Overfishing protections off the coast of California helped recover populations of animals that hunted sea urchins, which have been laying waste to the region’s kelp forests. Yet rather than suppressing the urchins’ prevalence by creating more predators, new evidence shows that the protections expanded their numbers, revealing more nuanced ecosystem dynamics than expected.
In a paper published Feb. 17 in Proceedings of the Royal Society B, marine biologists analyzed nearly 30 years of monitoring data to determine how marine reserves established in 2003 affected sea urchins and kelp in the region. They said their findings challenge pervasive assumptions about marine reserves and whether changing the balance of predators and prey creates rippling effects through the ecosystem, known as trophic cascades.
“In a classic trophic cascade model, links in a food chain are connected and removing one of those links can create a domino effect that cascades down to lower levels,” said Katrina Malakhoff, the study’s lead author and a Ph.D. student researching sea urchin ecology at the University of California, Santa Barbara.
Malakhoff referenced the work of ecologist Jim Estes, who documented the effects of sea otter population decline around Alaska’s Aleutian Islands. More than 90% of the urchin-eating marine mammals have disappeared, and sea urchins have subsequently been left unchecked as they consume and destroy vulnerable coral-like kelp reefs.
A similar scenario has been playing out around the Channel Islands, off the coast of Southern California. The threatened southern sea otter species is still recovering from centuries of pervasive hunting, and other urchin predators such as the spiny lobster and California sheephead fish have also been impacted by overfishing. Red and purple sea urchins, meanwhile, have exploded in population size and have left “urchin barrens” by overgrazing the kelp forests, which host biodiverse ecosystems.
The California Fish & Game Commission created several “marine protected areas” at the Channel Islands in 2002 that banned harvesting of sea life, which has led to recoveries in the spiny lobster and sheephead populations. Many scientists expected urchin numbers to fall as a result of the rising predator abundance in the marine reserves.
Malakhoff and her advisor, UC Santa Barbara research biologist Robert Miller, analyzed annual data on the reserve and non-reserve sites spanning 13 years before they were established and 15 years following, obtained from the U.S. National Parks Service’s Kelp Forest Monitoring Program. They tracked the urchin predators, one species of kelp and both species of sea urchin: red, which were fished before the reserves were established, and purple, which were not.
Their results defied the domino effect predicted by the trophic cascade model, showing urchin population density rose in marine reserves — alongside that of its predators — and rose nearly three times faster than at fished sites. The increase was mostly driven by red urchins, while purple urchins were unaffected by the reserves. A similar trend held true for small-bodied urchins, which are generally more vulnerable targets for predators.
Urchin predators had higher population densities in the reserves where sea urchins flourished the most, despite being expected to suppress the numbers of their prey. Predator data was not of a high enough quality to analyze how they were affected by the marine reserves, according to the researchers. The kelp, meanwhile, was unaffected by reserve status and urchin densities.
“Trophic cascades have been discussed so frequently in the context of marine reserves that they have almost reached paradigm status,” Malakhoff said. “Our results demonstrate, however, that trophic cascades are not always prevalent in marine ecosystems, and that fishing bans in marine reserves will not necessarily result in clear trophic cascades.”
Malakhoff said the surprising results were a sign of more complex factors at play, some not fully understood. The rebounding spiny lobsters and sheephead may not consume enough urchins as part of their diet to outweigh other influences, such as the benefits red urchins received from fishing protections. A trophic cascade may not strike the urchin populations until sea otters recolonize the ecosystem, she said.
The marine biologists also questioned the "widespread depiction" that predation necessarily controls whether the ecosystem produces kelp forests or urchin barrens. They noted that urchin grazing behavior is affected by several factors such as disease and physical disturbance, and that changes in different urchin populations around the world have been driven by other causes.
Malakhoff is currently researching urchins’ diets and feeding behavior, which could affect how they interact with kelp within their larger ecosystems, as well as the effects of environmental factors such as wave height and water temperature on both urchin species in relation with marine reserves.
In the paper, Malakhoff and Miller recognized the scientific value of marine reserves for testing ecological interactions on large scales but emphasized the need to more fully explore their effects on ecosystems.
“This doesn’t mean that marine reserves are a bad idea, or that we should not expand them,” Miller said. “They clearly work very well for protecting fished species.”
“It’s just that some of these popularly held ideas — like that they’ll cause trophic cascades to save the kelp — might be less valid,” he said.
The article, “After 15 years, no evidence for trophic cascades in marine protected areas,” was published Feb. 17 in Proceedings of the Royal Society B. The authors of the study were Katrina Malakhoff and Robert Miller, University of California, Santa Barbara. The lead author was Katrina Makahoff.