FIGURE 4.7 A typical dose-response curve from ecotoxicology.
Microcosms and/or mesocosms occupy the highest tier in this type of protocol, but they are seldom used by regulators because they can be expensive, time consuming, variable, and difficult to evaluate in terms of end points.
Most regulatory decisions are made based on the intermediate tier from single-species tests in which data from toxicity experiments are compared to estimated environmental exposure data. Thus, test populations of certain species are grown in the laboratory and tested for short-term (acute) vs. long-term (chronic), and lethal (causing mortality) vs. sublethal (causing stress but not mortality) dose experiments. The organisms most often used are the green alga Selenastrum capricornutum, the microcrustacean Daphnia magna (water flea), and the fish Pimephales promelas (flathead minnow). This selection of species provides a broad range of organismal responses to the chemicals being tested rather than focusing on a single taxonomic group. Typical acute tests would last 48 to 96 h and would test for end points in terms of survival of Daphnia and the flathead minnow or photosynthesis of the alga. Typical chronic tests would last up to a month and would test for end points in terms of reproduction of Daphnia and growth of the flathead minnow. Such tests are illustrated in Figure 4.7 with a dose-response curve. Thus, test populations are raised in a series of containers with increasing doses of the chemical that is being assessed (plotted along the x-axis of the figure) and their mortalities are recorded (plotted along the y-axis of the figure). The dosage of the end point (LD50 or lethal dose for 50% of the initial test population) is found by interpolation on the curve. This dosage is compared with the estimated environmental exposure dosage to complete the test. Note that the end point, death, is simple, definite, and easy to evaluate. The classic shape of the dose-response curve is sigmoid, though a u-shaped curve is also important for certain cases (Calabrese and Baldwin, 1999).
A controversy has arisen about the kinds of tests required in risk assessment of chemicals. A number of ecologists have insisted that single-species tests are inadequate for a full evaluation of ecosystem level impacts and that multispecies toxicity tests should be required. The principle issue is whether results from the single-species tests can be extrapolated to higher levels of ecological organization (Levin, 1998). Arguments against the ability to extrapolate have been provided by the Cornell University Ecosystems Research Center (Levin and Kimball, 1984; Kimball and
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