ABSTRACT
It is useful to have a mental model of the interactions between toxicants, other ecological factors, and the competition between two species. A useful model for evaluating these interactions has been the resource competition models of Tilman. Detailed derivations and proof can be found in Tilman's excellent monograph and by reading the ecological literature citing this seminal work. In order to describe the uptake of the toxicant by the organism, a resource consumption vector is constructed. The zero net growth point expanded to the two-dimensional resource space produces a zero net growth isocline (ZNGI). The shape of the ZNGI is determined by the utilization of the resource by the organism. If the resources are essential to the survivorship of the organism, then the shape is as drawn. Toxicants that do not directly affect the competing species but instead alter the availability of resources can alter the species composition of the community.
