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I I Introdudion I more susceptible to toxic effects. Conversely, the higher metabolic rates of creatures in conflict may help them dispose of toxins more readily. I These differences between natural and laboratory environments mean that measure- ments collected in natural settings are generally preferable to laboratory measure- ments for predicting toxic effects in natural systems. In cases where natural studies are lacking, though, the laboratory studies may provide the only useful guidance to possible toxic effects. Moreover, only in controlled laboratory studies can the effects of individual variables be studied, by holding all other factors constant. I I I I Interactions I The toxicity of an element or compound may be either reinforced or weakened through its interaction with other substances. In toxi- cology studies, such interactions are generally classified as being adversely additive, syner- gistic (greater than additive), or antagonistic (less than additive or even acting as antidotes to one another). For instance, various chapters in this volume describe synergistic relationships between boron and selenium, between copper and zinc, and between DDE and Arochlor, meaning that when both agents are present, their toxic effect is greater than would be expected just from adding together their individual effects. Elsewhere, these chapters describe antagonistic relationships between arsenic and selenium and between cadmium and copper: tests show these combinations of elements to be less toxic than either one would be by itself. In the case of selenium and mercury, however, the selenium chapter cites a study (Heinz and Hofbnan 1996) showing that these two elements are antagonistic to each other in their effect on adult mallards but synergistic in their effect on mallard reproduction. I I I I I I I I I I In some cases, two substances that interact antagonistically at first may eventually become synergistic with increasing concentrations. For instance, some interactions may transform a toxic compound to a less toxic, but also less soluble, form. These low-solubility compounds may then accumulate in the liver, the kidneys, or other bodily organs, eventually overtaxing the capacity of these storage sites. Physical damage may occur to organs storing too many solids. However, our understanding of biogeo- chemical interactions is still rudimentary. The potential combinations of trace elements are essentially infinite, and research thus far has defined the additive, antagonistic, and synergistic effects of only a few simple com- binations. Some compounds cause toxic effects by interfering in essential chemical metabolic pathways, yet different chemical species of the same two elements may interact on different metabolic pathways and produce a completely different result. Under present conditions it takes years of research-perhaps an entire career-to positively define just one or two complex metabolic chemical pathways. Many apparent discrepancies appear in the literature. Temperature All organisms have optimal temperature ranges in which they function most efficiently. Outside of these ranges they will be more susceptible to toxins. The DDT chapter, for instance, cites studies showing that both high and low temperatures increase the toxicity of DDT to the water flea Daphnia. Temperature fluctuations affect the rate of chemical reactions, the solubility of chemical species, and the metabolic rates of organisms. High temperatures generally increase the chemical reaction rate and the solubility of most solid substances. Oxygen and other gases, however, are more soluble in cold water than in warm. The effect of temperature on CJ