Sea urchins, spiny marine invertebrates, are experiencing significant population shifts globally. These changes are causing concern among scientists and conservationists due to the cascading effects on marine ecosystems. The question of “what happened?” to these populations involves a complex interplay of environmental factors and human activities.
The Urchin’s Ecological Role
Sea urchins are herbivores that play a fundamental role in maintaining the balance of marine ecosystems, particularly in kelp forests and coral reefs. They primarily graze on algae, including large kelp and turf algae, which prevents algal overgrowth. This grazing activity ensures that light and space remain available for other organisms, such as corals, to thrive.
Their feeding habits indirectly support a diverse array of marine life by preserving these habitats. In kelp forests, sea urchins consume drift kelp and even break down tougher pieces, making nutrients available for other detritivores on the seafloor. On coral reefs, they graze around 45% of algae, preventing it from smothering corals, which is vital for the health and resilience of marine environments.
Drivers of Population Change
Several factors contribute to changes in sea urchin populations, leading to either drastic declines or, in some instances, population explosions. Disease outbreaks represent a significant threat to sea urchins. For example, a severe die-off of long-spined sea urchins (Diadema antillarum) across the Caribbean Sea in 2022 was attributed to a microscopic single-celled parasite identified as Philaster apodigitiformis, a type of ciliate. This parasite caused declines of between 85% and 95% in affected areas.
The loss of natural predators also drives population imbalances. Sea otters, for instance, are significant predators of sea urchins in kelp forest ecosystems, and their presence helps control urchin numbers. When predator populations decline due to human activities, such as overfishing or disease, sea urchin populations can surge, leading to overgrazing of kelp forests and the formation of “urchin barrens.” This can transform vibrant underwater forests into barren seafloors.
Climate change impacts sea urchins through various mechanisms. Ocean acidification, caused by increased absorption of carbon dioxide, reduces carbonate ions essential for building their shells, spines, and teeth, leading to slower growth and larval abnormalities. Marine heatwaves also affect kelp productivity, reducing the food supply for sea urchins and increasing competition. Additionally, extreme weather events like torrential rainfall can decrease ocean salinity, negatively impacting sea urchins’ ability to attach and move, vital for their survival.
Pollution, particularly plastic pollution, poses another threat. Research indicates that chemicals leached from plastic particles can cause deformities and even death in sea urchin larvae, affecting their skeletal and nervous systems. While high concentrations of plastic are needed to cause these severe effects, such conditions can occur in localized areas of high pollution. Overharvesting for commercial purposes, like for their roe, also contributes to population declines in some regions.
Case Studies of Urchin Decline
The impact of altered sea urchin populations is evident in several marine environments. One prominent example is the transformation of kelp forests into “urchin barrens” in regions like California and Australia. In Northern California, a combination of a marine heatwave from 2014-2017 and a mass die-off of sea stars (a sea urchin predator) led to a significant increase in purple sea urchin populations. This resulted in a more than 90% reduction in bull kelp canopy along over 350 km of coastline, as urchins overgrazed the struggling kelp.
The Caribbean experienced a widespread die-off of long-spined sea urchins (Diadema antillarum) in 2022. This event, caused by a scuticociliate parasite, led to a decline of 85% to 95% of the population in affected areas within months. This die-off mirrored a similar, unexplained event in the 1980s that also drastically reduced Diadema populations, impacting coral reef health by allowing algae to proliferate.
Marine heatwaves have also triggered regional impacts. In Australia, warming waters have contributed to shifts in kelp ecosystems, affecting sea urchin dynamics. The vulnerability of red sea urchin populations to climate change, including temperature changes, varies by location, with some populations showing greater sensitivity to warming. These case studies demonstrate how different drivers can combine to create substantial ecological shifts.
Efforts Towards Recovery and Management
Scientists and conservationists are implementing various strategies to address the challenges facing sea urchin populations and the ecosystems they inhabit. Research and monitoring programs are underway to better understand the complex interactions between urchins, their predators, kelp, and environmental factors. For example, long-term studies track changes in kelp forest dynamics and sea urchin behavior to inform management decisions.
Restoration efforts often involve direct intervention to manage urchin populations. In areas where urchins are overpopulated and forming barrens, culling (removal or crushing) of sea urchins has proven effective in promoting kelp regrowth. Projects in California have shown that reducing urchin densities can lead to the restoration of kelp forests and the return of various marine species within months. For instance, at Tankers Reef in Monterey, permitting unlimited sea urchin removal aided kelp recovery, and the kelp forest has persisted.
Conservation of natural predators is another approach. Protecting and enhancing populations of sea otters and other urchin predators, such as California sheephead and spiny lobster, helps maintain a natural balance. Marine protected areas, where fishing of predators is restricted, can indirectly contribute to kelp forest resilience by allowing predator populations to thrive and control urchin numbers. These integrated management strategies aim to promote healthy marine ecosystems.