Sea urchins are spiny, spherical marine invertebrates that inhabit ocean floors across the globe, from shallow coastal waters to deep-sea environments. They are echinoderms, related to starfish and sand dollars, and are characterized by their hard, calcified shells, called tests, covered in movable spines. These creatures play various roles within their ecosystems, often grazing on algae and detritus. A common question is whether sea urchins are considered an invasive species or if they naturally belong in their marine habitats.
What Makes a Species Invasive?
An invasive species is defined as an organism not native to a particular ecosystem whose introduction causes or is likely to cause environmental or economic harm, or harm to human health. This classification is distinct from merely being a non-native species, as many introduced species do not cause significant negative impacts. For a species to be deemed invasive, it must be introduced outside its natural range and establish a population that actively threatens native biodiversity and ecosystem functions.
Common characteristics of invasive species include rapid reproduction and growth rates, high dispersal abilities, and the capacity to adapt to a wide range of environmental conditions. They often outcompete native organisms for resources, leading to declines in native populations. Human activities, whether intentional or accidental, are the primary means by which species are introduced to new environments, often through global trade and transport.
Sea Urchins in Balanced Ecosystems
In their native habitats, sea urchins play an important role in maintaining the health and balance of marine ecosystems, particularly in kelp forests and coral reefs. As herbivores, they primarily graze on algae, preventing overgrowth that could otherwise smother corals and other marine life. This grazing activity promotes biodiversity and creates suitable settlement space for new coral polyps and kelp.
Sea urchins also contribute to marine food webs, serving as a food source for a variety of predators. These include sea otters, spiny lobsters, crabs, starfish, and various fish species such as wolf eels and California sheephead. The presence of these predators helps regulate sea urchin populations, ensuring their numbers remain in check and their grazing benefits the ecosystem. In some cases, sea urchins can also act as “shredders” of kelp detritus, making nutrients available to other organisms on the seafloor.
When Sea Urchins Become a Problem
While sea urchins are integral to healthy marine environments, their populations can surge under certain conditions, transforming them into destructive forces. A primary factor contributing to these outbreaks is the decline of their natural predators. For instance, the historical overhunting of sea otters along the Pacific Coast significantly reduced a major consumer of sea urchins. More recently, a mysterious sea star wasting syndrome, which began around 2013, decimated populations of sunflower sea stars, another major predator of purple sea urchins (Strongylocentrotus purpuratus) in regions like California and Oregon.
Concurrent with predator loss, changing ocean conditions, particularly marine heatwaves such as “the blob” that affected the Northeast Pacific from 2014, have further exacerbated the issue. These warmer temperatures stress kelp, hindering its growth and making it more vulnerable to grazing. This combination of reduced predation and weakened kelp creates an environment where sea urchin populations can explode, increasing by over 10,000% in some areas since 2014. In the Caribbean, the long-spined sea urchin (Diadema antillarum) experienced mass die-offs in the 1980s, leading to algal overgrowth due to their absence as grazers. While their recovery has been slow, the ecological balance on reefs can shift dramatically when such herbivore populations fluctuate, potentially leading to overgrazing if they recover to high densities without adequate predatory control or sufficient food sources.
The Ripple Effect of Urchin Outbreaks
When sea urchin populations become unchecked, their intense grazing pressure can devastate entire marine ecosystems. This overgrazing transforms vibrant kelp forests into barren, desolate seafloors known as “urchin barrens.” These barrens are characterized by vast stretches of rock covered primarily by dense aggregations of sea urchins, with little to no kelp or other macroalgae remaining.
The loss of kelp forests has severe cascading effects on biodiversity. Kelp provides essential habitat, food, and shelter for numerous marine species, including fish, abalone, and various invertebrates. As kelp disappears, the species that rely on these underwater forests for survival also decline, leading to a significant reduction in overall ecosystem productivity and a collapse of localized food webs. This ecological shift not only impacts marine life but can also affect fisheries that depend on these healthy habitats.
The Ripple Effect of Urchin Outbreaks
When sea urchin populations become unchecked, their intense grazing pressure can devastate entire marine ecosystems. This overgrazing transforms vibrant kelp forests into barren, desolate seafloors known as “urchin barrens.” These barrens are characterized by vast stretches of rock covered primarily by dense aggregations of sea urchins, with little to no kelp or other macroalgae remaining. For example, 95% of California’s kelp forests have been decimated since 2014, replaced by these urchin barrens.
The loss of kelp forests has severe cascading effects on biodiversity. Kelp provides essential habitat, food, and shelter for numerous marine species, including various fish, abalone, and other invertebrates. As kelp disappears, the species that rely on these underwater forests for survival also decline, leading to a significant reduction in overall ecosystem productivity and a collapse of localized food webs. This ecological shift not only impacts marine life but can also affect fisheries that depend on these healthy habitats, such as those for red abalone and red sea urchins, which have seen significant declines.
Responding to Urchin Invasions
Efforts to manage excessive sea urchin populations and restore affected ecosystems involve a range of strategies. Direct intervention methods include manual culling, where divers physically remove or crush urchins, which can be effective for localized areas. Chemical application, such as quicklime, has also been tested to reduce urchin densities.
Another approach involves promoting the recovery or reintroduction of natural predators, such as sea otters and large lobsters, as their presence can help naturally control urchin numbers and maintain ecosystem balance over time. Commercial harvesting and “urchin ranching” initiatives aim to turn the problem into a solution by collecting wild, often starving, urchins from barrens and feeding them in aquaculture systems to produce valuable roe for the seafood market. This creates an economic incentive for urchin removal while supporting kelp forest recovery. These combined efforts contribute to restoring the health and biodiversity of marine environments impacted by urchin overpopulation.