coral reefs, and temperate rocky shores) have been and are undergoing heavy impacts from human activities. For the last three systems (shelf-slope systems, the deep sea, and the pelagic open ocean), a sense of urgency relative to what human activities may now be doing or could do in the near future is driven by profound ignorance of how diversity in these systems is created or maintained.
Estuaries and Bays
The majority of people in the world live within 100 kilometers of bays and estuaries (Norse, 1993), and such environments command enormous attention and use. Estuaries have long been associated with:
- some of the world's greatest fisheries for oysters, clams, shrimp, crabs, and fishes. In addition, bays, marshes, and seagrass beds act as well-known ''nursery grounds" for larvae and juveniles of many species (Weinstein, 1979; Boehlert and Mundy, 1988). Yet many of these notable fisheries are at or near collapse because of overfishing, disease, and pollution (e.g., Maurer et al., 1971; Newell, 1988).
- marshes, seagrass beds, and mangroves. Such coastal vegetation is ecologically critical as detrital and nutrient sources driving nearshore production, filters for land runoff, protection from coastal storms, sediment traps, and sediment stabilizers (Fenchel, 1977; Adam, 1990). Marshes also have important aesthetic and recreational value in their "pristine" condition (Teal and Teal, 1969; Chabreck, 1988). They are being rapidly lost, however, because of extensive draining and filling practices (Dahl et al., 1991).
- harbors and marinas. Dredged channels maintain open routes for economically important international shipping. In turn, international shipping means that ballast water is discharged frequently, facilitating the establishment of exotic species (see Boxes 6 and 7) that can significantly change ecosystem structure and function (e.g., Fig. 4). Moreover, floats, docks, and pilings have replaced mud-flats, marshes, and seagrass meadows to provide space for recreational pleasure boats (Zedler, 1994).
- pollution discharge. Estuaries and bays have long received the bulk of human-generated municipal and industrial wastes that enter the oceans from the land, rendering many of them unfit for fisheries production (Kennish, 1992; Schubel, 1994).
These and many other activities in the world's most populated areas adjacent to marine environments mean that estuaries and bays are where the greatest proportion of natural habitat has been destroyed or severely altered (Dahl et al., 1991). Thus, estuaries and bays, and their component marshes, seagrass beds, and mangroves provide striking opportunities for understanding how a profusion of human activities, acting singly and in combination, decrease, maintain, or increase biodiversity (e.g., see Box 5). In many estuaries, the original life is all