Trends in Ecology & Evolution
ReviewPredicting ecological consequences of marine top predator declines
Section snippets
Declines in marine top predators
Predators that occupy high trophic levels in marine habitats, including marine mammals, large teleosts and sharks, have been declining worldwide at a rapid pace 1, 2, 3, 4. Recent estimates suggest that populations of large sharks have declined regionally by 90% or more 3, 5. The status of large tuna, billfish and groundfish [2] and reef-associated predators in human-impacted areas [6] is equally dire. Although the magnitude of some declines is debated, few researchers doubt the generality of
Marine communities change when top predators decline
Predicting the ecological consequences of reductions in top predators is, in essence, an inquiry into the importance of top-down processes. From groundbreaking work on rocky intertidal shores [8] to the documentation of the keystone role of sea otters in kelp forests [9] and studies of the indirect effects of bird predation [10], among many other examples, there is little doubt that predators have a fundamental influence on the structure and function of marine communities. Hence, widespread
The importance of risk effects
Researchers of marine systems involving large-bodied predators often implicitly assume that trophic cascades occur via direct predation (so-called lethal effects) on mesoconsumers. Using this framework, the effects of predation could be fully quantified based on the diets, metabolic rates and abundances of predators and data on prey population dynamics [18]. Declines in top predator abundance should release mesoconsumers from predation and indirectly increase the mortality rate of resource
Risk effects of marine top predators
The influence of marine top predators such as tuna, sharks and marine mammals on the behavior of mesoconsumers has not yet been evaluated comprehensively [32], and few studies measure the indirect community effects that might arise from these interactions. However, those studies that have done so suggest that risk effects likely are common and might be transmitted to lower trophic levels. For example, the presence of New Zealand fur seals Arctocephalus forsteri causes a temperate reef fish
Identifying key interactions and species
Considering risk effects raises interesting questions that are relevant to understanding the ecological consequences of removing marine top predators. First, what factors influence the relative importance of risk effects and those of direct predation? Second, which top predators are most likely to have disproportionate effects on their communities (through both mechanisms) relative to their abundance (i.e. are keystone species, sensu [42])? Small-scale experiments and field studies in
Toward a predictive framework
Recent studies suggest that marine top predators can exert considerable effects on their prey that can cascade through marine communities. These overall effects might not be caused solely by direct predation; rather, risk effects can contribute a large component of overall predator effects. In most cases, therefore, predictions about how communities will respond to marine predator declines should be improved by an understanding of risk effects and behaviorally mediated indirect interactions.
Acknowledgements
We thank Pieter Folkens, Lindsay Marshall, Elliott Lee Hazen and Roger Hall (http://inkart.net) for artwork and photos. Julia Baum, Craig Layman, Ray Heithaus, Jim Estes and four anonymous reviewers provided helpful comments. This work was supported by NSF grant OCE-0526065 to M.R.H., a Lenfest Ocean's Program grant to B.W. and North Pacific Research Board and Prince William Sound Science Center grants to A.F.
Glossary
- Behaviorally mediated indirect interaction
- occurs when changes in the abundance of one species results in a change in the behavior of a second species (a risk effect) that in turn influences a third species.
- Density-mediated indirect interaction
- occurs when changes in the abundance of one species affect the density of another species through direct predation, which in turn changes densities of a third species.
- Direct predation effect
- effects of predator-inflicted mortality on prey populations.
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