Climate Change

Modeling the uncertain future of Northern California’s bull kelp forests

August 2, 2022

There is an ethereal quality to the kelp forests of coastal California — diving through the tangle of underwater canopy could reveal a hidden kingdom of octopuses, sea stars, red and purple sea urchins, abalone, harbor seals, and sometimes, if you’re lucky, sea otters. So if you find your mind drifting to thoughts of mermaids in a watery dreamscape, you could be forgiven for forgetting you were in a mathematical model. 

But that’s exactly what it is. For two years, scientists at UC Davis have been modeling a “Field of Dreams” hypothesis about bull kelp, a species of seaweed from the genus Nereocystis (Greek for mermaid’s bladder!), to understand what approaches could best help the recovery of the kelp forest ecosystem in Northern California. The Field of Dreams theory in ecology posits that partial efforts to create a suitable environment during the initial stages of a restoration project will kick-start the process of a full recovery and bring back a diversity of species to the environment. In other words, as in the 1989 film of the same name, if you build it, they will come. 

Between 2013 and 2015, multiple ecological and environmental stressors killed 95 percent of bull kelp forests along the Mendocino and Sonoma coasts. Bull kelp, an annual brown algae that can grow up to 60 feet tall, forms the foundation of ecosystems close to the rocky shore in the region. Ecosystems that in turn support recreational and commercial fisheries as well as cultural uses. The decline had many causes, including back-to-back marine heat waves that didn’t allow nutrient-rich cold water upwellings (which help the kelp growth cycle), and the local extinction of the sunflower sea stars that predate on urchins. Urchins feed on kelp, and the explosion in their population was a substantial cause for the kelp collapse. Of course, sea otters, another voracious urchin predator, are long gone from Northern California’s waters.

The rich kelp ecosystem has a high species diversity, but co-exists with another system in which purple sea urchins dominate––also a naturally occurring, low-diversity state. These alternating  ecosystems follow one another as the environment changes — for example, when a marine heatwave occurs or overfishing depletes the ocean’s resources. So if we think about the two states changing naturally from time to time, the question then becomes how to manage the system to make the shift to one that’s favorable for humans, and how to make it stay there before it switches again. 

After 2014, the natural resilience of the kelp ecosystem was compromised and seascapes dominated by the purple urchin, called “urchin barrens,”  caused an 80 percent reduction in commercial red urchin fishery, 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 baby bull kelp on green gravel and dropping it on sites where urchins had been removed, culturing heat tolerant strains of bull kelp and out-planting it, and finding the most efficient ways to remove urchins from the ocean floor. 

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

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

And so the model received inputs from other studies, many conducted at the Baskett Lab itself. One study looked at size structured interactions in Southern California: lobsters tend to prey on medium-size urchins, which fit in their mandibles. So if lobsters are harvested, how do those lobster–urchin interactions shape the overall system and how much lobster and red urchin can we sustainably harvest in Southern California? Another study, of the perennial giant kelp in the Channel Islands, looked at what happens in the ecosystem when there’s a lot of kelp that harbors several urchin predators. Urchins then tend to hide in crevices and feed on drift kelp. But as soon as kelp dwindles, urchins get hungrier and come out to attack the kelp. “That’s the model structure Jorge took and ran with, to ask what the relative efficacy of these different interventions could mean for the potential recovery of bull kelp,” Baskett says. “It is basically our little sandbox where we can play and say, ‘What if we fish this way? What if somebody managed it by focusing on urchin removal, focusing on kelp reseeding, doing one, then the other, or doing them together to different extents in different places?’” 

By running simulations based on these recorded behaviors, the team found that removing urchins together with increasing the density of adult bull kelp was most likely to kick-start the recovery process. Because of the counter-intuitive way that urchin feed — eating more when there is less food — keeping urchin numbers consistently below a threshold was more important to boosting kelp growth, rather than spreading the efforts over more time or space. In the Field of Dreams approach, the researchers say, “habitat quality is one of the main limiting factors of restoration success. In the case of kelp forest restoration, a suitable habitat is determined by the active grazer density (purple urchins in our study system).” 

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 depending on the amount of upwelling) and would make kelp mortality a function of heat stress. The species interactions data itself is also hard to measure, requiring careful empirical field studies. “We have an expectation of how we think these interventions are going to go. But the model is a simplification of reality, and so, by definition, it’s always missing something,” Arroyo-Esquivel acknowledges. 

Despite these challenges, models like these can help us make better management decisions in the face of a rapidly changing world. The glimpses of the possible futures they offer are dreamy enough that Baskett and her collaborators have received a National Science Foundation grant of $1.6 million over five years to continue this work with a model that asks and answers more complicated questions. They will include inputs from economists, ecologists, and social and political scientists, and take into account environmental fluctuations. This will enable them to link variability in the kelp and urchin ecosystem with variability in human responses — 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 is favorable for us humans, or shift it to the desirable state in the least painful way,” Arroyo-Esquivel says. 

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

Correction: A previous version of this article had two images — one of sea palm or Postelsia, and a kelp forest photo, neither of which were of bull kelp. Both images have been removed and an accurate image has been substituted.

About the Author

Mukta Patil is a freelance science writer based in the Bay Area. She loves the California poppy, Muir Woods, and Pacifica.