Crown-of-thorns starfish (COTS) are infamous in the marine world. Their toxin-loaded thorns give them a menacing appearance and they have a reputation as destroyers of coral reefs.
New research from the University of Sydney challenges the perception of COTS as purely destructive. The findings suggest controlled populations can have a positive impact on coral prevalence and diversity.
Coral-eating COTS are native to the Great Barrier Reef, where they have caused mass coral destruction. Since the 1960s, the Reef has recorded four major outbreaks of COTS, with outbreaks erupting approximately every 15 years. Diminishing populations of COTS predators – including tritons and multiple coral reef fishes – heighten the vulnerability of the reef, and marine environments worldwide, to COTS outbreaks.
Research published today in Ecology and Evolution reports the results of a survey tracking coral feeding scars and surviving coral tissue after COTS predation at the University's One Tree Island Research Station on the Great Barrier Reef. The surveyed sites are in protected areas, where healthy fish populations control COTS populations, preventing destructive outbreaks like those observed elsewhere on the reef.
At these sites, a research team led by Dr Shawna Foo found living coral tissue remained after almost 75 percent of the COTS feeding events examined.
"At One Tree Reef, COTS appear to consume a relatively consistent amount of coral during each meal and move on, rather than demolishing the entire coral colony," Dr Foo, a University of Sydney Horizon Fellow from the School of Life and Environmental Sciences , said. "This meant live coral tissue remnants were frequently left behind."
The findings suggest that, at low densities, COTS support coral biodiversity by preventing dominant coral species from monopolising reef space. The study found remnants of commonly found, fast-growing Acroporidae corals did not regenerate over COTS feeding scars. Regeneration occurred mostly among much slower-growing Montipora, and Porites corals.
"By feeding on fast-growing corals such as Acropora, low-density COTS populations create space for slower-growing coral species to persist, helping to increase coral diversity," Dr Foo said.
Dr Foo emphasised the findings do not diminish the significance of devastating COTS outbreaks which coral populations struggle to recover from. Rather, they highlight the upside of a low-density COTS population in a healthy marine ecosystem.
"It's important to distinguish between COTS populations performing their natural role as coral predators and outbreaks causing sustained reef decline."
Although largely conducted in areas with healthy fish populations not susceptible to COTS outbreaks, the research can inform culling practices for future outbreaks.
"The implications of our work can help us decide when and why culling should occur and the ecological endpoint of a cull. Once an outbreak is suppressed, the goal should not necessarily be to eliminate COTS entirely. Culling needs to be complemented by strong marine management to protect COTS predators and keep COTS populations in check."
Research co-author Professor Maria Byrne said: "The results are consistent with the intermediate disturbance hypothesis, an important framework in ecology and conservation." This hypothesis proposes that occasional, moderate disturbance in an ecosystem can help maintain biodiversity.
