SREL Reprint #2016






Toxicants as Selective Agents in Population and Community Dynamics

Ronald K. Chesser and Derrick W. Sugg


Although all sciences depend upon descriptive data and the technologies that aid their collection, synthesis of these data is equally important. Theory plays an important role in the synthetic process; however, its value is not limited to prediction. Modeling nature, at all levels of organization, provides much insight into the workings of our world. As with theory, models developed to explore theoretical issues are important not only for predictive purposes, but because they offer understanding. In this manuscript, we explore some of the theoretical issues of ecotoxicology by modeling some of the processes in ecosystems.

One may ask why ecotoxicology is so descriptive. The answer probably lies partly in our recent interest in some of the issues dealt with by ecotoxicologists. After all, the theory of natural selection was based on keen observations and descriptions made by Charles Darwin (1845), and only later synthesized into a coherent theory (Darwin, 1859, 1871). Another consideration is that ecotoxicology is the melding of two disciplines (ecology and toxicology), crossing the boundaries of many sciences (i.e., chemistry, geology, and biology). Ecotoxicologists often fail to recognize that these sciences and disciplines already have well-developed theories that can aid in our new pursuits. Novel questions associated with ecotoxicology have been the impetus for the new description and techniques that have proliferated in the literature. Description is a natural result of curiosity and, without that curiosity, it is unlikely that we would have even recognized the problems that ecotoxicologists are so concerned with today. As has been so eloquently and succinctly stated by Levin (1989), "theory in the absence of data is sterile, data without theory is uninteipretable." It is important that ecotoxicologists draw on all of the strengths,whether they be synthetic or descriptive, of other fields if it is to become a viable discipline, much as conservation biology has done. This approach is particularly important because much of the very existence of ecotoxicology is owed to the needs of regulatory bodies dealing with crises.

What role does theory have to play in ecotoxicology? One obvious answer to this question is that it will allow scientists, and regulators, to make predictions about the impacts of toxicants. However, too much emphasis may be placed upon the predictive powers of our theories (Ehrlich, 1986, 1989). It is through theories that we may observe, and ultimately understand, issues that are too complex to glean from descriptive data. Theory can indicate new types of data that must be collected, examine processes for their tractability and usefulness in explaining observed phenomena, and define the bounds under which these phenomena are operating. Ecotoxicologists should welcome theories that address any of these problems, regardless of their predictive abilities.

Obviously, ecosystems are complex entities that will require detailed information to be understood. Although there is considerable data on the composition and functions of ecosystems, a detailed understanding of these systems is lacking. Ironically, we may have too much data to develop a tractable model of ecosystem processes. It is imperative that we simplify the systems to avoid clouding the real issues with unimportant details (Levin, 1989). One method of attaining this goal is to devise standards that are useful for comparing the functions of different ecosystems (Kimball and Levin, 1985) while still maintaining the crucial functions shared by all. Evans (1956) suggested that one obvious simplification is to understand the flow of energy in ecosystems. The utility of this theme has been shown in some classical studies by Lindeman (1942) and Odum (1957). Indeed, many agencies view the flow of materials and energy to be central to understanding ecosystem processes (Schlesinger, 1989). Such a simplification will lead to a loss of predictability, but it does not preclude future improvements in the theories. The laws of thermodynamics have played, and will continue to play, an important role in our understanding of these complex systems. We are likely to benefit from even the most simplistic models as long as we recognize their strengths and weaknesses (Pimm and Gilpin, 1989).

Obvious areas for improvement of previous models of ecosystem functions are the relaxation of assumptions of equilibrium, constancy, and stability; these improvements may be more important than incorporating more complexity and increasing predictive power (Levin, 1989). Some attention must be paid to complexity; after all, simple Lotka-Volterra models poorly describe the massive amounts of energy actually affecting organisms in a complex community (Pimm and Gilpin, 1989). Herein, we develop a model for ecosystems that retains the importance of energy while relaxing the requirements for equilibrium conditions. The ecosystem is viewed as having very complex, nested interactions among the species, but we have limited the number of species to maintain tractability. We have also avoided the trap of closed communities by allowing immigration and recolonization; however, to simplify the system we only allow inclusion of the original member species. Finally, we introduce toxicants as a general selective agent. Our goals are not to provide a predictive model concerning the fate of ecosystems that have been impacted by a general toxicant. Instead, we are interested in understanding the roles that species redundancy, adaptation, and immigration play in the ability of an ecosystem to cope with such an insult. Our model is not perfect, it is even over parameterized for complete understanding, but it may offer some insight that could not be obtained by more classical approaches. This model is intended only to provide plausible functions for ecosystem dynamics. Other functions are possible, and they may alter the response of the ecosystem. More specific models can be developed to explore these details.

SREL Reprint #2016

Chesser, R.K. and D.W. Sugg. 1996. Toxicants as selective agents in population and community dynamics. p. 293-317. In Ecotoxicology: A Hierarchical Treatment, edited by M.C. Newman and C.H. Jagoe. Lewis Publishers. Chelsea, MI.

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