migration of the biota are affected, these uncontaminated sites cannot be reference sites. (3) The arrangement of the patches is critical to the dynamics of the system and the overall impact of a toxicant. (4) Due to the contagious distribution of the toxicant and the stochastic function describing exposure effect, multiple discrete outcomes often are possible from the same initial conditions. These outcomes can range from extinction to the reaching of the carrying capacity for a patch. (5) If sufficient cleanup is not possible, it may be necessary to isolate the contaminated patch, allowing the formerly connected patches to regain more typical population dynamics.