Effects of Chemicals in Communities and Ecosystems

In Chapter 3, the distribution of enviromnental chemicals through compartments of the gross environment was related to the chemical factors and processes involved, and models for describing or predicting environmental fate were considered. In the early sections of the present chapter, the discnssion moves on to the more complex question of movement and distribntion in the living environment— within individuals, communities, and ecosystems—where biological as well as physical and chemical factors come into play. The movement of chemicals along food chains and the fate of chemicals in the complex communities of sediments and soils are basic issues here. 

Ecotoxicology deals with the study of the harmful effects of chemicals in ecosystems. This includes harmful effects upon individuals, although the ultimate concern is about how these are translated into changes at the levels of population, commnnity, and ecosystem. Thns, in the conclnding sections of the chapter, emphasis will move from the distribntion and environmental concentrations of pollutants to conseqnent effects at the levels of the individnal, population, community, and ecosystem. The relationship between environmental exposure (dose) and harmful effect (response) is fundamentally important here, and full consideration will be given to the concept of biomarkers, which is based on this relationship and which can provide the means of relating environmental levels of chemicals to consequent effects npon individuals, populations, communities, and ecosystems. 

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The results of the traditional acute single-species toxicity tests conducted in the laboratory cannot be used alone to predict effects on natural populations, communities, and ecosystems. The cultural species in laboratory tests are different from those in most ecosystems. Conditions such as the size of the test species, its life stage, and nutritional state can have an effect on toxicity. Furthermore, the experimental conditions in laboratory tests cannot duplicate the complex interacting physical and chemical conditions of ecosystems, such as seasonal changes in water temperature, dissolved oxygen, and suspended solids. In addition to these environmental modifying factors, aquatic life is usually exposed simultaneously to numerous potential toxicants (mixtures). Although the toxicities of binary and ternary mixtures have been evaluated for some chemicals in laboratory toxicity tests, the resultant information has predictive limitations. 

Ecotoxicology is primarily concerned with effects of chemicals on populations, communities, and ecosystems, but the trouble is that field studies are expensive and difficult to perform and can only be employed to a limited extent. In the main, environmental risk assessment of pesticides and certain other chemicals has to be... 

Ecotoxicology Is concerned with die toxic effects of chemical and physical agents in living organisms, especially on populations and communities within defined ecosystems it includes transfer pathways of these agents and their interaction with the environment. 

Chemically mediated interactions have important direct and indirect effects on communities from both ecological and evolutionary standpoints.22 341 Chemical defense or communication cannot be properly understood unless it is viewed through the lenses of population, community, and ecosystem processes, and this requires consideration of both the biotic and abiotic components of the natural environment.342 For example, chemically mediated foraging is affected by water flow because it relies on water-soluble cues that are carried away from prey.343 345 Similar constraints likewise modify the effectiveness of other waterborne cues, such as alarm signals, sexual pheromones, and settlement cues, in both mobile and sessile organisms.244 345 350... 

When effects of pollutants are seen at the community or ecosystem levels, it may be too late to start reclamation activities, which become inefficient and extremely costly. That is why invertebrates have been effectively used as sources of biomarkers. They can be extrapolated to actual or potential changes at the population level, and considered as predictive tools to assess changes and consequences on community and ecosystem levels. Only some biomarkers identified in invertebrates are highly specific and sensitive, the majority are less specific to chemicals and then indicate the exposure or toxic effects of their mixture. In invertebrates biomarkers can be used as the diagnostic tools of their health or would give the basis for predicting the fate of stressed environment and to start remediation activities. 

Another important advantage of employing ecological models in risk assessment is that they provide outputs that are closer to the protection goals of the assessment (i.e., populations, communities, and ecosystems) than other approaches. Application of even very simple population models provides a better measure of response to chemicals than the individual-level endpoints (survival, reproduction, development) most widely in use for risk assessments today, not the least because such models integrate effects on all of the key life-history traits that determine population-level attributes (Forbes and Calow 1999). 

For the comparative screening of chemicals, and in particular for legal purposes, there is a need for standardised and reproducible methods, and present community and ecosystem approaches do not showthese features. Therefore legal testing procedures are based mainly on single-species testing, even if relatively complex end-points are often required (chronic effects, fertility studies, etc.). 

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