Environmental+Toxicology+Tox+2000+notes (2)

2007 similarly populations where reproduction and

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Unformatted text preview: s susceptible to stressors than populations were reproduction and survival are less density dependent. Measurement of stressor effects at the population level is best done by studying populations. While some use can be made o f field studies such as before and after and control and reference protocols, the range of substance/stressor concentration may not be wide enough to capture all the responses or allow the identification of a (NOAEC) for the substance/stressor. One way round this is to conduct controlled experiments under field conditions in micro - or meso-cosms. A number of procedures have been proposed for these types of tests and there are numerous examples of their utility (Hill et al. 1994). Most of this work has been carried out in aquatic systems but some terrestrial systems have also been used. The aquatic systems range from simple laboratory systems to complex flowing stream systems. EFFECTS OF PESTICIDES IN THE AQUATIC ECOSY STEM The difficulty of measuring population - or community-level effects is illustrated in the following discussion of the effects of pesticides in an aquatic ecosystem. The principles in this discussion illustrate some of the problems and questions which should be addressed in any study designed to assess the effects of a substance in the environment. This methodol ogy was developed at the University of Guelph and involves the use of enclosures and micr ocosms as an experimental technique to assess impact in aquatic ecosystems. Obviously, some of the experimental procedures do not apply to terrestrial ecosystems. However, the general principles are similar. Enclosures (limnocorrals) and microcosms ha ve become more widely used as a method or technique for studying the effects of a variety of substances in aquatic ecosystems. The use of these enclosures has its basis in two philosophies. The first of these is the perceived need to assess the effects of a potentially toxic substance in the field — either under actual conditions of use or under circumstances during which it may come into contact with biota. This philosophy is based on the dogma (essentially an admission) that one cannot duplicate all e nvironmental conditions in the laboratory. Laboratory assessments can therefore only act as a guide to developing hypotheses which must be subjected to testing in field assays. This dogma is widely accepted and is the raison d'etre for the work of many environmental toxicologists as well as those in the applied agri cultural sciences. Historically, these field observations have normally been made after the fact of contamination or release of a toxic substance. While these studies may have added much t o understanding the processes whereby such substances distribute in the environment and cause harm, they have done little to predict or prevent this damage from occurring in the first place. The clear need for adequate methods, as well as the examples in the systematic field trials of new crop varieties or agricultural chemicals by agriculturalists, has lead to the development of similar assessments for environmental stressors. These methods have begun to be accepted by regulatory authorities as well a s those who use or produce these s...
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