Ecosystem Modeling

Sharks are considered to be apex predators in marine communities and occupy similar trophic levels to carnivorous marine mammals (Cortés, E. 1999. Standardized diet compositions and trophic levels of sharks. ICES J. Mar. Sci. 56:707-717.). As such, they are hypothesized to have a major influence on the structure and function of marine communities (Stevens, J.D., R. Bonfil, N.K. Dulvy, and P.A. Walker. 2000. The effect of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems. ICES J. Mar. Sci. 57:476-494.).

There are conflicting views surrounding the ecological interactions between sharks and fisheries. One view suggests that removals of top predators or keystone species (Paine, R.T. 1980. Food webs: linkage, interaction strength and community infrastructure. J. Anim. Ecol. 49:667-685.) are thought to cause a cascading trophic effect within the remaining community (Carpenter, S.R., and J.F. Kitchell. 1993. The trophic cascade in lakes. Cambridge University Press, Cambridge, U.K.). These effects may involve changes in species composition among the prey or changes in the preferred prey of the predator. An alternate view has been suggested that the high diversity of oceanic systems may oppose strong top-down effects (Strong, D.R. 1992. Are trophic cascades all wet? Differentiation and donor-control in speciose ecosystems. Ecology 73:747-754.; Jennings, S., and M.J. Kaiser. 1998. The effects of fishing on marine ecosystems. Adv. Mar. Biol. 34:201-352.).

Ecosystem modeling, focusing on the role of sharks as predators, is being conducted by the Shark Population Assessment Group using Ecopath/Ecosim models to gain an improved understanding of the role of sharks in marine communities and an evaluation of the impacts of shark fishing on the marine ecosystem.

For an Ecopath-Ecosim model of Apalachicola Bay, Florida, used in Carlson (In Review), see:

This model works with the Ecopath software available for download at: www.ecopath.org.