and nonmigratory marine species commonly have a mechanism for dispersal through a reproductive larval stage that provides a level of insurance against their localized extinctions. As a consequence of these broad dispersal ranges, many marine species do not show genetic isolation even over large distances (Palumbi, 1992; also see Chapter 5).
Although few marine organisms are known to face extinction as a consequence of endemism and threatened habitat, there are important exceptions. The American Fisheries Society (AFS) recently recognized species vulnerable to extinction. These species generally are long-lived, mature slowly, have low fecundity, are closely associated with particular habitats, and are exceptionally vulnerable to fishing or other anthropogenic stresses. High-seas predators (e.g., tunas, marlins, swordfish, sharks), although not closely associated with seabed habitats, also are vulnerable. In a historic move, AFS has adopted policies that acknowledge the special needs of such species, which may become threatened or endangered if not managed wisely. AFS has recommended MPAs as one management tool to protect species at risk of extinction (Musick, 1999: Coleman et al., 2000).
In the marine environment, mobile species such as fish, marine mammals, and sea turtles, move in three dimensions and have a much greater ability to migrate over long distances than is common for organisms in terrestrial ecosystems. This makes it more difficult to identify discrete populations and blurs the apparent boundaries of marine ecosystems. Also, the relative openness and fluidity of marine ecosystem boundaries increase the likelihood that they will be subject to external influences such as pollution from surrounding lands and waters (Steele, 1985, 1991).
Another difference between terrestrial and marine ecosystems is that most seafood is obtained by fishing, not farming. Wild stocks of fish, not aquaculture, remain the major source of the world's seafood (New, 1997; Naylor et al., 1998, 2000), while land-based agriculture, not hunting, is the main terrestrial food source. Therefore, the continued supply of seafood for human consumption is dependent on sustainable fishing practices for the foreseeable future or until mariculture becomes independent of fish-based food sources. Finally, in contrast to the plants and herbivores that dominate terrestrial food production, most exploited fish species are carnivores, and their depletion may have cascading influences on marine food webs, such as the expansion of herbivore populations and subsequent declines in algal coverage from increased grazing pressure.
In socioeconomic terms, a fundamental difference between the use and management of resources in the sea and on land arises from historical perceptions or definitions of ownership and the laws and conventions that govern these activities. On land, problems arising from common property rights have been summa-