Ruling from the top-down: Sharks as Apex Predators and the Need for Better Management

by Tom Tascone, RJD Intern

An apex predator is defined as a predator residing at the top of the food web in its ecosystem.  Life at the top has its benefits – reigning supreme in its environment, the apex predator feeds on lower levels in the food chain and has no natural predators of its own, allowing it to enjoy the freedom that comes with being the hunter, not the hunted.  Many examples of apex predators exist in both terrestrial and marine habitats, and if you were to ask anyone, despite their scientific expertise, to name a marine apex predator, you would certainly get some type of shark as your most common answer.

The influence of apex predators like sharks on marine ecosystems, however, has been little studied.  Marine food webs are incredibly large and complex, and encompass a wide variety of species. Additionally, many of these species act as both predator and prey and interact with multiple other species in both capacities, complicating research that much further.  However, as anthropogenic (human-induced) changes to marine ecosystems through activities like commercial fishing continue to deplete many fish populations, sometimes upwards of 90%, growing concern has spurred new research in this area (Baum and Worm 2009).  Focusing on the potential effects that anthropogenic change can have on the structure of marine food webs, this research has identified just how important sharks are in their role as apex predators.

An example of the complexity of marine food webs. Wikimedia commons

A shark can influence its prey, and thus lower levels of the food web, in two ways. These are known to researchers as “direct predation effects” and “behaviorally mediated indirect interactions,” (Heithaus et al. 2008) but don’t worry too much about the technical terms.  Put simply, a shark influences its prey either by eating it or by scaring it.  A good example of this is one we use often at RJ Dunlap and seen on coral reefs near South Florida, where the local shark species feed directly on grouper.  Eating grouper directly reduces the population size of the grouper, which feed on parrotfish. If grouper numbers are larger, they will feed on more parrotfish, reducing the population of parrotfish on the reef.  Sharks, as a result, help to maintain the parrotfish population on a reef by keeping the grouper population in check.  Scaring your prey yields the same outcomes.  By simply being present in the environment, a shark can alter a prey’s behavior, reducing their rate of feeding or perhaps even influencing them to feed on different resources entirely.  The structure of a marine food web can then be altered by an apex predator without direct feeding (Heithaus et al 2008).

The collective influence of these direct and indirect effects on food web structure is known as a “trophic cascade” (Baum and Worm 2009, Heithaus et al. 2008, Schindler et al. 2002). Trophic cascades are natural and can be found in both terrestrial and marine habitats, and research continues to document numerous examples (Baum and Worm 2009). This research highlights not only the importance of the apex predator, but it often highlights the consequences of human actions on the marine environment as well, with haunting urgency.  A paper published by Myers et al. (2007) conveys this urgency all too well.  Using data from several different shark surveys conducted over the last 35 years, Myers and his colleagues compiled annual population estimates for 11 apex predatory sharks in the region.  These annual estimates stretched back to the early 1970’s and confirmed known declines in each species population, which have been linked to overfishing in the shark fin trade or by accidental catch in other commercial fisheries.  At the same time, they also documented the annual population estimates of the prey of these 11 shark species.  Included in their data was the cownose ray, whose explosive growth since the 1990’s has resulted in the collapse of North Carolina’s bay scallop fishery. The results of their study indicated a trophic cascade caused by the decline of these 11 sharks, linking them to the collapse of the bay scallop fishery. The cownose ray is a major prey item for all 11 of these shark species, and their declines allowed cownose ray populations to grow and feed freely on bay scallops, collapsing the fishery which has now been closed to harvesting since 2007.

“So, what’s the message here, Tom?”  I’m glad you asked. Sharks have a tremendous influence on marine food webs in their role as an apex predator, and changes in their abundance result in far-reaching consequences for marine ecosystems (Baum and Worm 2009). Slow growth rates as well as reproductive rates not only make a shark more sensitive to overfishing, but can inhibit recovery as well if their population numbers fall too low (Schindler et al. 2002).  Now more than ever sharks need our help. The time for innovative new management strategies has arrived; strategies that take into account both the direct and indirect ways that sharks influence marine food webs (Heithaus et al. 2008).  These strategies must also take into account the entire ecosystem and understand the complexity of predator-prey interactions (Baum and Worm 2009). Such strategies can help to conserve sharks and the ecosystems they inhabit, not only for them but for us. If we wish to continue enjoying the beauty of our coral reefs and the food that comes from our commercial fisheries, we must push for more proper management of our oceans.

REFERENCES

Baum, J.K and B. Worm. 2009. Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology 78: 699-714.

Heithaus, M.R., Frid, A., Wirsing, A.J., and B. Worm. 2008. Predicting ecological consequences of marine top predator declines. Trends in Ecology and Evolution 23 (4): 202-210.

Myers, R.A., Baum, J.K., Shepherd, T.D., Powers, S.P., and C.H. Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.

Schindler, D.E., Essington, T.E., Kitchell, J.F., Boggs, C., and R. Hilborn. 2002. Sharks and Tunas: Fisheries impacts on predators with contrasting life histories. Ecological Applications 12 (3): 735-748.