Status and Trends of the Nation's Biological Resources - Environmental Contaminants

side effect of human population growth and technological development. Unlike many other environmental stressors, casual observers often become aware of chemical pollutants only after some catastrophic event, such as an oil or chemical spill that kills many animals. Typically, chemical discharges that can be seen or smelled—or barren mine spoils (Fig. 1) and fishless streams—call attention to chemical pollutants. More commonly, sophisticated instruments are needed to detect toxic chemicals. The effects of chemical contaminants on individual organisms, populations, and ecosystems may also be difficult to detect. The imperceptible accumulation of DDT (Table 1) and its metabolites in bald eagles and other predatory birds led to eggshell thinning, reproductive failure, and, ultimately, greatly reduced populations over much of North America (for example, Carson 1962; Colborn 1991; Wiemeyer et al. 1993). Much of this happened before the problem was understood sufficiently to take the steps necessary to correct it; populations of bald eagles are now recovering (U.S. Fish and Wildlife Service 1994). Effects of other contaminants on less visible species may be more subtle and localized. In addition to killing plants and animals, contaminants may reduce growth and reproduction or cause animals to avoid contamiEnvironmental Contaminants