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Overexploitation, trophic skew, and the crossover between marine and terrestrial systems
The overexploitation of marine species for resource consumption is one of the most serious threats to coastal biodiversity. Examples of historic fisheries collapse are numerous (e.g. Boreman et al., 1997; Myers et al., 1997; Liu & De Mitcheson, 2008), and the unsustainable harvest of many species continues today (Coll et al., 2008). Nearly 60% of global fishery stocks are collapsed or overexploited, with another ~33% fully exploited (Froese et al., 2012), and the demand for fish for dietary protein is ever increasing (Pauly et al., 2002). Species losses appear biased toward higher trophic levels, with total biomass of marine predatory fish reduced by at least 80% in many marine ecosystems (Pauly et al., 1998, Worm & Duffy, 2003). This phenomenon is largely due to increased body mass and low reproductive rate (Byrnes et al., 2007). However, recent analyses suggest that fisheries collapse is biased toward small, low trophic-level species (Pinsky et al., 2011). In either case, overharvesting has severe direct impacts on targeted species.
The non-random loss of marine species also has several indirect impacts on marine ecosystems, including habitat loss and altered food webs (Botsford et al., 1997; Jackson et al., 2001). Prior research has focused on the top-down effects of the loss of marine predators. Reductions of top consumers can skew the distribution of biodiversity toward lower trophic-level species, affecting ecosystem function in several ways (Reynolds & Bruno, 2012). For example, in the Gulf of Maine, the loss of predator diversity in seagrass beds increased organic sediment loads (Duffy et al., 2003). In addition, the overharvesting of top and intermediate consumers, combined with the introduction of lower trophic-level non-native species, has suppressed the recruitment of many native fish species (Levin et al., 2002).
The effect of human disturbance on migrant birds is a conservation issue of international importance, as is determining if disruption has long-term population effects. Disruptions can occur during migration, wintering, breeding and foraging. Thousands of shorebirds migrate through Delaware Bay (Atlantic Coast of North America) in a four-week period each spring; this is the largest concentration of shorebirds in the continental USA. Ecotourists come to see them, creating the potential for disruption. Data available on shorebird/human interactions at a migratory stopover over a 20-year period were used to describe the interactions of shorebirds and people from 1982–2002 and examine trends in human disruptions and shorebird behaviour during this time. The rate of disruptions caused by people increased during the 1980s, declined slightly by the early 1990s, and declined sharply by 2002. The decline in human activity along the beach was directly related to the conservation efforts of the New Jersey Endangered and Nongame Species Program, New Jersey Audubon, and others interested in preserving the shorebirds. In the 1980s, birdwatchers concentrated on the beaches on which it was easy to walk and which had the highest shorebird counts, because there were no restrictions on human behaviour. During this time, the average disturbance duration was over 10 min, regardless of the type of intruder, and shorebirds were often disrupted for over 40 min hr−1. Even though the number of disruptions declined over the study period, the percentage of shorebirds that flew away (and did not return within 10 min) did not change during the 1980s, and increased in 2002. The average time that shorebirds were disrupted per hour by people declined during this period (mainly because there were fewer people on the beaches). The Endangered and Nongame Species Program placed signs on shorebird foraging beaches, restricted access, built viewing platforms to contain ecotourists, and eventually patrolled key beaches and issued summonses for infractions. These activities were so effective that only one bird watcher disturbed the birds in 2002. Education was also vital to encouraging local residents not to walk or fish along these beaches during the spring migratory stopover, and to keep their dogs on a leash. These data support the importance of actions on the part of state agencies and conservation organizations to limit disruptions to foraging shorebirds during critical migratory stopovers, a problem faced by shorebirds in many temperate regions of the world.
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