Defining Keystone Species
A keystone species is an organism that has a disproportionately large impact on its environment relative to its abundance. The concept was introduced in 1969 by zoologist Robert T. Paine, based on his research in the intertidal zones of the Pacific Northwest. Paine observed that removing the sea star Pisaster ochraceus led to a decline in biodiversity, as mussels, normally preyed upon by the starfish, overran the area. This demonstrated how some species are essential for maintaining ecosystem structure and diversity.
Sharks’ Pivotal Ecological Roles
Sharks function as keystone species as apex predators, occupying the highest levels of marine food webs. They regulate populations of other marine animals, including fish and marine mammals. This prevents any single prey species from becoming overabundant, which could destabilize the ecosystem. By preying on weak, sick, or injured individuals, sharks also contribute to the overall health and genetic fitness of prey populations.
Beyond direct predation, sharks influence the behavior and distribution of prey species. Their presence creates a “landscape of fear,” causing prey to avoid certain areas or alter foraging patterns. This behavioral influence helps prevent overgrazing of marine habitats like seagrass beds and coral reefs. For instance, tiger sharks deter grazers like sea turtles and dugongs from lingering, allowing seagrass to regrow and thrive.
Sharks also maintain the health of coral reefs. They regulate populations of mid-level predators, such as groupers, which prey on herbivorous fish. If these mid-level predators increase unchecked, they can decimate herbivore populations. Herbivorous fish graze algae that can overwhelm corals, so sharks’ indirect support of these grazers aids reef resilience. This top-down control by sharks ensures the food web remains balanced, supporting diverse marine life and healthy habitats.
Ecosystem Impacts of Shark Decline
The decline of shark populations can lead to imbalances within marine food webs, resulting in trophic cascades. When predatory sharks are removed, populations of their prey, particularly mid-level predators, can surge. This phenomenon, sometimes called “mesopredator release,” has consequences for the ecosystem.
An increase in mid-level predators can lead to overconsumption of species. For example, a decline in sharks that prey on cownose rays has led to an increase in ray populations, which heavily consume shellfish like scallops, causing fisheries to collapse. Similarly, on coral reefs, unchecked growth of mid-level predators like groupers can reduce herbivorous fish. With fewer herbivores to graze on algae, reefs can become overgrown, hindering coral survival and recovery.
The loss of sharks disrupts predator-prey dynamics and impacts marine habitat health and biodiversity. Degradation of habitats like coral reefs and seagrass beds, already vulnerable to environmental stressors, is accelerated without sharks’ regulatory influence. This ripple effect compromises the stability and resilience of marine ecosystems, impacting species that depend on these environments.