munities from other sources of variation, such as climate change (Jennings et al., 2001).

In some cases, removal of one species can have cascading effects on the rest of the ecosystem. For example, the combination of disease and high harvest rates over the past 150 years has reduced oyster density in the Chesapeake Bay to less than 1 percent. The loss of the filter-feeding oyster’s capacity to consume algae is hypothesized to be partially responsible for the proliferation of algal blooms. This appears to have shifted the composition of the pelagic community from mesozooplankton and fish to a community dominated by predatory jellyfish and comb jellies (Caddy, 1993; Ulanowicz and Tuttle, 1992).

RATES OF RECOVERY

Recovery is the return of an ecosystem to a state that existed before a disturbance, as measured by ecosystem processes, species composition, and species interactions. Recovery from trawling will depend on the type and extent of the habitat alteration, the frequency of the disturbance compared with natural changes, habitat characteristics, and species and life history characteristics. Recovery times vary according to the intensity and frequency of the disturbance, the spatial scale of the disturbance, and the physical characteristics of the habitat (sediment type, hydrodynamics). Superimposed on these human-related alterations are natural fluctuations, caused by storms or long-term climate changes, for example.

In most circumstances, only a first-order approximation of recovery rate is possible. Experimental evaluations recovery after cessation of trawling are limited and have focused on biotic recovery of small-bodied, short-lived invertebrates. Despite that, we can make some observations about the amount of physical disturbance that is sustainable in some types of habitat. The meta-analysis by Collie et al. (2000a) showed that recovery rate appears to be slowest in the more stable muddy habitats and biogenic (structurally complex) habitats. By comparison, mobile sandy sediment communities could be able to withstand 2–3 trawl passes per year without changing markedly. It is important to bear in mind, however, that although available data allow for prediction of the recovery rate for small-bodied taxa such as polychaetes (which dominate data sets for sandy sediment communities), less abundant, long-lived, and hence more vulnerable species could recover more slowly.

In some biogenic habitats, physical disturbance by dredging and trawling has a long-lasting effect. For example, clam dredging causes severe and persistent changes to seagrass ecosystems (Peterson et al., 1987; Stephan et al., 2000). After a single pass, seagrass biomass fell by about 65 percent below controls, and recovery did not begin for more than two years with seagrass biomass still roughly 35 percent below controls four years later (Peterson et al., 1987). The abundance of fish and shellfish that depend on seagrass for settling locations for protection from predators could be reduced where seagrass is damaged.

Environmental recovery after disturbance depends on the life histories of the organisms that live in or create the habitat. Recovery time is often one to five times the generation time of the organism (Emeis et al., 2001). Therefore recovery times could range from a few months—or less—to several decades (Hutchings, 2000). Many of the larger biogenic structure-forming organisms, such as soft corals and sponges, are slow growing and long-lived (Dayton, 1979; Leys and Lauzon, 1998). Empirical data about recovery times of corals and coral-line algae are sparse, but recovery times of decades to centuries can be inferred from the age of these organisms.

Recovery from trawling also depends on the size of the area disturbed (Thrush et al., 1998), and on the spatial pattern of the disturbance (Auster and Langton, 1999). Each trawl track is a small disturbance, but over a long enough period and with widespread coverage, the small changes can result in a large effect. The consequent habitat loss, and effects on resident species, depends to a large extent on its scale (Deegan and Buchsbaum, 2002). A single small loss might not, by itself, have an observable effect on species that are not directly damaged by trawling. However, the cumulative impact of many small losses may be quite significant at a regional scale (Odum, 1982). In some coastal ecosystems, mosaic-type damage could allow faster recovery than would a large-scale, isolated disturbance (Emeis et al., 2001).

Areas that are trawled with greater frequency could take longer to recover. Almost all studies have examined recovery after a single, acute pass by a trawl rather than after the multiple passes that are typical in frequently trawled, heavily fished areas. There, recovery would be expected to take longer because a larger fraction of the population is removed and immigration rates are lower (Figure 3.2). Results from the meta-analysis (Collie et al., 2000a) indicate that, on average, a single



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