A study from Nature journal finds that sharks are virtually absent on many of the world’s coral reefs. Out of 371 reef systems in 58 countries, sharks are “functionally extinct” in roughly 20% of them.
Sharks are vital to the balance of marine ecosystems (Ferretti et al. 2010). They are an invaluable keystone species with disproportionate effects on their biological community relative to their abundance. Sharks play their role as apex predators by removing weaker members of other species, avoiding overpopulation. Keystone species are especially crucial for ecosystems as they act as the protectors of evolution and biodiversity (Carrier, Musick, and Hetithaus, 2010).
Declines or removal of sharks from their environments can cause a cascading domino effect of overpopulation by their main prey species, which can sometimes cause entire ecosystems to shift into a very different state. Economically viable fish populations change with the ecosystem as well. The ecological domino effect is commonly known as a trophic cascade, a dramatic change in an ecosystem caused by the removal of a top predator, and it has already been proven to hinder local economies.
The results of the Nature study signify that sharks are not playing their typical role in the tested ecosystems. Almost no sharks were detected on any of the 69 reefs of the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles, and Qatar. Eight hundred survey hours resulted in only three shark sightings in these countries.
Sharks keep a coral reef balanced by hunting the predators that feed on small grazing fish. In return, grazers keep coral free of seaweed and other parasites that would otherwise out-compete the reef. A balanced community, with sharks atop the delicate peak of the food web, gives a coral reef significantly higher resilience in the face of damage and disaster (Attenborough, 2019). Along with climate change and ocean acidification, the removal of sharks has plagued the once luscious reef systems with green and brown algae, such as the reef systems off the coast of Akumal, Mexico in the Mesoamerican Barrier Reef (Nathan Perisic, personal communication, July 24th, 2018).
Coral reef systems are suffering immensely from direct climate change impacts, per se Heron et al. (2017); therefore, for the sake of the coral reef, shark populations need to remain minimally disturbed to contribute to the resilience of these ecosystems in the face of climate change. Positively, the study notes that shark populations remain healthy in the Bahamas and Australian waters, where they are protected from overfishing.
Congratulations to Aaron MacNeil, Demian Chapman, and Joshua E. Cinner for their hard work.