Sea otter digging promotes eelgrass resilience

Coastal habitats rely on undersea meadows

Eelgrass beds form productive underwater gardens that serve as the base of marine food webs, act as nurseries for juvenile fish, and reduce storm damage and erosion to coasts. These meadows of marine flowering plants play an important role in maintaining healthy coastal ecosystems and have been designated as an Essential Fish Habitat and a Habitat of Particular Concern by the United States federal government (under the Magnuson-Stevens Fishery Conservation and Management Act of 1996). Human infrastructure, fertilizer run-off, and warming ocean temperatures have led to eelgrass beds being classified as one of the most threatened and rapidly disappearing ecosystems in the world.  Ongoing restoration projects hope to recover many of the historic ecosystem services once provided by healthy eelgrass meadows in the United States, but could rebounding sea otter populations thwart these efforts?

Sea otters restructure ecosystems

Hunting decimated sea otter populations along the Western coast of the United States in the 18^th – 19^th centuries. But under the protection of the Marine Mammal Protection Act, sea otters have started to return to many coastal areas, restructuring ecosystems that saw drastic regime shifts in their absence. While the recovery of a keystone species like sea otters may seem like an obvious benefit to the ecosystem, scientists were unsure how they would impact other threatened species, like the imperiled eelgrass meadows.

Sea otters forage for prey like clams or other invertebrates by digging through the sediment around eelgrass meadows. This hunting behavior can break apart eelgrass roots and stems (called “rhizomes”), leaving behind a pock-marked seafloor. To see how sea otter digging impacted eelgrass meadows Erin Foster, a Research Associate at Hakai Institute and PhD student at the University of Victoria, led a research project investigating how the genetic diversity of eelgrass meadows changed in response to the presence of sea otters.

Foster and her collaborators used the gradual recovery of sea otters in the Northeastern Pacific as a natural experiment: as sea otters rebound, they have re-established populations in some, but not across all of their historic range. The team investigated eelgrass meadows in areas where sea otters had been present for decades, in areas where sea otters had only recently (within the past 10 years) become re-established, and in areas where sea otters are still absent after being driven away over 100 years ago.

The scientists were interested in studying the genetic diversity of the eelgrass populations, which can be an indicator of the health of a population. Eelgrasses can reproduce sexually (when the genetic material from one plant mixes with that of another) or asexually, producing genetic clones of the parent plant. Clones are not inherently unhealthy, but sexual reproduction reshuffles the genetic deck, producing new combinations of genes within the population. By increasing the genetic diversity, sexual reproduction can make a population more resilient to change: Some individuals may be susceptible to increased ocean temperatures even if others carry a genetic combination that makes them more heat tolerant. When a heat wave hits, a population of susceptible clones would be decimated, but some of the individuals from the genetically diverse population would survive and repopulate the area.

Increasing genetic diversity in eelgrass meadows

Foster and her collaborators weren’t sure if sea otter digging would decrease genetic diversity in an area by favoring only plants with a specific genetic make-up that made them resilient to the digging disturbance (but not necessarily resilient to other disturbances) or if the digging might encourage the eelgrass to sexually reproduce, thereby increasing genetic diversity. When they compared the genetic make-up of eelgrass populations, the researchers found that eelgrass genetic diversity increased at sites where otters were present, and that genetic diversity increased most in areas where otters had been living the longest. It appears that continued disturbances caused by otter digging promotes the eelgrass to flower and sexually reproduce.

The team proposes that the relationship between otter disturbances and eelgrass sexual reproduction evolved over thousands of years, before otters were removed from the ecosystem. In their absence, the eelgrass meadows were largely undisturbed, promoting asexual cloning instead of the sexual reproduction induced by sea otter digging. Foster also points to another possible reason for the shift to clonal reproduction: European colonization. Eelgrass rhizomes were often harvested by indigenous people living near eelgrass meadows. This practice, which likely mimicked sea otter digging and promoted the sexual reproduction in eelgrass meadows, declined with colonization.

As sea otters recover, it is possible eelgrass populations will regain some of their genetic diversity and become more resilient to environmental disturbances. This research also highlights the wider role large animals serve in ecosystems – influencing the behaviors, interactions, and even the genetic make-up of species at all levels of the food chain.