Dr. Rikke Jeppesen is an estuarine ecologist at Elkhorn Slough National Esturary Research Reserve. Her current work focuses on water quality monitoring and large-scale marsh restoration. Dr. Jeppesen has been studying green crabs on both coasts of the US since the early 2000s but recently made headlines when her recent study revealed native sea otters are controlling an invasive green crab population in the Slough.

GreenCrab.org Executive Director Mary Parks had a chance to interview Dr. Jeppesen and learn more about her research with green crabs, the importance of keystone species, and why this heartwarming story wouldn’t exist without long-term research and federal funding.

Before diving in, we wanted to share a few definitions that might help navigate this conversation:

Keystone Species: in ecology, a species that has a disproportionately large effect on the communities in which it lives; many are apex predators - Britannica
Apex Predator: a predator that sits at the top of the food chain - National Geographic
Estuary: a water passage where the tide meets a river current - Merriam-Webster
Slough: Along the West Coast, sloughs are often named for the quiet, backwater parts of bays and therefore, they are part of the estuary, where freshwater flows from creeks and runoff from land mix with salty ocean water transported by the tides - NOAA

Dr. Rikke Jeppesen with a crab trap in Elkhorn Slough shot by Amy Ritter

How did your research on green crabs lead to otters?

During my dissertation, I did green crab population surveys all across the West Coast and East Coast. I then went to their native range and completed surveys in Denmark, the Netherlands, and France. Populations in the native range (in Europe) had similar abundances as the East Coast. These surveys helped us understand that green crabs were much more successful on the East Coast with abundances about 10x compared to the West Coast.

We also found that since 2003/2004 when green crab populations peaked at the Slough, the population level remained high for a decade and then dropped to fairly low relative abundances in the following decade. We realized green crab populations were lowest in the Slough in areas inaccessible to otters and after 4 submissions our paper on the subject was finally published in Biological Invasions. What strengthened our study was looking at Elkhorn Slough in comparison to bays and estuaries north of us. Over 5-10 years we found that in areas without otters (Bodega Bay, San Francisco Bay, and Tomales Bay) green crab populations increased. Whereas, in Elkhorn Slough, the green crab population remained low.

Dr. Rikke Jeppesen harvesting green crabs in 2006

Could you talk a little bit more about the Elkhorn Slough ecosystem and some other species and anthropogenic factors that shape it?

Elkhorn Slough has been highly impacted by human activity. We’ve built roads and added riprap, pilings, and docks. The Union Pacific Railroad also runs through the Slough. Full tidal flow is cut off from about 50% of the estuary meaning a lot of water flows through culverts and tide gates and that changes the water quality. What we’ve seen is green crabs are more abundant in those tidally restricted areas with poor water quality, which are also the areas where we’ve found the highest abundances of green crabs. In other words, we have inadvertently created a great habitat for invasive green crabs.

Impacted species include any crab smaller than a green crab. During my dissertation, I wanted to find out if green crabs directly eat other crabs. I set up experiments with native crab species vs. green crabs. What I found was that the species is not as important as size. Larger crabs eat smaller crabs. However, green crabs are often larger than the native mud crabs and other native crab species. Green crabs also eat other mud-flat invertebrates that would otherwise be available to other native crustaceans, fishes, and shore birds. Green crabs also really love oysters. Studies from Europe have shown how green crabs can decimate oyster beds. My boss Kerstin Wasson does Olympia oyster restoration at Elkhorn Slough, so we don’t want the green crabs to impact the restored oyster population. Crabs in general, including green crabs can eat any small invertebrate. My former colleague Brent Hughes published a paper on the trophic cascade of how otters eat crabs (many species), crabs eat inveterate grazers (like sea hares or nudibranchs) that graze epiphytes off of eelgrass. Eelgrass grows better without epiphytes growing on it so when otters are present and they remove crabs from the system, the invertebrate grazers are happy and graze epiphytes off of the eelgrass, and the eelgrass is happy. Green crabs also directly damage eelgrass beds by chopping them up when searching for food.

How did you decide to focus on otters as a potential apex predator that could impact green crab populations?

As is the case with a lot of research, you set out to study one thing and end up finding something new. Field observations and curiosity are key.

20 years ago when I was still a grad student, my advisor said to me “That would be such a cool story of a native species helping eradicate an invasive”. But we didn’t have the data, we mostly had qualitative observations. USGS has otter foraging studies where volunteers collect data with high-powered scopes, recording otters’ feeding behavior including what prey they catch. We worked together with Tim Tinker from USGS. He knows how much an otter eats every day, how much they dive, for how long, and how many prey items an otter can. He found that otters could eat a substantial amount of green crabs, about 330 green crabs per day in the slough, given the current otter population size. That amounts to somewhere between 50,000 and 120,000 green crabs consumed by otters in a year. Tim Tinker also built a model that compared Elkhorn Slough sites with and without otters. We found that green crab populations were higher in areas where otters were absent than in areas where otters were present. Lastly, we found that otters were eating 20-30 green crabs in an hour, while we were doing fieldwork in an area.

What really strengthened our study was collaborating with our Northern neighbors in bays and estuaries without otters. In these areas, green crab populations have been increasing in the last 5-10 years. However, in Elkhorn Slough which is home to otters, the green crab population has remained low in the last 10 years.

Although you don’t ever prove anything in science, by collecting enough evidence, we were able to build a trustworthy story.

What are some knowledge gaps or future directions this research could take?

I think you could make a case (and this might be controversial) for reintroducing otters to places where they used to be. Maybe otters could help control invasive crab populations in places like Oregon and Washington with huge green crab problems.

In terms of knowledge gaps, green crabs have a bipartite life history where they have a pelagic larval phase, they spend their larval stages in the ocean, then they return to a nearshore environment to settle and turn into adults. I think research related to genetics and understanding local retention is important. I’d like to know if we have a small self-sustaining population or some outside input. I saw a talk from Carolyn Tepolt several years ago that discussed this subject.

Can you define an apex predator vs. keystone species

An apex predator is at the top of the food chain.

A keystone species can be also an apex predator but it might not be. A keystone is an architectural piece that if removed can topple an entire structure. A keystone species is a species of critical importance and if you remove it the whole ecosystem can change. It’s been shown through the trophic cascade study that if you remove the otter from the system at Elkhorn Slough, it’s bad for the eelgrass.

There are also other ocean examples of otters as keystone species. Otters eat sea urchins and sea urchins eat kelp seedlings. If there aren’t otters to each the urchins the sea urchin populations explode and destroy kelp forests.

This story comes from years and years of research and collaboration. Why was long-term research and federal funding important for this project?

We’re part of the National Estuarine Research Reserve System. The purpose of this system is to establish baselines for the nation’s estuaries and support long-term monitoring. Historically long-term monitoring is not sexy, as it doesn’t produce quick results, and it’s difficult to procure funding for. Being part of the National Estuarine Research Reserve System (NERRS) has really enabled us to conduct long-term studies. The work itself is not glamorous. You tromp around in the mud and it’s tiresome and hard but if you persist you can tell a very compelling and maybe even splashy story that serves as an example for others. I hope our future governments continue to fund and support research like this because we’re federally funded. The NERRS provides the framework for success and it’s not always an easy sell but once you get to the story, people are interested.

I also think what this study illustrates is that you can get multiple benefits from supporting apex predators and looking beyond just the invasive. In this case, the otter helps control an invasive species and keeps the whole system in balance.

Lastly, in restoration ecology and invasive species ecology, there are often two approaches: bottom-up and top-down. Historically California has lost over 90% of its wetlands over the past few centuries. The way we try to mitigate that is by restoring habitats like salt marsh or seagrass beds and then we can benefit the otters. That’s an example of a bottom-up approach. On the flip side, by supporting apex predators and their habitat we can mitigate an invasive species indirectly using top-down control, and at the end of the day you have a healthier food web, you support a threatened species, and you end up with fewer invasive species

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