General results of microcosm research seem to indicate that replication becomes more difficult to achieve as the microcosm becomes larger and as the experiment lasts longer, but there is clear scientific value in large and long-term microcosms. Curiously, they conclude that "the better the microcosm the worse it replicates and, therefore, the worse it is as an experimental tool." They suggest long time period observations as the "way out of this morass," which is the opposite of most microcosm research that relies on short-term studies to avoid the almost inevitable divergence of replicate microcosm over time. Every bay and cove is to some measurable extent unique." They discuss the dilemma of trying to develop a set of microcosms that replicate well but still exhibit the variability inherent in nature. Every patch of water follows its own course to some extent, exchanging all the while with its surroundings. For example, they state: "The existence of variability is a severe problem, becoming even philosophically difficult to deal with. Pilson and Nixon (1980) provide many practical insights on the issue. Hurlbert's (1984) discussion is valuable in this regard as are papers reviewing the statistical treatment of microcosm research (Chapman and Maund, 1996 Gamble, 1990 several papers in Graney et al., 1994 Sheehan, 1989 Smith et al., 1982). Clearly, there is some subjectivity here, and there are no clear cut criteria on how similar replicate microcosms must be for use in experiments. Furthermore, Whittaker (1961) in one of the first microcosm studies introduced the term aquarium individuality for "the marked differences between aquaria with similar conditions which result from minor, uncontrolled factors and are a major limitation on reproducibility and statistical adequacy of the data." Ironically though, Abbott (1966) quotes another pasage from Whittaker's paper as justification for the microcosm approach. However, Abbott's study was criticized on the very same grounds of coefficient of variation by Hurlbert (1984) and Pilson and Nixon (1980). This means that groups of parallel systems can be established and studied in rigorously defined experiments, but only on a statistical basis." This was the first study devoted to the question of microcosm replicability, and it has been often quoted in more recent studies as justification for the experimental use of microcosms. under proper conditions aquatic microcosms show replicability comparable to that found among other types of statistical trials. For example, Abbott (1966) studied replicability of 18 5-gal (19 l) glass carboy microcosms of an estuarine bay and found that, based on coefficient of variation calculations, ". While most studies and reviews focusing on replicability have found that it can be satisfactorily achieved in microcosms (Conquest and Taub, 1989 Giesy and Allred, 1985 Isensee, 1976 Levy et al., 1985 Takahashi et al., 1975), there is disagreement. (1999) found that only 65% of the studies reported the number of replicate systems per treatment, which implies that many researchers take replication for granted. In a review of 360 microcosm experiments, Petersen et al. Problems do arise when considering replication of microcosms. As has been noted, cross seeding is used in microcosm research to reduce this variability and thus "enhance" replication (Beyers and H. The number of replicates required for an experiment depends on the variability of the data being collected. As noted by Sheehan (1989) "the main purpose of replication is the supply an estimate of variability (error) by which significance of treatment and control comparisons can be judged." Thus, replication is needed to distinguish between natural variation and variation due to a treatment (such as introduction of a toxin into a microcosm) in an experiment. In fact, replication is one of the "minimal requirements of experimental design in ecology" (Hairston, 1989) as in any application of the scientific method. It is included in this discussion because of its critical nature to the "microcosm method" (Beyers, 1964) and because its consideration leads to fundamental questions of variability (possibly chaos) of ecosystems. Replication is an issue of experimental design rather than technical design of a microcosm.
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