The return of sea otters can increase the genetic diversity of eelgrass

Publication in Science found that otter-disturbed eelgrass meadows were up to 30% more genetically diverse than areas without otters.

Contrary to how it may appear, it turns out that sea otters ripping up eelgrass actually does the plant – and the nearshore ecosystem – a favour.

Researchers have found that eelgrass meadows where otters had been established for 20-30 years off the coast of British Columbia showed 30% higher allelic richness and 6% higher genotypic diversity than areas where otters were present for less than 10 years or absent for more than 100 years. 

The research was led by Erin Foster of the Hakai Institute and the University of Victoria, and a team including Raincoast scientists Chris Darimont and Lauren Henson. The resulting paper, “Physical disturbance by recovering sea otter populations increases eelgrass genetic diversity,” was published in the journal, Science

When looking for buried food like clams, otters often dig in eelgrass meadows, leaving pits in their wake. This digging disturbs the rhizomes of eelgrass. Rhizomes are underground “roots” of eelgrass, which they use to reproduce via asexual cloning. These clones are exact genetic copies of the single parent plant.

When sea otters rip up rhizomes, eelgrass kicks in its other mode of reproduction: sexual reproduction through pollen and flowers – resulting in seeds which are all genetically different from one another. The otter pits provide nice growing conditions for the dropped seeds, and voila – baby eelgrass with genetic material from two parents! With increased flowering rates, eelgrass builds up genetic diversity via the ‘recombination’ of genes that occurs during this sexual route of reproduction.

The researchers speculated that sea otters have likely had an effect on the evolution and ecology of eelgrass meadows in the area since the mid-Pleistocene epoch, for some 600,000 to 700,000 years. With the onset of European colonization and the fur trade, however, otters were extirpated from the whole BC coast. Several decades ago, otters were reintroduced to some parts of the coast, and their populations have grown since. 

A two panel graphic illustrates how sea otters increase eelgrass genetic diversity.
Graphic by Josh Silberg and Erin Foster.

The research illustrates how the restoration of predators can have far-reaching effects on ecosystems. Typically, the ecological benefits are studied, but in this case researchers found genetic benefits. This research illustrated the evolutionary influence of predators.

The associated ecological benefits of genetically diverse eelgrass, however, are profound. Eelgrass provides among the most important habitats in coastal ecosystems. The meadows serve as nurseries for the juvenile forms of many species of enormous cultural and economic importance (like crabs and fishes). Eelgrass is also a major player in sequestering carbon. But it is also sensitive to change wrought by climate change and other stressors in a rapidly changing world.

Having genetically diverse eelgrass, a benefit delivered by sea otters, helps to future-proof our coast.

Erin Foster dives for data.
Erin Foster measures sea otter pit length in an eelgrass meadow. Photo by Jane Watson.

Citation

Foster, E., Watson, J., Lemay, M. A., Tinker, M. T., Estes, J. A., Piercey, R., Henson, L., Ritland, C., Miscampbell, A., Nichol, L., Hessing-Lewis, M., Salomon, A. K., & Darimont, C. T. (2021). Physical disturbance by recovering sea otter populations increases eelgrass genetic diversity. Science (New York, N.Y.), 374(6565), 333–336. https://doi-org.libaccess.lib.mcmaster.ca/10.1126/science.abf2343.

Abstract

Most knowledge regarding the role of predators is ecological in nature. Here, we report how disturbance generated by sea otters (Enhydra lutris) digging for infaunal prey in eelgrass (Zostera marina) meadows increases genetic diversity by promoting conditions for sexual reproduction of plants. Eelgrass allelic richness and genotypic diversity were, respectively, 30 and 6% higher in areas where recovering sea otter populations had been established for 20 to 30 years than in areas where they had been present <10 years or absent >100 years. The influence of sea otter occupancy on the aforementioned measures of genetic diversity was stronger than those of depth, temperature, latitude, or meadow size. Our findings reveal an underappreciated evolutionary process by which megafauna may promote genetic diversity and ecological resilience.

Select figures

Figure 1

Sea otter research figure one.
Fig. 1. Digging by otters changes eelgrass meadows.(A) Otters absent; (B) otters established; and (C) an otter-foraging pit.

Figure 2

Sea otter research figure 2.
Fig 2. Eelgrass populations and locations.Meadow locations with sea otters absent (pale blue circles), recent (<10 years; dark blue circles), and established (20 to 30 years; red circles). STRUCTURE analyses identified two eelgrass populations (K = 2); genetic divergence is likely driven by distance. Pale and dark green bars show the probability of individual population assignment.

Figure 4

Sea Otter research figure 4.
Fig. 4. Established otters enhance eelgrass genetic diversity.Posterior distributions of estimated parameters from the best-supported model predicting eelgrass allelic richness (A) and genotypic diversity (C). Shaded areas show 90% credible intervals (CI). Marginal means of sea otter effects on allelic richness (B) and genotypic diversity (D); bars indicate 95% CI. Colors and abbreviations as in Fig. 3.

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