differences in spatial scales of the habitats and contrasts between life strategies in water and on land. Marine ecosystems also may be more variable than terrestrial ecosystems, especially on shorter time scales. Marine ecosystems are subject to the physics of the surrounding medium and respond to forces such as tides, circulation patterns, and decadal shifts in overall productivity, whereas terrestrial ecosystems are more internally controlled by the life processes of the dominant organisms (e.g., trees) and may change only slowly, sometimes on century time scales, unless humans intervene (Steele, 1985, 1991, 1996).
On land, survival of rare or endangered species is especially dependent on habitat, which often plays a decisive role in identifying areas worthy of protection. The case for protection of a terrestrial area to save a species from extinction has provided powerful arguments for garnering public support. Habitat destruction accounts for about 36% of animal extinctions whose cause is known (compared to 23% due to hunting and 39% due to introduced species) and is thought to be even greater for the extinction of terrestrial species where the cause is unknown (Groombridge, 1992). As people increase their use of the land, habitats to support terrestrial species will continue to decline, both from destruction and from fragmentation into areas too small to support indigenous populations.
Human populations appear to have less impact on marine habitats because people do not live in the ocean and thus are less aware of the change. The loss of marine habitat, except for wetlands and estuarine marshes, has been documented infrequently, and population declines or extinctions in marine species are more often attributed to overexploitation. Historically, the concept of conserving critical habitat for endangered marine species has been applied mostly to marine mammals, sea turtles, and sea birds, with only occasional application to endemic fishes or invertebrates (Kelleher and Kenchington, 1992). However, the dramatic loss of coastal wetlands (NRC, 1992) and recent descriptions of the impacts of trawling gear on the seabed (Watling and Norse, 1998), among other stresses, have led to increased attention to the vulnerability of some marine species to extinction from loss of habitat (Roberts and Hawkins, 1999). Even more common is the decrease in genetic diversity from the loss of distinct populations associated with habitat at a discrete site.
In selecting areas for protection, several concepts applied to terrestrial reserves are also important for marine reserves, including sources and sinks, dispersal range, and metapopulations (see Chapter 6). When the range of a species is large and the density of the population is relatively low, it may be impractical to design a reserve that is large enough to protect the species. On land, the solution may require establishing several reserves connected by corridors that allow the physical passage of species. In the ocean, water provides the corridor, and the design issue rests on an understanding of currents and circulation patterns or other oceanographic features that will either facilitate or impede the dispersal of individuals among reserves (see Chapter 6). Also, even sedentary