The Nature of Multispecies Toxicity Tests

As discussed in Chapter 2, ecological structures including multispecies toxicity tests have the fundamental property of being historical. Brooks et al. (1989), in an extensive literature review and detailed derivation, concluded that ecological systems are time-directed, or in other words, irreversible with respect to time. Drake (1991) has experimentally demonstrated the historical aspects of ecological structure in a series of microcosm experiments. Design 

Evolutionary events also occur within multispecies toxicity tests. Species or strains resistant to xenobiotics do arise. Simple microbial microcosms (chemostats) are often used to force the evolution of new metabolic pathways for pesticide and xenobiotic degradation. 

Microcosms do not have some of the characteristics of naturally synthesized ecological structures. Perhaps primary is that multispecies toxicity tests are by nature smaller in scale, thus reducing the number of species that can survive in these enclosed spaces compared to natural systems. This feature is very important since after dosing, every experimental design must make each replicate an island to prevent cross contamination and to protect the environment. Therefore the dynamics of extinction and the coupled stochastic and deterministic features of island biogeography produce effects that must be separated from that of the toxicant. Ensuring that each replicate is as similar as possible over the short term minimizes the differential effects of the enforced isolation, but eventually divergence occurs. 

Coupled with the necessity of making the replicates similar is the elimination of a key ingredient of naturally synthesized ecological structures the spatial and temporal heterogeneity. Spatial and temporal heterogeneity are one key to species richness, as in The Paradox of the Plankton (Hutchinson 1961). Environmental heterogeneity is key to the establishment of metapopulations, a key factor in the persistence of species. 

The design of multispecies toxicity tests runs into a classical dilemma. If the system incorporates all of the heterogeneity of a naturally synthesized ecological structure, then it can become unique, thereby losing the statistical power needed for typical hypothesis testing. If multispecies toxicity tests are complex systems and subject to community conditioning, then the tests are not repeatable in the same sense as a single-species toxicity test or biochemical assay. 

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