Sharks as Keystone Species

Why it is Important to Protect Sharks?- Sharks Are Important Keystone Species in a Marine Environment: 

Sharks as apex predators can regulate species abundance, distribution and diversity, which in turn can impact the health of marine habitats. Additionally, they provide essential food sources for scavengers and remove the sick and weak from populations of prey species. The decimation of these important shark species can have cascading effects throughout the ecosystems they inhabit, resulting in economically and ecologically devastating consequences. 

Tiger sharks in Shark Bay, Australia are an important keystone species

An important example of how sharks serve as the keystone species in a marine ecosystem is the tiger shark that directly influence the bottom sea grass community in Shark Bay, Australia.

A food web of the Shark Bay ecosystem (OF:Original Figure)

As the top predator, tiger sharks directly or indirectly limit the behavior and population of other species in the food web. Dugongs and green sea turtles are staples of the tiger shark diet and the presence of a shark intimidates these two species to graze lower quality sea grass near the patch edges and continually modifying their distribution. This allows the sharks to prevent overgrazing of the sea grass and preserving the benthic community which relies on the sea grass.
Tiger sharks also influence the location and distribution of bottlenose dolphins even though they are rarely consumed by the sharks. In their presence, dolphins avoid feeding in the productive shallow waters. Once the sharks leave, dolphins and other piscivorous species are free to feed in all habitat types. Not only do they exert control on sea grass grazing, tiger sharks also influence and limit feeding of the fish community. Removal of the keystone species would lead to an imbalance in the food web by having over-exploitation of sea grass and fish by uncontrolled populations of turtles, dugongs, and dolphins. 

Sources:
 http://oceana.org/sites/default/files/o/fileadmin/oceana/uploads/Sharks/Predators_as_Prey_FINAL_FINAL.pdf
http://education.nationalgeographic.com/education/activity/keystone-species-shark-bay/?ar_a=1