than anticipatory, simply because we did not have sound information that would enable prediction of consequences of our actions. In effect, our management of resources has been a series of large scale—and often irreversible—experiments from which we often learned little because of the lack of information on baseline conditions or monitoring of trends. Now, with more sophisticated experimental research and modeling tools and information, we are better able to base decisions on anticipatory planning.

Third, ecological knowledge helps to define the types of policy tools and resource management practices that can be used to achieve various social and ecological objectives. For example, between the 1950s and 1970s, fisheries management relied on deterministic fisheries population models that were used to calculate harvest levels based on the maximum sustained yield of the population. As knowledge of fisheries population dynamics has grown, managers now realize that the non-linear nature of the population dynamics of many species, combined with the substantial uncertainty surrounding factors influencing recruitment and survival, requires different approaches to fisheries management—specifically, reliance on adaptive management techniques and highly conservative quotas.

Do we know enough about ecological systems to be confident of the environmental management goals that we set? As our understanding of ecological systems has increased, we have refined environmental management goals and, in some cases, substantially changed them. For example, many of the regulations that were initially issued under the Clean Air Act and Clean Water Act in the early 1970s were based on limited knowledge of dose-response curves, environmental thresholds, and relative risks. With further research and experience with the consequences of various pollutants, those goals have been modified. More generally, though, how firm is the science on which we base our choice of environmental goals today?

Biodiversity conservation. Advances in scientific understanding of the importance of biodiversity and the threat that it faces is contributing to significant rethinking of biodiversity conservation objectives and goals. When the Endangered Species Act (ESA) was enacted in 1973, the goal of biodiversity conservation was straightforward—to save endangered species from extinction. The goal become substantially more complex, however, as scientists began to recognize the value of protecting all levels of biological diversity—genes, species, and biological communities. It became still more complex as the full magnitude of the species extinction problem in the U.S. was revealed. Where it may have seemed conceivable to protect all of the nation's species from extinction based on the knowledge available in the late 1960s, it is clear now that this would be extremely difficult and costly—not a surprising conclusion given the extensive habitat transformation that has taken place in the U.S.