Sea urchins are widespread marine invertebrates that play a complex role in ocean ecosystems as herbivores, grazing primarily on algae. The question of whether they are an invasive species is not a simple yes or no, as the answer depends on the specific context and location. In some areas, destructive population booms are caused by native species, which is an ecological disruption, while in other regions, the problem is caused by a truly non-native species introduction. Understanding this distinction is fundamental to marine conservation efforts and the development of effective management strategies.
Understanding Ecological Status
To be classified as a true invasive species, an organism must meet two strict ecological criteria: it must be non-native or alien to the ecosystem, and its introduction must cause or be likely to cause environmental or economic harm. Many species that cause massive destruction, including problematic sea urchin populations, are native to the area. A native species that experiences a population explosion that damages its habitat is often referred to as a pest or an outbreak species, but it does not fit the scientific definition of an invasive species.
The vast majority of destruction caused by sea urchins in places like the North American Pacific coast involves native species, such as the purple sea urchin (Strongylocentrotus purpuratus). These outbreaks are the result of an imbalance in the food web, rather than the arrival of a foreign organism. The ecological status of a species is defined by its origin and its impact, which dictates the appropriate response from environmental managers.
The Mechanism of Urchin Barrens
The most common and destructive scenario involves native sea urchins transitioning from a balanced grazer population to an overwhelming destructive force, often resulting in “urchin barrens.” This shift is triggered by the loss of keystone predators that naturally keep urchin numbers in check, creating a phenomenon known as a trophic cascade. On the Pacific coast of North America, for instance, sea otters and the sunflower sea star were historically the primary predators of sea urchins.
Human activities, such as historical fur trading, decimated sea otter populations in many regions, removing a natural control mechanism. More recently, the massive die-off of the sunflower sea star due to Sea Star Wasting Syndrome, an epidemic that began around 2013, further eliminated a major predator. Without this predatory pressure, native urchin populations, like the purple sea urchin, have multiplied unchecked, reaching densities up to 60 times higher than normal levels.
When urchin populations explode, they graze voraciously on the holdfasts, or anchor structures, of kelp, effectively clear-cutting the underwater forests. This destructive grazing transforms a biodiverse kelp forest into an urchin barren, a collapsed ecosystem state dominated by bare rock and coralline algae. Once the kelp is gone, the urchins can enter a “zombie-like” state, surviving for years without food. This stable, barren state is maintained by the high density of starved urchins, which quickly consume any new kelp growth, making it extremely difficult for the kelp to re-establish.
Identifying Truly Invasive Species
While many problems stem from native overpopulation, true invasive sea urchins exist when a species is transported outside its natural geographic range and successfully establishes a damaging population. One high-profile example is the black long-spined sea urchin, Diadema setosum, which is native to the Indo-Pacific but has been introduced to the Mediterranean Sea. This species is likely a Lessepsian migrant, having entered the Mediterranean via the Suez Canal and thriving as water temperatures increase.
In Australia, the long-spined sea urchin, Centrostephanus rodgersii, is native to New South Wales but has expanded its range south to Tasmania and Victoria, driven by the warming East Australian Current. Although native to the continent, this species is considered a range-extending invasive species in its new southern habitat, where it is creating extensive barrens. These non-native grazers lack natural predators in their new locations and outcompete native species, leading to a profound alteration of the local ecology and biodiversity.
Control and Management Strategies
Addressing the problematic sea urchin populations, whether they are native outbreaks or true non-native invasions, requires a multi-faceted approach focused on restoring ecological balance. One long-term strategy involves the restoration and protection of natural predators, such as reintroducing sea otters to areas where they were historically eliminated, or implementing fishery management practices to increase the size and abundance of rock lobsters and large predatory fish. These efforts aim to re-establish the top-down control that naturally regulates urchin numbers.
For areas already devastated by barrens, more direct intervention is necessary to reduce the immediate grazing pressure. Active removal or culling of urchins, often through manual crushing by divers or targeted harvesting, is employed to temporarily reduce density and create windows of opportunity for kelp recovery. In some regions, there is an effort to develop a commercial fishery for the urchins, harvesting them for their roe, or “uni.”
Successful long-term recovery depends on combining direct removal with physical restoration, which involves replanting kelp in cleared areas. No single management strategy is a complete solution; measures must also address the underlying cause, such as protecting the entire predator community to ensure the kelp forest’s resilience against future outbreaks.