Sharks, skates, and rays belong to a taxonomic subclass called Elasmobranchii, sometimes referred to Elasmobranchs. Four hundred million years ago, way back in the Devonian era, these cartilaginous fishes first appeared in the fossil record (Fowler et al., 2005). And they have hardly had to change their basic form since (Fowler et al., 2005)!
Sharks are essential to healthy ecosystems. With over 400 species of elasmobranchs, sharks and rays hold a variety of roles within their ecosystems as apex and meso predators. Apex predators dominate their ecosystems with highly specialized predator-prey relationships and exert top-down control over their community structure (Estes et al., 2011). This means their presence in the ecosystem is what keeps the ecosystem in balance, like a keystone in an archway. They are often referred to as keystone species because of this. Meso-predators are mid-level predators that are generalists. They have a much more diffuse prey pattern than apex predators and do not influence other species within their community as strongly (Heupel, Knip, Simpfendorfer, & Dulvy, 2014). So while species like the great white shark (Carcharodon carcharias) are apex predators, most species of sharks and rays are mid-level predators and fall under the meso-predator category (Heupel, et al., 2014).
Over the last century, global shark and ray populations have begun to see sudden declines in populations (Robbins, Hisano, Connolly, & Choat, 2006). In the North Atlantic Ocean, documented declines in large bodied sharks have led to top-down trophic cascades (Heupel, et al., 2014). For example, following a collapse of large shark populations on the eastern coast of the United States, another elasmobranch species, the cownose ray (Rhinoptera bonasus), filled in the gap. This is referred to a mesopredator release: when a mid-level predator population increases due to the lack of predation and competition from an apex predator. The cownose ray population increase eventually led to the depletion of its prey source: the nearshore bay scallop (Baum & Worm, 2009).
According to the International Union for Conservation of Nature (IUCN), nearly 25% of elasmobranchs are vulnerable to extinction (Magiera, 2014). Bony fishes exhibit r-selected species characteristics, meaning they have high growth rates, are quick to sexual maturity, and have high rates of reproduction. However, sharks and rays are k-selected species, which means they have slow life history characteristics. They have long life spans with slow growth rates, late in life sexual maturity, and low fecundity- or ability to produce offspring (Chin, Kyne, Walker, & McAuley, 2010). All of these traits leave them vulnerable to rapid changes in their environments such as habitat destruction or loss, climate change, and over exploitation due to fishing. All of these anthropogenic influences are likely driving factors of steep population declines in elasmobranchs in the last century (Knip, Heupel, & Simpfendorfer, 2010; Ward-Paige et al., 2010).
Over the next few weeks I will be exploring each of these anthropogenic influences: Habitat loss, Climate Change, and Fishing Pressures. I encourage you to follow along, get involved, ask questions, and leave comments!
Featured Image Source
Tiger Shark [Digital Image] Wiki Media Commons. Retrieved From https://upload.wikimedia.org/wikipedia/commons/5/5c/Tigershark3.jpg
Baum, J. K., & Worm, B. (2009). Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology, 78(4), 699–714.
Chin, A., Kyne, P. M., Walker, T. I., & McAuley, R. B. (2010). An integrated risk assessment for climate change: Analysing the vulnerability of sharks and rays on Australia’s Great Barrier Reef. Global Change Biology, 16(7), 1936–1953.
Estes, J. a, Terborgh, J., Brashares, J. S., Power, M. E., Berger, J., Bond, W. J., … Wardle, D. a. (2011). Trophic downgrading of planet Earth. Science (New York, N.Y.), 333, 301–306.
Fowler, S. L., Cavanagh, R. D., Camhi, M., Burgess, G. H., Cailliet, G. M., Fordham, S. V., … Musick, J. A. (2005). Sharks, rays and chimaeras: The status of the chondrichthyan fishes. IUCN- The World Conservation Union (Vol. 14).
Heupel, M. R., Knip, D. M., Simpfendorfer, C. A., & Dulvy, N. K. (2014). Sizing up the ecological role of sharks as predators. Marine Ecology Progress Series, 495, 291–298.
Knip, D. M., Heupel, M. R., & Simpfendorfer, C. A. (2010). Sharks in nearshore environments: models, importance, and consequences. Marine Ecology Progress Series, 402, 1–11.
Magiera, E. (2014). A quarter of sharks and rays threatened with extinction. Retrieved March 23, 2017, from https://www.iucn.org/content/quarter-sharks-and-rays-threatened-extinction
Robbins, W. D., Hisano, M., Connolly, S. R., & Choat, J. H. (2006). Ongoing collapse of coral-reef shark populations. Current Biology, 16(23), 2314–2319.
Ward-Paige, C. A., Mora, C., Lotze, H. K., Pattengill-Semmens, C., McClenachan, L., Arias-Castro, E., & Myers, R. A. (2010). Large-scale absence of sharks on reefs in the greater-Caribbean: A footprint of human pressures. PLoS ONE, 5(8).