Sea otters (Enhydra lutris) are marine mammals native to the coastal regions of the northern and eastern North Pacific Ocean. These animals, weighing between 14 and 45 kilograms, are the heaviest members of the weasel family and among the smallest marine mammals. Historically abundant across their range, from Japan to Russia and along the North American coast to Baja California, extensive hunting for their dense fur between 1741 and 1911 nearly drove them to extinction. Conservation efforts and legal protections, such as the 1911 International Fur Seal Treaty and the 1972 Marine Mammal Protection Act, have led to a significant rebound, though they remain listed as an endangered species by the IUCN.
Sea Otters as Keystone Species
Sea otters are recognized as a keystone species, an organism whose presence and activities have a disproportionately large effect on its environment. Without such a species, the ecosystem can undergo dramatic changes, often leading to decreased biodiversity. Sea otters exert this influence primarily through their feeding habits, acting as top predators that regulate other populations. Their role initiates a cascade of effects throughout the food web, maintaining balance in nearshore marine environments.
Guardians of Kelp Forests
One of the most recognized contributions of sea otters is their role in maintaining the health of kelp forests. Sea otters primarily feed on marine invertebrates, including sea urchins, a favored food source. Sea urchins are voracious grazers that consume kelp, and unchecked populations can decimate entire kelp beds. This overgrazing leads to “urchin barrens,” areas largely devoid of kelp and other marine vegetation.
By preying on sea urchins, sea otters keep these herbivore populations at manageable levels, preventing them from overwhelming kelp forests. In areas with healthy sea otter populations, kelp forests flourish, providing complex three-dimensional habitats. These thriving kelp forests serve as shelter, food sources, and nursery grounds for a wide array of marine species, including fish, crustaceans, and other invertebrates. Healthy kelp ecosystems also help buffer wave action, protecting coastlines from erosion.
Boosting Seagrass Health
Beyond kelp forests, sea otters also contribute to the well-being of seagrass meadows. Sea otters forage for infaunal invertebrates, such as crabs and small snails, within these underwater grasslands. This foraging activity helps prevent overgrazing on seagrass by smaller herbivores or stops them from destabilizing the sediment. For example, otters consume crabs, which are predators of smaller invertebrates like sea slugs and isopods that graze on algae. A reduction in crab populations allows these algae-eating invertebrates to thrive, keeping seagrass blades clean and ensuring they receive sufficient sunlight.
The physical disturbance caused by otters digging for clams in seagrass beds can also encourage the sexual reproduction of seagrass, making the meadows more resilient to environmental changes. Healthy seagrass beds are productive ecosystems that serve as crucial nursery habitats for numerous fish and shellfish species. They also filter pollutants and excess nutrients from the water, improving water clarity, and their dense root systems help stabilize coastal sediments, reducing erosion.
Wider Ecological Contributions
The maintenance of healthy kelp and seagrass ecosystems by sea otters extends to broader ecological benefits, including carbon sequestration. Both kelp forests and seagrass meadows are significant “blue carbon” sinks, absorbing and storing large amounts of carbon dioxide from the atmosphere and oceans. Kelp forests can sequester substantial quantities of carbon, with some estimates suggesting they remove up to 190 million tons of carbon dioxide annually worldwide. These marine ecosystems can be more efficient at carbon absorption per unit area than many terrestrial forests.
Seagrass meadows also contribute to carbon storage, trapping organic material in their sediments for potentially thousands of years. Their capacity to absorb carbon can be up to 35 times faster than tropical rainforests per hectare. By enabling these crucial habitats to flourish, sea otters indirectly enhance the ocean’s capacity to mitigate climate change. This preservation of complex habitats also leads to enhanced biodiversity, as healthy kelp and seagrass beds provide shelter and food for a wide range of marine life.