The sea otter, Enhydra lutris, is a marine mammal with immense influence on the North Pacific coastal environment. Ecologists recognize it as a keystone species—an organism that has a disproportionately large effect on its ecosystem relative to its abundance. The removal of a keystone species causes dramatic changes, often leading to a loss of biodiversity. Requiring it to consume up to 25% of its body weight daily, the sea otter’s voracious appetite makes it a top-down predator that structures entire underwater habitats.
Protecting Kelp Forests
The most widely studied function of the sea otter is maintaining the health of kelp forests through a trophic cascade. This cascade involves a three-level food web linking the predator, the herbivore, and the primary producer. Sea otters prey heavily on sea urchins, which are voracious grazers of kelp.
Where otters are absent, urchin populations often explode in number and size. These unchecked herbivores consume kelp at an unsustainable rate, creating vast, desolate areas known as “urchin barrens.” The presence of sea otters keeps the urchin population low enough to allow the kelp to grow and flourish.
By controlling these herbivores, sea otters indirectly safeguard the kelp forest ecosystem. This protection ensures the kelp grows tall and robust, forming an underwater canopy essential for the habitat’s structure and function. This top-down force maintains stability in the ecosystem.
Enhancing Seagrass Health
Sea otters contribute to the health of seagrass meadows through a four-level trophic cascade. Unlike in kelp forests, the otter does not directly protect the plant from a primary herbivore. Instead, otters primarily consume intermediate predators, particularly crabs, which are abundant in estuarine seagrass beds.
The reduction in the crab population allows small invertebrates like sea slugs and isopods to increase. These small grazers feed on the epiphytic algae that grows on the seagrass blades. Algal overgrowth can smother the seagrass, blocking sunlight and hindering photosynthesis, a common problem in nutrient-rich estuaries.
By boosting the numbers of these tiny grazers, sea otters ensure the seagrass blades remain clean and absorb sufficient sunlight to thrive. This indirect cleaning effect has been shown to reverse patterns of seagrass loss in places like Elkhorn Slough in central California. The recovery of these underwater meadows demonstrates the otter’s beneficial influence on estuarine ecosystems.
Supporting Overall Marine Biodiversity
The maintenance of thriving kelp forests and seagrass meadows benefits the entire marine community. These vegetated habitats create structural complexity, offering a multitude of niches for various marine life. Kelp forests are among the most productive and biodiverse ocean habitats, supporting hundreds of species.
The dense structures of kelp provide refuge from predators and shelter from strong currents for fish, invertebrates, and other marine mammals. Flourishing kelp and seagrass are also feeding grounds and nursery areas for the eggs, larvae, and juveniles of commercially important fish and shellfish. The presence of sea otters can elevate reef fish population densities due to the enhanced habitat quality.
The Significance of Their Return
The ecological role of sea otters is understood against the backdrop of their near-extinction during the maritime fur trade of the 18th and 19th centuries. Historical overhunting decimated their populations, demonstrating the effects of losing this keystone predator. The recovery of otter populations in certain areas has provided scientists with real-world experiments illustrating their capacity for ecological restoration.
The otter’s presence is recognized as a natural defense against climate change due to its influence on carbon sequestration. Healthy kelp forests and seagrass beds are highly efficient at absorbing and storing carbon dioxide from the atmosphere. The protection otters provide to kelp can nearly double the amount of carbon stored in these marine forests, strengthening the resilience of coastal ecosystems.