Ecotoxicology the best-adapted individuals Chemical/physical stress on an ecosystem will therefore lead to a shift in the balance between r- and K-strategists in favor of r-strategists This principle has been used successfully to monitor environmental pollution and disturbance using colonizer/persister (c-p) indices applied to nematodes (Bongers, 1990) or perturbations of rhizophere microbial communities (De Leij et al., 1993) Another useful way of looking at communities and ecosystems is by classifying the organisms that inhabit that ecosystem by their ‘‘trophic function.’’ Using this approach, organisms can be classified (aboveground) as primary producers (plants and algae), herbivores, predators, and top predators, while organisms that feed on a variety of food sources are classified as omnivores In general, food chains have no more than four trophic levels because less and less energy is available higher up the food chain Furthermore, dependence of predators and top predators on exclusive feeding sources make them vulnerable to perturbations lower down the food chain Using the concept of trophic levels, it can be argued that specialized feeders are more sensitive to environmental stress (when this stress affects their food source) than organisms that are less specialized Not surprisingly, when recalcitrant chemicals enter the food chain it is the specialized feeders (for example, peregrines and sparrow hawks that feed exclusively on birds) that are the first to be affected by toxins Beside the fact that they are exposed to relatively high levels of a pollutant, these organisms are often very skillful specialized hunters Any impairment of these skills due to a toxin will result in starvation Therefore, these species can be sensitive bioindicators of toxins, even at sublethal concentrations Another important ecological concept that can be used to simplify ecosystems in a meaningful way is the concept of keystone species The idea is that even though there is dependency between all species that are involved in a certain food web, some species are more important than others It is important in this context to make a distinction between dominant species that derive their importance to the ecosystem in terms of biomass or energy flow and keystone species Keystone species are often relatively rare but have a major impact because of their key regulatory function in the system as a whole This is because these species are involved in more links or interact with parts of the ecosystem that maintain living conditions for a wide variety of other species In other words, the species diversity of an ecosystem is determined in large part by a few keystone species on which many others depend A detrimental effect on these species will result in the collapse of the whole ecosystem (or at least a large part thereof) Several different categories of keystone species have been recognized, such as keystone predators, keystone prey, and keystone habitat modifiers One of the best-studied examples of the importance of keystone predators is the starfish (Pisaster) and predatory whelks (Nucella) in pools along the rocky inter tidal zone Removal of these predators led to the disappearance of 80% of the prey species and the nearly complete dominance of the prey community by mussels (Mytilus) Sea otters (Enhydra lutis) have also been labeled as keystone species as they limit the density of sea urchins, which, in turn, eat kelp and other macro algae that provide the habitat for a large variety of species Without sea otters the sea 125 urchin population explodes resulting in the destruction of the kelp forests leading to the loss of habitat for those species that depend for their survival on the kelp forests for reproduction, shelter, and food The way keystone species are affected by toxins is an important consideration in ecotoxicology, because they represent the Achilles’ heel of an ecosystem Not surprisingly, keystone species such as earthworms and honeybees are often used in toxicity testing because they are important for ecosystem function in general Clearly a negative effect on ecosystem function is often due to alterations of complex interactions within the ecosystem itself In most cases, toxic effects are buffered due to the compensating capacity of an ecosystem However, occasionally toxic effects can affect species that are responsible for essential functions in that ecosystem Further insight in the way that different species contribute to ecosystem function is an essential requirement for the development of the most appropriate biomonitoring tools for assessing the ecotoxicological effects of chemicals Conclusion Currently there is growing concern internationally about environmental toxicants affecting biodiversity and human health Biodiversity has come to the fore by the United Nations meetings, the first of which was in Rio de Janeiro in 1992 The medical profession is increasingly concerned about a range of conditions that might be induced by chemicals in the environment This is resulting in moves to increase legislation The study of ecotoxicology seems therefore certain to grow in importance, as well as there being a stimulus to prevent problems occurring in the first place via the development of clean technology and stimulation of sustainable land use On the other hand, ecotoxicology can help to identify existing pollution problems, which in many cases can be cleaned up using a variety of remediation strategies See also: Air Pollution Ecological Footprint, Concept of Environmental Impact, Concept and Measurement of Greenhouse Effect Keystone Species Soil Biota, Soil Systems, and Processes References Alloway B (1995) Heavy Metals in Soil Glasgow: Blackie Academic and Professional Bongers T (1990) The maturity index: an ecological measure of environmental disturbance based on nematode species composition Oecologia 83: 14–19 Carson R (1965) Silent Spring Harmondsworth, England: Penguin Books De Leij FAAM, Whipps JM, and Lynch JM (1993) The use of colony development for the characterisation of bacterial communities in soil and on roots Microbiol Ecology 27: 81–97 Everts JW, Aukema B, Hengeveld R, and Koeman JH (1989) Side effects of pesticides on ground dwelling predatory arthropods in arable ecosystems Environmental Pollution 59: 203–225 Gupta SK (1991) Metabolisable metal in antropogenic contaminated soils and its ecological significance In: Vernet JP (ed.) Impact of Heavy Metals in the Environment, pp 299–310 Amsterdam: Elsevier