survive by playing a kind of hide-and-seek game within a landscape; they must feed on ephemeral plants before some other herbivores eat the plants and must also try to stay one jump ahead of their predators.
Many declining species-such as the California least tern, the mangrove cuckoo, many frogs and salamanders, and some puddle ducks-are broadly distributed but are rare or threatened at individual sites. These species have evolved to find and use specialized habitat that appears at regionally scattered sites. Regional long-term climate trends and human conversion of these sites have significantly reduced the animals' opportunities to find suitable habitats. A minimum number of sites is needed to maintain a viable population. Most resource management agencies, however, focus on individual sites, rather than on the regional distribution of aquatic types and their summed functions. Future restoration projects would benefit from a large-scale integrated management approach that seeks to help managers understand what attracts and supports mobile species within a target area.
Gosselink and Lee (1989) discuss the question of the appropriate scale for managing aquatic ecosystems for specific species, focusing on the problem of understanding and evaluating cumulative losses to the ecosystems. They suggest that the area of concern for any given species should be its range. For example, they propose the term duckshed as analogous to watershed for the unit of management for a local population of ducks. A duckshed would include any area where individuals of the population might have to go to survive under the worst conditions, as well as the ecosystem that supports the population. The range may have within its boundaries considerable space that is used regularly by the particular species. Other areas may serve as occasional habitat and still others as refuges in which the species can survive during periods of natural stress. These ''stress shelters" may be of marginal use to the species most of the time, but because these refuge areas appear marginal, they are often not properly protected and are lost. Much of the nation's waterfowl habitat has been lost in this way.
A method that may be useful for planning aquatic ecosystem restoration programs is the Adaptive Environmental Assessment (AEA) approach of Holling (1978) (Walters, 1986). The AEA is a process for involving scientists, resource managers, policy analysts, and decision makers interactively in designing resource management programs.