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).