Résumé
Sharks and their relatives fill many roles in their ecosystems, including as predators, competitors, and prey, as well as facilitators of other species. In this chapter, we review and synthesize empirical studies of the ecological importance of elasmobranchs within a theoretical context and relative to key marine ecosystems. Although sharks and their relatives do not always play critical roles in ecosystems, a growing number of studies provide insights into where, when, and why they might be important to overall ecosystem dynamics. Research gaps remain, but in general elasmobranchs likely play a greater role in marine communities when (1) they prey upon, or threaten, longer-lived species that are more likely to invest in anti-predator behavior or are more susceptible to predation; (2) they are the primary predator on a limited number of prey and, therefore, could have larger impacts on population sizes of these species; (3) they prey upon or threaten species that are likely to play important ecological roles themselves (e.g., large herbivores); (4) their predation is focused on life-history stages of prey where density-dependent selection occurs (although trophic cascades are possible in the absence of effects on meso-consumer population sizes); and (5) they are transporting nutrients in generally oligotrophic ecosystems. Further work is needed to definitively make the link between loss of sharks and ecosystem disruption and to predict when and where it will be most pronounced.