A Predator in Moderation
AFew animals have a worse reputation on the Great Barrier Reef than the crown-of-thorns starfish. Covered in sharp spines and able to grow wider than a dinner plate, it feeds on living coral by pushing its stomach out of its body and digesting the soft tissue of the coral beneath it. When the starfish appear in huge numbers, they can strip large areas of reef of their living coral. Not surprisingly, the species is usually described as one of the reef's greatest threats. New research from the University of Sydney, however, suggests that this picture is incomplete, and that in small numbers the starfish may actually play a useful part in the life of a reef.
BThe study was led by Dr Shawna Foo, a Horizon Fellow in the university's School of Life and Environmental Sciences, and was published in the journal Ecology and Evolution. The team carried out its fieldwork around the university's One Tree Island Research Station on the Great Barrier Reef. The study sites were chosen with care. They lay within protected areas where fish populations were healthy and where crown-of-thorns starfish were present only in relatively low numbers. This allowed the researchers to observe how the starfish affect corals when they are part of a balanced ecosystem, rather than during a population explosion.
CThe scientists examined the scars that the starfish left on corals after feeding and recorded how much living tissue remained. The results were unexpected. In nearly three-quarters of the feeding events they observed, at least some living coral tissue survived. This challenges the common image of the starfish as an animal that leaves nothing behind but bare coral skeletons. According to Dr Foo, the starfish appear to eat a fairly consistent amount of coral at each meal and then move on. As a result, patches of living tissue are frequently left behind, giving the coral a chance to recover.
DWhat happened next depended on the type of coral. Surviving fragments of fast-growing corals from the family Acroporidae did not grow back over the damaged areas. By contrast, regrowth was seen mainly in slower-growing corals belonging to the groups Montipora and Porites. In other words, feeding by the starfish had very different consequences for different kinds of coral. This difference matters. When fast-growing corals are left unchecked, they can spread rapidly and take over much of the available space, leaving little room for slower competitors. By holding back these dominant species, a small population of starfish may create room for slower-growing corals, which could increase the variety of corals on a reef.
ECo-author Professor Maria Byrne points out that the findings are consistent with an established idea in ecology known as the intermediate disturbance hypothesis. This theory proposes that a moderate level of disturbance can stop any one species from dominating an ecosystem. The idea has been used to explain patterns of diversity in a range of habitats, from tropical rainforests to coral reefs. If disturbance is very rare, a few strong competitors tend to take over, while if it is very frequent or severe, only the hardiest species survive. Between these extremes, the theory suggests, a wider range of species is able to live side by side. Seen in this light, the crown-of-thorns starfish may act as a natural source of moderate disturbance, preventing a single type of coral from monopolising the reef.
FNone of this means that the starfish are harmless. The Great Barrier Reef has suffered four major outbreaks since the 1960s, occurring roughly every 15 years, and these remain a serious threat to the reef. One reason reefs have become more vulnerable is that the natural predators of the starfish, such as the triton snail and several species of reef fish, have declined. The researchers stress that there is a clear difference between the starfish acting as a natural predator at low density and an outbreak that causes the long-term decline of a reef.
GThe study also has implications for how the reef is managed. Culling has long been used to bring outbreaks of the starfish under control. The new findings suggest that once an outbreak has been suppressed, the aim should not necessarily be to remove every last starfish. Instead, culling should be combined with strong marine management that protects the natural predators of the starfish, so that the starfish population is kept in check. Such an approach would treat the starfish not simply as a pest to be destroyed, but as a natural member of the reef community. In this way, the reef might keep the benefits of a predator in moderation while avoiding the damage caused by a predator in excess.