Biodiversity of Ecotoxicological Responses in Animals

Although the results of ecotoxicological research have had a great influence on environmcntal policy, the scientific basis of the field is still underdeveloped, in particular regarding the formulation of theory and general rules based on causal mechanisms. This is partly due to the fact that the insight in conditions that determine toxicity is much better developed on the side of chemical substances than it is on the side of biological receptors. To strengthen the field of ecotoxicology, it is necessary to know the properties of ecological receptors on which environmental chemicals may act. An ecological receptor may be defined as a species that is vulnerable through a combination of high exposure, inherent sensitivity and weak ability to recover. Sensitivity is determined by processes such as internal distribution of chemicals, metabolism, detoxication and excretion. A possible way to map patterns of sensitivity is to compare the activity of detoxication enzymes such as cytochrome P450 and glutathion-S-transferase. There seem to be differences within the group of terrestrial arthropods, where species with a broad feeding range seem to have the best developed detoxication system. Over the whole animal kingdom it appears that aquatic species on the average have a lower activity than terrestrial species. Ecological effects of environmental toxicants do, however, not only depend on the inherent sensitivity, but also on the population consequences of intoxication, including the ability to recover. A way to express population-level effects may be found in the ratio of the LC50 to the no-effect-level for reproduction; this ratio is called sublethal sensitivity index, SSI. Reviewing data from the terrrestrial environment it may be concluded that SSI varies from 3 to 300, where a high value is characteristic for a susceptible species-toxicant combination. The life-history of a species, which reflects the priorities put in the energy budget during life, may be subject to selection by environmental contaminants; this is exemplified by a study on isopods living in metal-contaminated sites. In conclusion, the biodiversity of ecotoxicological responses requires an evolutionary explanation and the effects of toxic chemicals must be seen in the context of the complex of adaptations of animals to the environment.