Sea Otters are a Keystone Species

Caitlin Dempsey

Updated:

Sea otters (Enhydra lutris) play a crucial role in maintaining the health of California kelp forests through a process known as a trophic cascade. A trophic cascade is an ecological phenomenon where changes in the abundance of one species at the top of a food chain can have profound effects on the entire ecosystem, including species further down the chain.

Otters Help Maintain the Health of Kelp Forests

This effect of sea otters on the health of kelp forests is what makes them a keystone species. Keystone species are organisms within an ecosystem that have a disproportionately large impact on the structure and function of that ecosystem, often far greater than their abundance or biomass would suggest.

The relationship between sea otters and kelp forests

Sea otters primarily feed on sea urchins, which are herbivores that graze on kelp. By keeping sea urchin populations in check, otters prevent these herbivores from overgrazing kelp beds. Without otters, sea urchin populations can explode, leading to the overconsumption of kelp and causing extensive damage to kelp forests.

Many fish species, invertebrates, and other marine organisms rely on kelp forests for shelter, breeding grounds, and food sources. Thus, the presence of otters indirectly benefits these species by preserving their habitat.


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Why are kelp forests important?

Kelp forests are a critical component of many temperate and boreal coastal ecosystems.  Giant Kelp provide food and habitat for a large number of fish, invertebrates, birds, and marine mammals which in turn are important a variety of economic and commercial activities.  

Kelp forests are also an important source of blue carbon storage. Kelp is an excellent carbon sink, as it absorbs and stores large amounts of carbon dioxide from the atmosphere.

Thriving kelp forests act as natural buffers against coastal erosion and storm damage. They dampen wave energy, reducing the impact of storms on coastal areas and protecting shorelines.

Sea otters also protect salt marshes

Sea otters have also been shown to be ecologically valuable when it comes to warding off salt marsh erosion. For example, researchers, in a study (Hughes et al., 2024) recently published in the journal Nature, found that the presence of sea otters slowed the erosion of salt marsh edges in Elkhorn Slough estuary in Central California.

The reason why? Sea otters are predators of Striped shore crabs (Pachygrapsus crassipes) which eat away at the roots of pickleweed (Salicornia pacifica). Pickelweed is an important plant stabilizer of the salt marshes’ sandy banks. By hunting these crabs, sea otters play a valuable role in controlling erosion of these salt marshes by allow denser patches of these plants to thrive.

Where are Sea Otters Located?

There are 13 species of river and sea otters located around the world. The United States is home to the North American river otter and the sea otter.

A sea otter swimming in Monterey Bay, California.
A sea otter swimming in Monterey Bay, California. Photo: Tania Larson, U.S. Geological Survey. Public domain.

Sea otters are native to marine environments in the North Pacific Ocean.  90 percent of the world’s sea otters live in coastal Alaska. To the south, sea otters can be found along the mainland coastline of California from San Mateo County to Santa Barbara County, and San Nicolas Island.

Conservation Efforts for Sea Otters

Heavy hunting for their fur between 1741 and 1911 led to a precipitous drop in numbers of this species.  The passage of the Marine Mammal Protection Act has helped to protect sea otters from extinction.

Conservation efforts have helped the number of sea otters rebound from a low of around one to two thousand to a little over 100,000 today.

A raft of sea otters. Photo: USFWS, public domain
A raft of sea otters. Photo: USFWS, public domain

Sea otters are what are known as a keystone species. Keystone species are plants or animals whose presence in an ecosystem plays a crucial role in its health.  

The term was first coined by Robert T. Paine, a professor of zoology at the University of Washington in 1969 who used the term to describe the importance of Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel.  

 Kelp seen in Monterey Bay, California.
Kelp forests are often called the rainforest of the sea. They support wide varieties of marine life. Kelp seen in Monterey Bay, California. Photo: Tania Larson, U.S. Geological Survey. Public domain.

By keeping benthic (sea floor) herbivores such as sea urchin populations in check, sea otters preventing an over consumption and die off of sea kelp.  Kelp is critically important as it provides habitat and food for a large number of fish, invertebrates, birds, and marine mammals.

How Sea Otters Sleep in the Ocean

Sea otters spend the majority of their lives in the water, eating, playing, and even sleeping in the ocean.  

Living in a fluid environment, sea otters have come up with various ways to keep from floating away while sleeping.  

Sea otters may wrap themselves in sea weed to anchor themselves.  Another strategy is to hold hands to keep family groups together.  

A group of sea otters is known as a “raft”. Sea otters in a raft will hold paws while sleep to keep the group from floating apart.

Milo and Tanu, two sea otters housed at the Vancouver Aquarium demonstrate this adorable trait:

Facts About Sea Otters

The last week of September each year has been designated Sea Otter Awareness Week in order to promote the essential role that these excessively cute marine mammals play in the health of nearshore ecosystems.  

Sea otters use rocks as tools to break open clams and mussels. When not in use, the sea otter will store the rock in a flap in their armpit for safe keeping.

Sea otters spend quite some time diving in the ocean foraging for food. Sea otters have an incredible lung capacity and can hold their bath for up to five minutes at a time although dives typically last 1-2 minutes.

Sea otters have very dense fur that keeps them warm in the cold Pacific Ocean waters. The fur of a sea otter is 800,000 to one million hairs per square inch.

When not sleeping or foraging for food, sea otters can be seen grooming their fur. Grooming helps to dry the hair to fluff it up to trap air. The trapped air gets heated by the body temperature of the sea otter to keep the animal warm.

Natant Otter Pups

Natant is an adjective taken from the Latin word natare, which means “to swim.”  The word is used to reference something or someone floating or swimming in water.

When otter pups are first born, they are unable to swim.  They are, however, equipped with a buoyant coat of fur that keeps the pups afloat.  

To keep them safe and to prevent them from floating away in the ocean, the mothers will wrap their babies with seaweed.  

This gives the mothers freedom to hunt for food by anchoring their natant offspring among the kelp in the Pacific Ocean.

This video from PBS shows natant otter pups in off the coast of California and in the tidal slough and estuary of Elkhart Slough.

This article was originally published on May 5, 2023 and has since been updated.

References

A Keystone species, the sea otter, colonizes Glacier Bay. (n.d.). NPS.gov (U.S. National Park Service). https://www.nps.gov/glba/blogs/a-keystone-species-the-sea-otter-colonizes-glacier-bay.htm

Hughes, B. B., Beheshti, K. M., Tinker, M. T., Angelini, C., Endris, C., Murai, L., Anderson, S. C., Espinosa, S., Staedler, M., Tomoleoni, J. A., Sanchez, M., & Silliman, B. R. (2024). Top-predator recovery abates geomorphic decline of a coastal ecosystem. Nature626(7997), 111–118. https://doi.org/10.1038/s41586-023-06959-9

Paine, R.T. (1969). “A Note on Trophic Complexity and Community Stability”. The American Naturalist 103(929): 91–93. doi:10.1086/282586JSTOR 2459472.

Sea otters | Alaska region. (n.d.). U.S. Fish and Wildlife Service. https://www.adfg.alaska.gov/index.cfm?adfg=seaotter.printerfriendly

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.