chemical does not necessarily indicate a threat; its concentration must be considered 

 in relation to its biological activities. These two "lanes" constitute the exposure and 

 hazard portions of the assessment roadway. Together they yield risk assessment, 

 which is based on the primary dictum of toxicologists extending to Paracelsus: "Dose 

 always determines toxicity." 



Some hard realities intrude in efforts to build such a system for evaluating 

 chemicals specifically and pollution in general. We had decades to build the Panama 

 Canal and a decade of national effort to put a man on the moon. The law says that we 

 should already be enforcing all aspects of the several acts, but there are not enough 

 scientists and laboratories in existence to test the 60,000 known toxic substances and 

 thousands of product waste streams for all known adverse effects. Some roads can be 

 constructed for limited purposes — pesticides, drugs, etc. — but it is not possible to 

 build sufficient bridges for all the traffic. 



There are many steps in the sequence of events from the time a scientist detects 

 some adverse response in the laboratory or the field to the time at which a regulatory 

 agency or private body can take effective action. Bridges must be built from one solid 

 foundation to another, linking a specific test response to projections of risk through 

 exposure and the explicit implications of the consequences of that risk. 



During the past decade, the basic component for building these bridges has been 

 developed in the laboratory: the single species toxicity test. In the laboratories of the 

 Fish and Wildlife Service and the former U.S. Public Health Service groups (later to 

 become the U.S. Environmental Protection Agency), in private industry and non- 

 profit institutions, and in colleges and universities, means have been found to test the 

 acute (short term or single-exposure) and chronic toxicity of practically the entire 

 taxa of plants and animals. 



The use of the single species assay is the dominant feature of modern ecotoxi- 

 cology. By carefully selecting test species and controlling the environmental 

 conditions of exposure, a powerful tool has been created that by extension can serve 

 from the early stages of evaluation (identification of bioactivity) on through to more 

 complex aspects (economic and ecologic evaluation). 



As the single-species assay was becoming the primary tool of ecotoxicology, 

 advances in ecology and environmental chemistry served to focus attention on the 

 systems-level aspects of the fate and effects of chemicals. This approach was 

 promoted by two familiar causes. It became theoretically imperative to examine the 

 set of interacting processes and components (biological and chemical) that produced 

 ecosystem response, and it became increasingly feasible to do so. The success of the 

 single-species assay, advances in chemical separation and quantification, improve- 

 ments in controlled environments, and increasing ease of high speed computation of 

 statistical mathematical models contributed to the practicality of systems-level 

 attack on pollution problems. Recognition of the need for attaining better under- 

 standing in the laboratory provided the impetus. 



This paper examines the strengths and weaknesses of these building blocks of 

 understanding and how they fit into the logical framework that may get us across the 

 swamp. 



LABORATORY MICROCOSM TESTS 



A systems-level laboratory attack on the problems of pollution can begin with an 

 excised portion of the "real world" brought into the laboratory,, with an artificial 

 assemblage, or with a mathematical model of processes. Such a model is usually a 

 product of laboratory measurements of biological and physiochemical processes and 

 characteristics or species functions. Hence, our attention should be directed first to 

 the place where artificial and natural assemblages are maintained and studied in the 

 laboratory. 



Ecosystems have certain critical biological functions^ and characteristic structures 

 that may be threatened by stress and pollutants. These threats include: 



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