Patterns and ecosystem consequences of shark declines in the ocean.
Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding. Large sharks in particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities. We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure. The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top-down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects. In conclusion, large sharks can exert strong top-down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.
Ferretti
Francesco
F
Biology Department, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia B3H4J1, Canada. ferretti@dal.ca
Worm
Boris
B
Britten
Gregory L
GL
Heithaus
Michael R
MR
Lotze
Heike K
HK
D000818:Animals; D044822:Biodiversity; D003247:Conservation of Natural Resources; D005398:Fisheries; D020387:Food Chain; D008954:Models, Biological; D009792:Oceans and Seas; D011157:Population Dynamics; D011235:Predatory Behavior; D012754:Sharks / Q000502:physiology*