Tags: Biology,  Environment 

Sharks' Role in the Oceans

Sharks play a very important role in the oceans in a way that an average fish does not. Sharks are at the top of the food chain in virtually every part of every ocean. In that role, they keep populations of other fish healthy and in proper proportion for their ecosystem.  How do sharks keep the oceans healthy?

Sharks keep food webs in balance

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Sharks have evolved in a tight inter-dependency with their ecosystem. They tend to eat very efficiently, going after the old, sick, or slower fish in a population that they prey upon, keeping that population healthier. Sharks groom many populations of marine life to the right size so that those prey species don’t cause harm to the ecosystem by becoming too populous. 

The ocean ecosystem is made up of very intricate food webs.  Sharks are at the top of these webs and are considered by scientists to be “keystone” species, meaning that removing them causes the whole structure to collapse.  For this reason, the prospect of a food chain minus its apex predators may mean the end of the line for many more species.  A number of scientific studies demonstrate that depletion of sharks results in the loss of commercially important fish and shellfish species down the food chain, including key fisheries such as tuna, that maintain the health of coral reefs.

Sharks Role Slideshow

See how eliminating the apex predator affects the entire ecosystem

Sharks keep prey populations healthy

Predatory sharks prey on the sick and the weak members of their prey populations, and some also scavenge the sea floor to feed on dead carcasses.  By removing the sick and the weak, they prevent the spread of disease and prevent outbreaks that could be devastating. Preying on the weakest individuals also strengthens the gene pools of the prey species.  Since the largest, strongest, and healthiest fish generally reproduce in greater numbers, the outcome is larger numbers of healthier fish.

Sharks keep sea grass beds and other vital habitats healthy

Through intimidation, sharks regulate the behavior of prey species, and prevent them from overgrazing vital habitats.  Some shark scientists believe that this intimidation factor may actually have more of an impact on the ecosystem than what sharks eat.  For example, scientists in Hawaii found that tiger sharks had a positive impact on the health of sea grass beds.  Turtles, which are the tiger sharks’ prey, graze on sea grass.  In the absence of tiger sharks, the turtles spent all of their time grazing on the best quality, most nutritious sea grass, and these habitats were soon destroyed.  When tiger sharks are in the area, however, turtles graze over a broader area and do not overgraze one region.

An important lesson: we need sharks!

Where sharks are eliminated, the marine ecosystem loses its balance.

In the parts of the ocean where sharks have been fished out of existence, we can see the dangerous result of removing the top predator from an ecosystem.

The lesson is important. Sharks are being killed for their fins for shark fin soup, a food that has assumed cultural value but is not important for human survival or health. However, removing the sharks can result in the loss of important foods that we do depend upon for survival.

Sharks have survived for 450 million years, but may be gone within the next decades. Life within the oceans, covering 2/3rds of our planet, has enjoyed a relationship with sharks for about 450 million years. Our growing demand for shark fin soup has increased the slaughter of sharks to such a great extent that many shark species are already nearing extinction.

What will the health of oceans be like when such an important group of animals have been destroyed? Do we want the destruction of sharks and the oceans to be the legacy we leave for our children?

Additional Resources

Declines in large shark populations on the East Coast of the US led to the collapse of North Carolina's century-old bay scallop fishery. (Source: Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean - Ransom A. Myers - Dalhousie University - 2007).

A study by Enric Sala of Scripps Institute of Oceanography suggests that the loss of sharks may have contributed to the decline of reefs in the Caribbean, most of which are now dominated by algae. (Source: New Scientist - April 23, 2005)

Studies of remote, pristine ecosystems demonstrate the positive impacts of the presence of sharks, including greater biodiversity, larger numbers of fish and healthier sea grass beds in areas with healthy populations of sharks as compared to similar systems in which the sharks have been overfished. (Source: High apex predator biomass on remote Pacific Islands. Stanford University)

“When a shark fishery in Tasmania experienced a population collapse, octopus, which they had fed on, exploded. Octopi feed on spiny lobsters, so lobsters crashed, and fishermen were put out of work.” (Helfman. 2007. Fish Conservation: A Guide to Understanding & Restoring Global Aquatic Biodiversity & Fishery Resources”)

[From the removal of tiger sharks] “Some unexpected outcomes also occurred: a total and rapid crash in the abundance of tuna and jacks, and an increase in bottom fishes.”  (The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems. – ICES Journal of Marine Science, 57: 476–494.See p. 488 for the tuna reference. Stevens, J. D., Bonfil, R., Dulvy, N. K., and Walker, P. A. 2000)

Observations from Palau, which is now a Shark Sanctuary, indicate that biodiversity has improved since they began to protect sharks. (Based on a conversation with an official with the Palau Mission to the United Nations)

Predicting ecological consequences of marine top predator declines. A Summary of New Scientific Analysis:  Heithaus, M.r, Frid, a., Wirsing, a.j and Worm, B. 2008. Trends in Ecology and Evolution 23(4):202–210.

The role of sharks in the ecosystem by Mike Bennett 2005 - School of Biomedical Sciences, The University of Queensland, St. Lucia, Queensland, 4072 Australia

Coupled changes in structure and function in reef fish assemblages: how predators increase fisheries production: Sandin, S. A. Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0202

Ecologically justified charisma: preservation of top predators delivers biodiversity conservation
Journal of Applied Ecology 2006, FABRIZIO SERGIO, IAN NEWTON,LUIGI MARCHESI, and PAOLO PEDRINI

High apex predator biomass on remote Pacific islands
Charlotte Stevenson, Laure S. Katz, Fiorenza Micheli, Barbara Block, Kimberly W. Heiman, Chris Perle, Kevin Weng, Robert Dunbar, Jan Witting

Ongoing Collapse of Coral-Reef Shark Populations
Current Biology 16, 2314–2319, December 5, 2006, William D. Robbins, Mizue Hisano, Sean R. Connolly, and J. Howard Choat

Predators as Prey: Why Healthy Oceans Need Sharks
Griffin, E., Miller, K.L., Freitas, B. and Hirshfield, M. July 2008
Fish Conservation: A Guide to Understanding & Restoring Global Aquatic Biodiversity & Fishery Resources  - Helfman, 2007

Cascading top-down effects of changing oceanic predator abundances
Journal of Animal Ecology. Vol. 78. p. 699. Baum & Worm. 2009.

Loss of top predators causing surge in smaller predators, ecosystem collapse
Oregon State University, William Ripple

Direct and indirect fishery effects on small coastal elasmobranchs in the northern Gulf of Mexico
Travis D. Shepherd and Ransom D. Myers.

Status and Conservation of Oceanic Pelagic sharks
Dulvy et al. 2008

Long-term declines in Two Apex Predators, Bull Sharks (Carcharhinus leucas) and Alligator Gar (Atractosteus spatula) in Lake Pontchartrain, an Oligohaline estuary in SE LA.
O’Connell et al. 2007

Effects of Losing Top Predators - Predicting ecological consequences of marine top predator declines.
Lenfest Ocean Program  - A summary of new scientific analysis –  Heithaus, Wirsing, Worm 2008

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