Sea Otters as Keystone Species

Isaac Cox, Morgan Hartman, Melinda Owens, and Timothy Passalacqua

One sea otter holding another.

Figure 1. One otter in water holds another otter.


Sea otters are more than the cute, cuddly creatures seen in many zoo exhibits. These fluffy creatures hold great importance in their ecosystem, naming them a keystone species. These sea dwellers, while great for their environments, are not flourishing in population numbers, still recovering from near extinction in the 18th and 19th centuries. Now that scientists are aware of the impact of these marine mammals, they have been protected under the Marine Mammal Protection Act and Endangered Species Act, providing protection from their main threats, humans. Sea Otters promote the overall health of not only their ecosystems, but surrounding species populations as well (1).


Sea Otters (Enhydra lutris) are primarily found along the coasts of the Pacific Ocean in North America and British Columbia. Their diets are primarily carnivorous, but at times can be omnivorous (1). Sea otters are members of the family Mustelidae which includes other animals like badgers and weasels. The similarities between otters and weasels likely isn’t anything surprising if you were to just look at them. They live mostly in giant kelp forest and all daily activities such as eating, resting and grooming are done on the water’s surface. While they can dive upwards of 45 meters(145 ft), they prefer waters closer to 30 meters(100 ft). They also vary in size depending on where in the world they live. A specific example of the size difference is that of the Alaskan sea otter which weighs a good 10 kg(22 lbs.) more than their Californian counterpart. Sea otters also do not have any insulating fat and must rely on their fur to maintain heat. Everyone is familiar with otters using rocks for tools to open different foods, but were you aware they have patches of loose skin below their forearms to store these tools? Sea otters are unique compared to other carnivores in only having 4 lower incisors that are used for cutting food (7).

Why Sea Otters Are A Keystone Species

Sea otters play a vital role in biodiversity, food webs, and trophic cascades. In all ecosystems food webs are present. Food webs are defined as the relationship between all food chains, interconnected as one. It is the complex network in which matter and energy is transferred from prey to predator to decomposer. Our food web starts with producers who convert solar energy into sugar through the process of photosynthesis. Next in the web comes the primary consumers which consume producers. Above the primary consumer is the secondary consumer, then tertiary consumer, apex predators, and finally decomposers who break down and digest dead or decaying organic matter. In relation to sea otters, a simple food chain breaks down as such, great white sharks and killer whales eat sea otters, sea otters eat sea urchins, and sea urchins eat giant kelp (1). The balance of this cycle allows for greater biodiversity and healthy populations of organisms.

Impact of sea otters on Elkhorn Slough ecosystem.

Figure 3. Left: a muddy ecosystem with less plants more crabs and no sea otters, right: a clean ecosystem with more plants, less crabs, and sea otters.

Keystone species are considered species that the ecosystem as a whole depends on and the removal drastically changes these ecosystems. Sea Otters are considered keystone species because of their ability to exert a trophic cascade or in other words top down pressure. By directly consuming Sea Urchins they indirectly promote kelp growth because there are less Sea Urchins to graze on the kelp. Consequently, other species such as crabs and abalone that depend on kelp can share this vital resource. Kelp serves not only as a vital food source, but also as a shelter and habitat for a myriad of fish and invertebrate populations. The presence of kelp forests invites greater biodiversity and safer nesting grounds. Furthermore, Sea Otters indirectly promoting kelp growth allows for kelp structures which reduce storm driven tides. These kelp structures help prevent erosion from shores by fencing invading erosion (3). This is important to reduce pollution, stabilize marine structures, and prevent degradation of shores.

What Our Ecosystems Are Like Without Sea Otters

A study conducted in Australia observed that grazing rates of kelp were greater in temperate Australia. Correspondingly, temperate Australia lacks Sea Otter presence. This same study found that North Pacific areas lacking Sea Otters showed even greater grazing rates of kelp, more so than areas with Sea Otters and more so than temperate Australia (4).

Balance between otters, kelp, and sea urchins.

Figure 2. Otter ecosystem showing the balance of sea otter, sea urchins, and kelp forests.

What We Can Do

As previously stated sea otters were near extinction in the 18th-19th century this was primarily caused by the fur trade. It was not until 1977 that the sea was listed under the Endangered Species Act (ESA) which was established in 1973. However as of today sea otters have not fully bounced back even with certain protections in place. These protections include being listed under the Endangered Species Act, protecting them under the International Fur Seal Treaty and the Marine Mammal Protection Act (MMPA). However there is more that can be done to help these hand holding animals(5). The largest current threats against sea otters is white sharks, contaminants, oil spills, and human disturbance (6). Certain things are difficult to control such as the white sharks because the threat is not created by humans, however there are many other things we can do. The decreased use of fertilizer and pollutants by people would decrease the contaminants that negatively affect the sea otters. Also there are chances to volunteer to clean and help rehabilitate sea otters that have been affected from oil spills. It’s best to avoid disturbing sea otter habits and harassing the animals by trying to get their attention. If people want to help, but do not know they can always donate to programs such as the California Sea Otter Fund which provides support to sea otter research and conservation (6).

A group of floating sea otters.

Figure 4. Group of many sea otters lying on their back in the water.


The loss of any keystone species is catastrophic, but the loss of sea otters can cause problems that will devastate a number of organisms. The conservation and protection of sea otters is utterly important to conversely benefit the surrounding organisms such as kelp, urchins, whales and sharks. This correlation between species is an important relationship that must be understood when preserving biodiversity. Protecting estuaries in which otters inhabit are especially important in terms of the fishing industry, recreational sport and biodiversity as a whole. Protection and conservation should be the first priority when ensuring the survival of sea otters, rather than damage control after catastrophic events, such as the fur trade.

Image of a sea otter eating a sea urchin.

Figure 5. Sea otter biting a sea urchin.



Text Citations

  1. Sea otter. Defenders of Wildlife. (n.d.). Retrieved April 10, 2022,
  2. Womble, J. (2016, July 29). A keystone species, the sea otter, Colonizes Glacier Bay. National Parks Service. Retrieved April 10, 2022,,the%20presence%20of%20sea%20otters.
  3. Greg Helms Author, Helms, G., Author, Frey, M., Hogge, K., Brandon, A., Lewis, J., & Perez, J. (2021, July 12). Kelp’s mighty role in our ocean. Ocean Conservancy. Retrieved April 10, 2022,
  4. Steinberg, P. D., Estes, J. A., & Winter, F. C. (1995). Evolutionary consequences of food chain length in kelp forest communities. Proceedings of the National Academy of Sciences, 92(18), 8145-8148.
  5. Conservation & Research. SeaWorld Parks & Entertainment. (n.d.). Retrieved April 10, 2022,
  6. Sea otter conservation. (n.d.). Retrieved April 10, 2022,
  7. Allegra, J.; R. Rath and A. Gunderson 2012. “Enhydra lutris” (On-line), Animal Diversity Web. Accessed April 10, 2022
  8. What are Keystone Species? National Marine Sanctuary Foundation. (2020, May 11). Retrieved April 10, 2022,

Image Citations


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