In today’s Academic Minute, Dr. Jonathan Ruppert of York University describes the connection between shark population and reef health.
Jonathan Ruppert is a postdoctoral research associate in the Department of Biology at York University in Toronto, Canada. His current research project involves the long-term monitoring of the health of reefs on Australia’s northwest coast. He holds a Ph.D. from the University of Toronto.
Dr. Jonathan Ruppert - Sharks and Reef Health
It has been well established that sharks do well on coral reefs with very little human disturbance. That is, you will tend to find a high abundance of sharks where human activities are minimized such as fisheries. Fisheries, including those involved with the shark-fin soup trade, can be quite damaging to shark populations as many species of sharks will require centuries to recover from even very low fishing pressures. As the effect of this trade has become global, we are now currently observing worldwide declines in these top-predators on coral reefs.
Further complicating this story is the presence of many different environmental stressors that can impact species and ecosystems at the same time. On coral reefs, fisheries and benthic disturbances are some of the most prevalent. While fisheries directly remove individuals from the reef, benthic disturbances, such as cyclones and coral bleaching, transform the colourful, biodiverse reef surface to a reef dominated by green algae.
Recently, I was part of an international collaboration to determine what effect declines in sharks may have on coral reef ecosystems that experience benthic disturbances such as cyclones and coral bleaching. We found that shark-fin fisheries are causing dramatic declines in shark populations and this subsequently impacts species that are found lower down in the food web, an effect that is termed a trophic cascade. With declines in sharks we observed increases in smaller fish eating carnivores and decreases in herbivores that consume algae. This last point is crucial, as herbivores are critical to the removal of algae produced from disturbances that result in the loss of coral cover. The consumption of algae by herbivores allows coral to re-establish and for coral reefs rebuild after disturbance events. Thus, sharks play an important role by increasing herbivore abundance and creating ecosystems that are more resilient to benthic disturbances.