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The current research takes advantage of multi-dimensional isocline graphical methods to describe competition for resources. These models proved useful based on the assumption that the toxin impacts the efficiency of at least one metabolic pathway. This loss of efficiency subsequently transforms the consumption vector and causes an increase in the minimum levels of resources needed to result in a zero net growth of the population.
The models produced a variety of results. Regions of equilibria expanded, contracted, or shifted to new regions of the resouce space. Interestingly, conditions that produced the largest regions of competitive equilibria are similar to conditions that theoretically could lead to extinction of a species. Spatial and temporal heterogeneity also masked the impact of a toxin from indexes of diversity or other measures of community structure. The impact of a toxin was also dependent on the class of resource (substitutable, essential, switching, etc.) affected. Genetic diversity could also be added as a component to the system.
Higher order trophic interactions were also modeled. A two-dimensional resource space can be altered to include the growth rate of the populations being modeled as the third axis. The resource supply for the next trophic level can then be calculated. Effects of the addition of the toxicant can then be followed through the organization of a biological community.
An important aspect of this particular approach is that the models are deterministic and the data necessary to construct the graphs can be obtained experimentally. The models also point to severe shortcomings in the toxicity assays. In acute assays, reproductive parameters are not addressed. Common reproductive assays do not look at the effects of varying resource levels. A major use of the model can be in the design of toxicity testing methods to predict community-level effects.
resource competition, modeling, xenobiotics, ecotoxicology, community structure, genetic diversity, trophic interactions, aquatic toxicology
Research Biologist, Toxicology Division, Research Directorate, Chemical Research and Development Center, Aberdeen Proving Ground, MD