Effects on Communities and Ecosystems

The effects of exploitation at the level of the community and ecosystem are often complex. They can generally be classified into direct and indirect effects. Direct effects o ^

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Figure 2 Simplification of marine ecosystem by exploitation. Note the sequential depletion of large, medium, and small species, and the concomitant destruction of seafloor habitats. The latter is often caused by the use of destructive exploitation methods, such as trawling or dynamite fishing. Adapted from Pauly D and MacLean J (2003) In a Perfect Ocean. Washington, DC: Island Press.

Figure 2 Simplification of marine ecosystem by exploitation. Note the sequential depletion of large, medium, and small species, and the concomitant destruction of seafloor habitats. The latter is often caused by the use of destructive exploitation methods, such as trawling or dynamite fishing. Adapted from Pauly D and MacLean J (2003) In a Perfect Ocean. Washington, DC: Island Press.

concern the removal of the target species, often primarily large species. This often shifts the size structure of the community from large, long-lived species to smaller, shorter-lived species (Figure 2). Indirect effects are secondary changes that concern nontarget species or habitats. For example, we often observe release of prey species after their predators' density has been reduced by exploitation. Other species may increase because their competitors have been targeted. Others may decrease because a positive interaction (facilitation or mutualism) with another species is disrupted by exploitation of that species. Thus, indirect effects of exploitation often arise through the alteration of trophic, competitive, or positive interactions. This, in turn, can lead to ripple effects across food webs, such as trophic cascades.

Trophic cascades can be envisioned as a domino effect, where the removal of a predator causes fluctuations in prey populations, across two or more trophic levels. The overexploitation and collapse of Atlantic cod (Gadus mor-hua) stocks in eastern Canada, for example, has triggered increases in shrimp, crab, and herring populations. Herring and other small pelagic fish, however, through their predation on cod eggs and larvae, may inhibit the recovery of cod from overfishing. As such, overexploitation may shift an ecosystem to a permanently altered state, which may be difficult to reverse. Similar effects have been seen on land, where the removal of predators such as wolves across North America was followed by large increases in the ungulate prey, such as deer and moose. These large grazers could then locally control the vegetation, and slow forest growth or recovery of deforested areas.

Another indirect effect of exploitation is the incidental harming of nontarget species and habitats (Figure 2). This clearly depends on the method used for extraction. For example, dragging large fishing nets (called bottom trawls) across the seafloor or clear-cutting large tracts of land often leads to the destruction of habitats, harming other species, which are not the primary focus of exploitation. Thereby, the effects of exploitation can become much more widespread than originally envisioned. The question of unintended by-catch is particularly problematic in industrialized fisheries. The large-scale (10-100 km), mechanized extraction of fish and invertebrates with bottom trawls, longlines, or drift nets can produce more by-catch than target catch, for example, in tropical shrimp fisheries, where the shrimp catch is often less than 10% of the total. Because fisheries are typically managed to optimize harvest of the target species, by-catch species which are not landed or monitored, such as sharks, albatrosses, or sea turtles, may be decimated to very low levels before this is even noticed.

Exploitation of animals often progresses in a systematic way through the food web, from top to bottom (Figure 2). In aquatic ecosystems, this trend has been called fishing down food webs. It can cause significant damage to ecosystems by depleting top trophic levels, and switching to progressively lower trophic levels, as these become more abundant. As lower levels succumb to increased exploitation pressure, recovery ofthe top levels, which depend on lower levels for food, becomes more difficult. A well-documented example is the outer Bay of Fundy in eastern Canada where intensive fishing over the last 200 years progressed from very large cod (>1m average length) to small cod, to herring, to invertebrates, and finally seaweeds. Similar trends were seen in the archeological record of terrestrial ecosystems, where exploitation of land mammals by human hunters progressed over time to smaller and smaller species.

The removal of entire trophic levels by exploitation can also affect species diversity and the functioning of ecosystems. Removal of large predators or herbivores in particular can unleash fast-growing prey species from consumer control. These may then outcompete other species, causing a decline in diversity, and changes to ecosystem functioning. Coral reefs, for example, are often heavily affected by fisheries exploitation. It has been shown that the exploitation of large herbivorous fishes can cause fast-growing algae to increase in abundance and replace corals. The algae-dominated reef often has less species and very different functioning than the coral dominated reef. Such a fundamental change in species composition, diversity, and ecosystem functioning is sometimes referred to as a regime shift.

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