S M Bard, Dalhousie University, Halifax, NS, Canada © 2008 Elsevier B.V. All rights reserved.
Ecotoxicology is a relatively young field that was first defined by Rene Truhaut in 1969 as ''the branch of toxicology concerned with the study of toxic effects, caused by natural or synthetic pollutants, to the constituents of ecosystems, animal (including human), vegetable and microbial, in an integral context.'' Ecotoxicology is multidisciplinary and aims to primarily combine the study of ecology (species richness, abundance, and distribution) and toxicology (toxic effects caused by anthropogenic or natural substances). The scope of ecotoxicology encompasses the interaction, transformation, fate, and effects of xenobiotics ('foreign compounds') on the organism, population, community, and ecosystem, from the regional to global level. To elucidate such broad scientific questions, the study of ecotoxicology incorporates concepts contributed from diverse fields including analytical and environmental chemistry, biochemistry, molecular biology, microbiology, immunology, physiology, behavioral ecology, soil science, limnology and oceanography, atmospheric science, environmental and chemical engineering, economics, public environmental policy, and other disciplines. At the level of the organism, ecotoxicological studies include elucidating the cellular and molecular defense mechanisms that wild species deploy against toxicants. Such defense mechanisms include biotransformation of toxicants by drug-metabolizing enzymes (e.g., phase I cytochrome P450, phase II glutathione ^-transferase, etc.) and transmembrane toxin efflux transporters (e.g., multidrug resistance proteins) that facilitate the elimination toxicants and their metabolites via bodily excretion. Other ecotoxicological endpoints include endocrine disruption, genotoxicity, immunomodulation, and other biological responses that are considered biomarkers of exposure to toxicants. Examining these measures in large numbers of individuals can permit an evaluation of the health effects of toxicants across a population. In highly toxic environments, some populations can develop chemical resistance either through adaptive response or genetic selection in which only the hardiest individuals survive. Elucidating the influence of toxicants on whole communities involves assessing biological diversity (species richness and abundance) along a pollution gradient. Near polluted sites, a few hardy species can tolerate the extreme chemical conditions and may even thrive; at sites of intermediate exposure, sensitive species begin to appear and species diversity increases; but many species are pollution intolerant and can only survive at uncontaminated sites. Understanding the pollution tolerance of species within a community can permit development of biological indices to rate pollution exposure at sites based on the species present. In addition to the toxicological sensitivity of different species, the trophic role of individual species plays a strong role in structuring communities, in particular the presence of keystone predators (e.g., Pisaster seastar), species that provide physical habitat for other species (e.g., mussel beds or leafy seaweed cover), and organisms that facilitate other species' breeding success (e.g., seagrass nurseries). Ecotoxicological studies at the ecosystem level include examining the biomagnification of persistent organic pollutants through the food web. Lipophilic ('fat-loving') anthropogenic contaminants can be absorbed from the environment and bioaccumulate in oily deposits in low-trophic-level organisms. These compounds bioconcentrate through the food web such that top predators such as marine mammals, polar bears, and humans in turn accumulate the highest levels of these contaminants in their own bodies and can discharge them in fat-rich breast milk. The biomagnification of persistent organic pollutants is not restricted to one region, as these chemicals can be transported thousands of kilometers from the tropics to concentrate in the polar regions by both atmospheric processes and migratory species to create a chemical source and sink problem of global significance.
See also: Atmospheric Deposition; Biogeochemical Approaches to Environmental Risk Assessment; Body Residues; Ecotoxicological Model of Populations, Ecosystems, and Landscapes; Environmental Tolerance; Soil Ecology.
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