Sizing up the ecological role of sharks as predators

Abstract

Top predators and large-bodied megafauna are often the most affected elements of exploited ecosystems, both on land and in the sea; and the top-down control these predators exert on prey species can significantly alter community structure (Pinnegar et al. 2000, Laliberte & Ripple 2004).The loss of predation and the resultant indirect effects of mesopredator release and trophic cascades have resulted in widespread trophic downgrading of ecosystems (Crooks & Soulé 1999, Baum & Worm 2009, Estes et al. 2011).For example, both the loss of wolves from Yellowstone National Park in the USA and declines in largebodied shark populations of the western North Atlantic Ocean have been reported to cause mesopredator release and trophic cascades (Berger & Conner 2008, Baum & Worm 2009, Ferretti et al. 2010).Recently, much of the research focus has been on the response of prey to predators; here, we focus our attention on the ecological role of the predator.Despite some compelling and widely-cited case studies, our understanding of the dynamics of predators in regulating prey populations is still limited, yet predation is recognised as a key ecological and evolutionary process (Estes et al. 2011).Particularly in the marine realm, gaining sufficient knowledge of large, highly mobile predators to define their roles is challenging.The widespread nature of these species makes them difficult to target, handle and experimentally manipulate for the purposes of research.Thus, defining the impacts of large marine predators is problematic.Our understanding of the ecological role of predators is often constrained by the limited range of ways to classify the trophic role of species and individuals.Predators are most commonly defined by their spe-