Creature Feature: The kelp forest—an underwater housing crisis

Let’s take a dip into the dark, chilly waters of Northern California, where upwelling (i.e., deep ocean water being brought to the surface by winds, currents, and the earth’s rotation) supplies nutrients to a forest canopy of slick, bulbous kelp tendrils, tangles of verdant sea grasses, and richly iridescent seaweeds. Kelps are considered “ecosystem engineers” as they provide habitat structure and modulate nutrient dynamics for a diverse host of tenants. Each tenant competes for resources like food, space, and shelter, yet they all depend on each other to keep their ecosystem community balanced [1]. Ecologists use the term “trophic cascade” to describe these relationships because if there is disturbance at one trophic level (trophic levels are levels of an ecosystem that share spots in the food web), the change cascades to other trophic levels, potentially throwing off the balance of an ecosystem [2].

Trophic Cascade: A classic example of a trophic cascade involves sea otters, urchins, and kelp. In a nutshell (or should I say snail shell), these species can co-exist together to balance a healthy ecosystem; algae and kelp are pruned by Pacific purple sea urchins (Strongylocentrotus purpuratus), which promotes new kelp growth. The sea urchins are in turn eaten by sea otters (Enhydra lutris), ensuring sea urchins don’t “over-prune.” When top predators, like sea otters, are removed from the equation, sea urchins can take over, decimating the kelp and throwing the ecosystem off balance. Image credit: Karli Chudeau

Kelp forests are a naturally resilient ecosystem: they tolerate seasonal climate changes like El Niño and endure trophic changes caused by unpredictable events (e.g., rough storms) or humans (e.g., overfishing). Unfortunately, historic sea otter fur trade created intense ecosystem shifts in northern California, and now climate change has created additional serious stressors on the kelp forests. There has been a 90% decrease in the kelp canopy along more than 350km of coastline—an unprecedented rate of decline, creating an underwater housing crisis [3,4]. The underwater housing crisis began with a record-breaking marine heat wave, known as “The Warm Blob”, that arrived in California in 2014 and triggered oceanic changes resulting in nutrient poor, warm water (2.5 °C warmer than normal). The Warm Blob sat in the Pacific all year and greeted an El Niño in 2015, during which the cool upwelling that kelp forests rely on ceased [5]. The prolonged heat of the Warm Blob plus the 2015 El Niño reduced the growth and reproduction of large marine algae such as bull kelp (Nereocystis lueketana) and giant kelp (Macrocystis pyrifera). With these ecosystem engineers in peril, how are the kelp forest tenants faring amidst this underwater housing crisis?

Bull kelp (Nereocystis lueketana; left) and giant kelp (Macrocystis pyrifera; right). Photo Credit: Karli Chudeau.

Tenant No. 1: Evicted.

Sea stars (Class: Asteroidea), specifically Sunflower stars (Pycnopodia helianthoides), are colorful predators that feed on sea urchins, snails, and other invertebrates. Historically found along the coasts from Alaska to Southern California, sunflower stars are among the world’s largest sea stars and have an arm span up to 1m (3.3ft)! Tragically, before the Warm Blob came to town,  Sea Star Wasting Syndrome, a disease that causes lesions and tissue decay to the point of body fragmentation, had devastated sea star populations [4]. Sunflower stars were hit especially hard by both the disease and loss of their kelp forest habitat and are now considered locally extinct on the Northern California coast since they haven’t been observed since 2015 [3,4]. With sea stars evicted from the kelp forest, other tenants eagerly began moving in.

Reefcheck, a nonprofit that collaborates with citizen scientists and SCUBA divers to survey reef populations, found that in Northern California, the number of sunflower stars (in blue) decreased and the number of sea urchins (in purple) increased, with the pivotal year being right around when the Blob came to town. Image credit: McHugh et al., 2018

Tenant No. 2: Literally eating everyone out of house and home.

Pacific purple sea urchins (Strongylocentrotus purpuratus) are described by UC Davis faculty member Dr. Cynthia Catton as “the goats of the sea.” These omnivorous pin cushions—with many, tiny tube feet for locomotion, purple, calcium carbonate spines for protection, and mouths on the underside of their bodies—scuttle around the reef, mowing through sponges and algae. With the sickly eviction of their sea star predators, urchin tenants surged into the kelp forest. Marine ecologists began seeing little armies of purple spike-balls creating “urchin barrens” that can extend up to 1000s of kilometers in some areas [6]. Urchin barrens are areas of reef dominated by urchins that have overgrazed algae and can persist for decades. Kelp-craving sea urchins may wreak havoc on precious real estate but they are not the villain in this ecosystem story. Urchins are merely a species taking advantage of a situation that happens to involve bulldozing the already-struggling bull kelp, making the living environment uninhabitable by other kelp forest tenants [4].

Urchin barrens can be small patches of reef or extend for miles and miles. Photo credit: Karli Chudeau

Tenant No. 3: Struggling to make rent.

One kelp forest tenant struggling to hang on (literally) is a large, red sea snail. Now, before you dismiss a snail, red abalone (Haliotis rufescens) are a central part of Native Chumash culture, coveted by seafood lovers for their meaty “foot”, and known for their pearlescent inner shells used in jewelry. These special sea snails are slow-growing herbivores, relying on kelp for food and shelter, making them susceptible to the underwater housing crisis.

Red abalones are fascinating marine invertebrates with a lifespan between 35–54 years old, but they are slow to sexually mature and have specific environmental needs for successful reproduction. They become sexually mature between 3–7 years old, with larger individuals producing more gametes (eggs and sperm) than smaller individuals. As broadcast spawners (i.e., animals that simply release gametes into the ocean and fertilization occurs outside the body), abalones need to be close to each other so their gametes are more likely to mix [7,8]. Successful reproduction also relies on water temperature, with warmer water hindering sperm production and larval development [9].

Surveys of red abalone (in red) and bull kelp (in green) populations over the years show a continuous decrease in numbers. The authors note that the spike of red abalone in 2014 is likely due to abalone emerging from more sheltered spaces to search for food. These graphs show strong connections between species and how different trophic levels depend on each other. Image credit: McHugh et al., 2018

The increased competition for food, lack of habitat, and warmer waters are making it difficult for red abalone populations to make ends meet. In search of disappearing kelp, they must travel further away for food and shelter, meaning their growth is slower and reproduction is less likely to be successful. Marine ecologists that survey kelp forests have described a very disheartening underwater scene: empty abalone shells strewn across the reef, live abalones with smaller, weaker feet that are no longer able maintain hefty suction because they are starving, and unusual behaviors of exposing their vulnerable foot while climbing up kelp stalks searching for food (as depicted below). Over the past several years, abalone populations have declined so much (an 80% reduction) that the red abalone fishery, worth approximately $44 million dollars, closed in 2018 [4].

Red abalone ascends a strand of kelp. Image credit: Karli Chudeau

These tenants need (k/h)elp!

The confluence of all these issues plaguing these vital underwater ecosystems in California is quite serious. The coastal marine community is trying to keep morale high, but seeing kelp forests collapse is depressing on many levels.

Fortunately, researchers, wildlife managers, conservationists, community stakeholders, and governmental agencies are rallying together to brainstorm viable solutions to save not just one species, but a whole ecosystem! The California Department of Fish and Wildlife and Greater Farallones Association have set up a comprehensive bull kelp recovery plan (check out the engaging figures and graphs) and the California Ocean Protection Council is providing $500,000 of funding in 2020 to begin addressing the role of human intervention in active kelp forest restoration via removal of purple urchin from targeted reefs. Additionally, California Sea Grant has just announced additional funding for research (including research conducted at UC Davis’ Coastal and Marine Sciences Institute), habitat monitoring, and stakeholder engagement (e.g., the non-profit conservation organization Reef Check), and community-level involvement.

Defying terrestrial adaptations to take a peek at our underwater ecosystems is highly encouraged. Video credit: Karli Chudeau

Conservation is a team sport that thrives when we work together for a common goal. Kelp reforestation (like many other environmental issues) is complex and can often be overwhelming. However, whether it is research, civil engagement, educating yourself, communicating science to others, or making more sustainable behaviors a part of your everyday life, we can all find ways to contribute to ending this underwater housing crisis!

To read about ways you can contribute to conserving ecosystems such as kelp forests, check out this previous Creature Feature and scroll down for a list of actionable items for wildlife conservation. If you are a Northern California local, consider becoming a Reef Check citizen science diver!

Karli Chudeau is a PhD candidate in the Animal Behavior Graduate Group and a part of the UC Davis Coastal Marine Science Institute and Center for Animal Welfare. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.


  1. Foster, S. & Schiel, R. (1985). The ecology of giant kelp forests in California: A community profile. US Fish & Wildlife Service Biological Report, 85, 1–150.
  2. Terborgh J., Estes J. A. (2010). Trophic Cascades: Predators, prey and the Changing Dynamics of Nature. Island Press.
  3. McHugh, T., Abbott, D. & Freiwald, J. (2018). Phase shift from kelp forest to urchin barren along California’s North Coast. Sonoma-Mendocino Bull Kelp Recovery Plan For Greater Farallones National Marine Sanctuary and California Department of Fish and Wildlife.
  4. Hohman, R., Hutto, S., Catton, C. & Koe, F. (2019). Sonoma-Mendocino Bull Kelp Recovery Plan. Plan for the Greater Farallones National Marine Sanctuary and the California Department of Fish and Wildlife. San Francisco, CA. 166 pp.
  5. Oliver, E. C. J. et al (2018). Longer and more frequent marine heatwaves over the past century. Nature Communications 9, 1324.
  6. Filbee-Dexter, K., & Scheibling, R.E. (2014). Sea urchin barrens as alternative stable states of collapsed kelp ecosystems. Marine Ecology Progress Series, 495, 1–25.
  7. Haaker, P.L., Henderson, K.C., & Parker, D.O. (1986). California Abalone, Marine Resources Leaflet No. 11. State of California, The Resources Agency, Department of Fish and Game, Marine Resources Division, Long Beach, CA.
  8. Prince, J.D., Sellers, T.L., Ford W.B., & Talbot S.R. (1988). Confirmation of a relationship between the localized abundance of breeding stock and recruitment for Haliotis rubra Leach (Mollusca: Gastropoda). Journal of Experimental Marine Biology & Ecology, 122, 91–104.
  9. Morse, A.N.C., Froyd, C., & Morse, D.E. (1984). Molecules from cyanobacteria and red algae that induce larval settlement and metamorphosis in the mollusc Haliotis rufescens. Marine Biology, 81, 293–298.

Main image of kelp forest by Karli Chudeau.

Edited by Jessica Schaefer.

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