Restoring Balance: What Sea Urchins Reveal About Ecosystem Health
By: Rachel Plunkett
June 25, 2026
On a coral reef in Florida Keys National Marine Sanctuary, divers carefully place long-spined sea urchins onto the seafloor. Thousands of miles away off California, divers remove purple sea urchins from rocky reefs. In both places, the goal is the same: restore ecological balance.
At first glance, these actions may seem contradictory. But they illustrate a fundamental principle of ecosystem health: the same organism can play very different roles in an ecosystem depending on the relationships and balance of predators, prey, habitat, and environmental conditions around it.
Sea urchins are essential herbivores, but whether they help stabilize an ecosystem or contribute to its decline depends on those relationships. From coral reefs in the Florida Keys to kelp forests along the California coast, their story offers a powerful lesson in how ecosystems function—and what it takes to restore them when balance is lost.
Why Herbivores Matter in Coastal Ecosystems
In coastal ecosystems, herbivores play a quiet but powerful role.
“Herbivores help maintain balance and ecosystem functioning across coastal ecosystems, particularly those dominated by vegetation, such as kelp forests,” said Dr. Kristen Elsmore of the California Department of Fish and Wildlife’s Marine Region.
By grazing on algae, they contribute to ecological stability by keeping vegetative growth in check, creating space available for other species to survive and grow.
"It is the strength of this very relationship that can, at times, support healthy ecosystem dynamics, but when disrupted, highlight the capacity of some herbivores to dramatically alter the structural complexity of the habitat," Elsmore added.
On coral reefs and in kelp forests, that balance is essential. When herbivores keep algae in check, corals and kelp have room to grow, reproduce, and build the complex habitats that support fish, invertebrates, and entire food webs. When that grazing pressure is lost or becomes too intense, the system can quickly shift.
This dynamic is part of a broader concept scientists call ecological balance: a system of checks and balances in the food web among predators, herbivores, and habitat. Predators help regulate herbivores, herbivores control plant growth, and together they shape the structure of the ecosystem.
When one part of that system changes—such as the loss of a key predator or herbivore—the effects can cascade through the ecosystem. Scientists refer to this as a trophic cascade a chain reaction that can alter everything from species composition to habitat structure.
Florida Keys: When Grazers Disappeared
A Reef Without Its “Lawnmowers”
In the early 1980s, coral reefs across the Caribbean experienced a sudden and widespread loss of one of their most important herbivores: the long-spined sea urchin (Diadema antillarum). Between 1982 and 1983, populations declined drastically. A second die-off in the early 1990s further reduced populations in the Florida Keys. The exact cause was never definitively identified, but scientists believe a bacteria or virus spread rapidly through the region.
“The first die-off was dramatic, with losses of up to 95 to 99% in most locations across the Caribbean,” said Dr. Andy Bruckner, research coordinator for Florida Keys National Marine Sanctuary. “That was a major turning point for reefs that depended on Diadema to keep algae under control.”
When the urchins disappeared, that balance began to shift. In many places, fleshy algae quickly expanded across the reef, covering open space and competing with corals for light and room to grow. Other herbivores, such as parrotfish, helped slow this transition in some areas, but they could not fully replace the ecological role of Diadema.
“One reason the shift was so pronounced is that the Caribbean has less functional redundancy than the Indo-Pacific,” Bruckner said. “In other words, there are fewer herbivores that can step in and fill the same role when one is lost.”
At the same time, reefs were facing additional stress from coral disease, hurricanes, bleaching, and other disturbances. As coral cover declined, more bare space became available for algae to colonize. Because algae grow faster than corals, they were able to spread quickly and overgrow surviving colonies, making it harder for baby corals to settle and survive. Over time, many reefs shifted toward algae-dominated conditions—a change scientists often describe as a phase shift.
Today, reestablishing herbivores is a key part of reef recovery efforts in Florida Keys National Marine Sanctuary. Through large-scale initiatives like Mission: Iconic Reefs, practitioners are working to reintroduce long-spined sea urchins in targeted areas to help control algae and improve conditions for coral growth.
“Our goal is to improve habitat quality,” Bruckner said. “By bringing urchins back, we can help keep macroalgae in check, create a cleaner benthic environment, and support conditions that allow new corals to settle and survive.”
However, the goal is not simply to add more urchins.
“We’re testing what density of urchins is enough to control algae effectively, but not so much that they start harming corals or bioeroding the reef framework,” Bruckner said.
Reintroducing Long-Spined Sea Urchins
On January 14, divers placed 101 long-spined sea urchins onto Horseshoe Reef in Florida Keys National Marine Sanctuary. Before the release, no Diadema were observed at the specific outplanting sites, and the reef showed the macroalgae-dominated conditions that have persisted across much of the Florida Keys since the species’ die-off in the 1980s. In the weeks that followed, however, researchers began to see a 42% retention rate and localized grazing patches in some areas where the urchins were deployed, offering an early sign that the animals were beginning to feed and alter the reef surface.
Those grazing patches may be small, but they point to a much bigger question: how many urchins are needed to make a meaningful ecological difference? Historically, Caribbean reefs supported roughly one to two long-spined sea urchins per square meter, far more than are found on most reefs today.
When asked what success looks like in the next year or so, Dr. Katey Lesneski, research and monitoring coordinator for Mission: Iconic Reefs, said that “In the near term, success means learning. We want to conduct enough trials to understand what release strategies and densities lead to better retention alongside live coral.”
While the team is just getting started at testing optimal methods and urchin densities at Iconic Reef sites, previous research provides a starting point for understanding what densities may be needed to produce meaningful ecological benefits.
“Research suggests that even densities of around one Diadema per square meter may begin to produce meaningful grazing pressure, but densities closer to three to four per square meter are often considered more effective for reducing macroalgae,” said Jim Brittsan, director of Sustainable Ocean and Reefs Inc. (SOAR), the Mission: Iconic Reefs partner rearing native urchins at in-situ nurseries and leading the reintroduction efforts.
For the long-term outlook, Lesneski said researchers “hope to see an increase in density of urchins to well above 0.01 individuals per square meter at Mission: Iconic Reefs sites, coinciding with a decrease in algae cover and an urchin population with demographics indicating a self-sustaining population.” A self-sustaining population would mean that new urchin larvae are successfully settling onto the reef and enough individuals survive through juvenile and adult life stages to maintain the population without continued human intervention.
To measure whether the effort is working, Brittsan explained that teams are tracking how far the urchins move from their release sites, how many remain over time, the habitat characteristics of the modules they were released with, and the presence and extent of grazing patches. They also document possible predation events, such as spine piles or empty tests, to better understand what helps released urchins survive and contribute to reef recovery.
For Brittsan, the work is both scientific and personal.
“It’s a bit of a love-hate relationship—you definitely get spined occasionally,” he said. “But knowing the role they play, and seeing grazing patches appear after release, makes it incredibly rewarding.”
The outplanting at Horseshoe Reef represents more than the return of a single species. It is the first effort within Mission: Iconic Reefs to restore herbivorous grazers alongside coral restoration activities, recognizing that healthy reefs depend not only on corals, but also on the ecological processes that support them. This work is part of Phase 1B of the program’s 20-year restoration plan, which also includes supplementing other grazers such as Caribbean king crab, alongside efforts to improve diversity on the reef by restoring branching, star, brain, and pillar corals.
Brittsan explains the significance of this aspect of the project and the connection between grazers and genetic diversity on the reef, saying that, “Grazers like Diadema help create open substrate that supports successful settlement of coral larvae, which is important for maintaining genetic diversity and potentially increasing resilience to stressors such as temperature and disease.”
Nearly 200 additional urchins were translocated at the end of May.
“We only released the largest individuals with the smaller individuals being placed within our in situ nursery,” Brittsan explained. The team plans to release more in the future, depending on how the populations at Pickles Reef are looking.
California: When Balance Tips the Other Way
From Kelp Forests to Urchin Barrens
In healthy kelp forests off California, sea urchins are part of the natural ecosystem. They graze on algae and help recycle nutrients along the seafloor. But when the balance is disrupted, urchin populations can surge and the entire system can begin to unravel.
That is what happened along parts of the California coast beginning in 2014. A combination of sea star wasting disease, unusually warm ocean conditions, and declines in kelp created what researchers have described as a major ecological regime shift. Sunflower sea stars and other predators that normally help keep urchin populations in check declined dramatically. Around the same time, a powerful marine heatwave known as “the blob,” followed by El Niño conditions, stressed kelp forests and caused widespread canopy loss.
As both giant and bull kelp disappeared, purple sea urchins did not disappear with it. Instead, they persisted in huge numbers, even in areas where food was scarce.
“Urchins are able to enter a dormant state when their food source is unavailable,” said Danielle Lipski, research ecologist at Greater Farallones and Cordell Bank National Marine Sanctuaries. “Their growth and metabolism slow, which allows them to survive through lean times until food becomes available again.”
Even when kelp begins to return, intense grazing pressure can prevent young algae from reaching maturity, especially in the absence of predators that once kept urchin populations in check. In many areas, this dynamic locks reefs into a persistent state known as an “urchin barren,” where the forest cannot reestablish itself.
The result is a seafloor that looks and feels dramatically different from a thriving kelp forest. Instead of tall fronds reaching toward the surface and fish weaving through a dense canopy, the underwater landscape becomes open, exposed, and bare.
“Imagine floating through the most biodiverse rainforest you’ve ever seen,” said Tyler Mears of Greater Farallones and Cordell Bank National Marine Sanctuaries and Greater Farallones Association. “Now imagine that same forest after a wildfire—exposed, still, and stripped of life. An urchin barren feels very similar.”
This shift has affected far more than the underwater scenery. Kelp forests support biodiversity, absorb wave energy, and provide habitat for many species. Their collapse has also had economic consequences, including major impacts to California’s commercial red sea urchin fishery. Without enough kelp to eat, red urchins produced poor-quality gonads, making them less marketable and in some cases leading to mortality. In northern California, these losses were severe enough to contribute to federal fishery disaster declarations and funding for recovery, research, and restoration.
"These changes impact not only the dynamics below the surface, but the connections people hold with these habitats and coastal systems," Elsmore said.
In many areas, kelp forests have struggled to return on their own, held back by persistent grazing pressure, shifting ocean conditions, and the loss of key predators that once helped keep ecosystems in balance.
These challenges have prompted scientists and restoration practitioners to take a more active role in helping kelp recover.
Testing Targeted Urchin Removal
Today, restoration practitioners in California are working to reverse these trends in some areas by removing excess purple urchins to reduce grazing pressure and give kelp a chance to recover. Early results from experimental removal sites show promising signs of progress.
These efforts often focus on experimental removal plots, where dense aggregations of purple sea urchins are reduced and scientists monitor how the ecosystem responds. By comparing treated areas to nearby reefs where urchins remain abundant, researchers can better understand how changes in grazing pressure affect kelp growth, biodiversity, and overall ecosystem health.
So far, the program has removed over 183,441 pounds of urchins with varied results measured across restoration sites since 2022. Average annual purple sea urchin densities measured during surveys range from as high as 98.3 and as low as 9.3 urchins per square meter across all sites; and from 11.2 to 32.9 urchins per square meter at the primary restoration sites Fort Ross Cove and Timber Cove.
“In areas where dense urchin populations have been reduced, we’ve seen increases in kelp density and canopy cover,” said Mears. “We’re also seeing bull kelp grow to reproductive adults, which is a key milestone for recovery.”
Even with these gains, recovery is not guaranteed.
“This is something we are working to figure out,” said Lipski. “Another major important factor that we cannot control is large-scale ocean conditions.”
At its core, the goal is not to eliminate sea urchins. They are a natural and important part of kelp forest ecosystems. Instead, restoration practitioners are working to reduce their numbers enough to restore the predator-prey balance that historically kept these systems in check.
"We continue to learn more about the system and the relationships between grazers and kelp through these experimental approaches, which build upon efforts dating back to the 1950s here in California," Elsmore said. "A growing body of work has shifted recent thinking around grazer management. It may be more than just a numbers game—urchin behavior may be another lever to target."
The work underway in Greater Farallones National Marine Sanctuary is part of a broader effort to restore California's kelp forests. In 2019, Greater Farallones Association led the development of the Sonoma-Mendocino Bull Kelp Restoration Plan, a collaborative roadmap created with scientists, resource managers, Tribes, fishermen, nonprofit organizations, and local communities. The plan identified a range of restoration strategies, including targeted urchin removal, kelp outplanting, improved monitoring, and efforts to rebuild ecological resilience in areas most affected by kelp loss.
Greater Farallones Association and Greater Farallones and Cordell Bank National Marine Sanctuaries are also part of the Pacific Coast Ocean Restoration Initiative, an $18 million NOAA-funded effort to advance coordinated marine restoration along the West Coast by supporting the recovery of kelp forests, sunflower sea stars, and other key species.
At the same time, the California Department of Fish and Wildlife and Ocean Protection Council are developing a statewide Kelp Restoration and Management Plan for giant and bull kelp. Rather than focusing on a single species, efforts are aimed at rebuilding the relationships between predators, grazers, and habitat, such as allocating funds to reintroduce sunflower sea stars into the environment.
“The plan will provide proactive approaches for managing, protecting, and restoring kelp forests in the face of changing ocean conditions,” said Elsmore.
Restoring Relationships, Not Just Species
In Florida, the loss of herbivores allowed algae to outcompete coral, prompting scientists to restore grazing. In California, an overabundance of herbivores led to overgrazing, prompting scientists to reduce grazing pressure.
At first glance, these approaches may seem contradictory. But they are guided by the same principle: healthy ecosystems depend on balance. Rather than focusing on a single species or problem, both efforts take an ecosystem-based management approach—considering the relationships among predators, herbivores, habitat, and environmental conditions that shape the health of the entire ecosystem. This a foundational approach for how national marine sanctuaries works to maintain and restore marine ecosystem health, services, and function.
Sea urchins are not inherently beneficial or harmful. Their role depends on the broader web of relationships that surrounds them: predators that keep populations in check, habitats that provide food and shelter, and environmental conditions that allow ecosystems to function as they evolved to.
Across America's National Marine Sanctuary System and surrounding waters, sea urchins are revealing an essential lesson: that predators, grazers, and habitat work together to shape whether coral reefs flourish or kelp forests thrive. As multiple stressors continue to disrupt these systems, restoration efforts are increasingly focused on rebuilding those relationships—restoring not just species populations, but the balance that allows entire ecosystems to function and endure.
Rachel Plunkett is the content manager and senior writer/editor for NOAA’s Office of National Marine Sanctuaries