Local modeling

Modeling the uncertain future of kelp forests

There’s an ethereal quality to California’s coastal kelp forests – diving into the tangle of underwater canopy could reveal a hidden realm of octopuses, starfish, red and purple sea urchins, abalones, harbor seals and sometimes, if you’re lucky, sea otters. So if you find your mind drifting to thoughts of mermaids in an aquatic dreamscape, you could be forgiven for forgetting you were in a mathematical model.

But that’s exactly what it is. For two years, UC Davis scientists have been modeling a “field of dreams” hypothesis about bull kelp, a species of algae in the genus Nereocystis (Greek for mermaid’s bladder!), to understand which approaches could best aid in the recovery of the kelp forest ecosystem in Northern California. The dream field theory in ecology posits that partial efforts to create an appropriate environment during the early stages of a restoration project will trigger the full recovery process and return a diversity of species to the environment. In other words, like the 1989 movie of the same name, if you build it, they’ll come.

Between 2013 and 2015, multiple ecological and environmental stressors killed 95% of kelp forests along the Mendocino and Sonoma coasts. Bull kelp, an annual brown algae that grows up to 60 feet tall, forms the basis of the region’s rocky near-shore ecosystems. Ecosystems that in turn support recreational and commercial fishing and cultural uses. The decline had many causes, including back-to-back marine heat waves that did not allow upwellings of nutrient-rich cold water (which aids the kelp growth cycle) and the local extinction of sunflower starfish that are predating sea urchins. Sea urchins feed on kelp and their population explosion has been a significant cause of the kelp collapse. Of course, sea otters, another voracious predator of sea urchins, have long since disappeared from northern California waters.

The rich kelp ecosystem has a high diversity of species, but coexists with another system in which purple sea urchins dominate – also a low diversity natural state. These alternating ecosystems follow one another as the environment changes – for example, when a marine heat wave occurs or overfishing depletes the ocean’s resources. So if we think the two states naturally switch from time to time, then the question becomes how to manage the system to switch to a human-friendly state, and how to make it stay there before it switches again.

After 2014, the natural resilience of the kelp ecosystem was compromised and purple sea urchin-dominated seascapes known as “urchin barrens” led to an 80% reduction in commercial red sea urchin fishing and a complete ban on recreational red abalone fishing. In response, several kelp recovery plans, programs and funds have been created. In 2020 alone, the California Sea Grant provided $2.1 million for six research projects. These included growing bull kelp on green gravel and laying it on sites where sea urchins had been removed, growing heat-tolerant strains of kelp and planting them, and finding the best ways to more effective at removing sea urchins from the ocean floor.

To assess what might work best, Jorge Arroyo-Esquivel designed a population model as part of his doctoral research at UC Davis’ Alan Hastings Lab. How effective are the three NOAA-proposed interventions—removal of sea urchins, seeding kelp with spores, and planting cultivated kelp in target areas—in bringing back kelp-dominated shorelines? In what order should they take place, to what extent and for how long? To answer this, the model tracks the distribution of kelp and sea urchin through their life stages: survival, reproduction and dispersal. “A model is essentially a set of rules that describe a process that takes place in nature,” says Arroyo-Esquivel. “In this case, we’ve streamlined it to focus on the most important terms in the system we’re studying: the interactions between kelp and sea urchins across time and space.”

The first step to the future is to understand the past. And Marissa Baskett, an Arroyo-Esquivel collaborator and professor in the Department of Environmental Science and Policy at UC Davis, has modeled these species interactions throughout her research career. “Before that, we were looking at what was going to dominate the system — kelp or sea urchins — and how to sustainably fish those undersea forests to avoid kelp decline,” Baskett says. “Now we’ve leveraged those modeling frameworks to help us inform these very urgent new management decisions.”

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And so the model has received contributions from other studies, many of which have been conducted at the Baskett Lab itself. One study looked at size-structured interactions in southern California: lobsters tend to feed on medium-sized sea urchins, which fit in their mandibles. So if lobsters are harvested, how do these lobster-urchin interactions shape the overall system and how many lobsters and red sea urchins can we sustainably harvest in Southern California? Another study, on perennial giant kelp in the Channel Islands, looked at what happens to the ecosystem when there is plenty of kelp that harbors multiple sea urchin predators. The sea urchins then tend to hide in crevices and feed on drifting kelp. But as soon as the kelp dwindles, the sea urchins get hungrier and come out to attack the kelp. “This is the model structure that Jorge took and ran with, to ask what the relative effectiveness of these different interventions might mean for potential kelp recovery,” Baskett says. “It’s basically our little sandbox where we can play and say, ‘What if we fish this way? kelp, doing one and then the other, or doing them together in different degrees in different places?”

By running simulations based on these recorded behaviors, the team found that removing sea urchins along with increasing the density of adult kelp were most likely to initiate the recovery process. Due to the counter-intuitive way urchins feed – eating more when there is less food – it was more important to keep urchin numbers consistently below a threshold to stimulate kelp growth, rather than spreading the effort over more time or space. In the Field of Dreams approach, according to the researchers, “the quality of the habitat is one of the main factors limiting the success of the restoration. In the case of kelp forest restoration, suitable habitat is determined by the density of active grazers (purple sea urchins in our study system).

Kelp Forest in Northern California. (Photo by Mac Gaither, https://unsplash.com/photos/PwJgkyFDNJA)

Like any model, however, Field of Dreams is only as accurate as its inputs, and the enormous complexity of an ecology means great uncertainty in modeling. For example, right now, the environment around these animal interactions remains constant. A more realistic scenario would make kelp growth a function of nitrogen in the water (which varies with the amount of upwelling) and make kelp mortality a function of heat stress. Data on species interactions themselves are also difficult to measure, requiring extensive empirical field studies. “We have an expectation of how we think these interventions will play out. But the model is a simplification of reality, and therefore, by definition, something is always missing,” acknowledges Arroyo-Esquivel.

Despite these challenges, models like these can help us make better management decisions in the face of a rapidly changing world. The glimpses of possible futures they offer are dreamy enough that Baskett and his collaborators were awarded a five-year, $1.6 million National Science Foundation grant to continue this work with a model that poses and responds to more complicated questions. They will include input from economists, ecologists, and social and political scientists, and will take environmental fluctuations into account. This will allow them to link the variability of the kelp and sea urchin ecosystem to the variability of human responses, which is important because different stakeholders along the northern California coast have different goals. “Depending on how you frame the model, you can manage the system to keep it in the state that’s favorable for us humans, or move it to the desirable state in the least painful way,” says Arroyo- Dodge.

This “desirable state” can mean different things to different people: strengthening the current ecosystem to maintain it in the historically represented state or trying to shift it to survive under increased stress in the future. They are extremes on a continuum, requiring different interventions that may or may not work. “We know that marine heat waves are coming in the future, and these stressors will continue,” Baskett says. “So we’re trying to figure out what local stakeholders and decision makers are looking for in that future.”

Featured Image Flickr CC