Paper: Clements CS, Hay ME (2017) Size matters: Predator outbreaks threaten foundation species in small Marine Protected Areas. PLoS ONE 12(2): e0171569. doi:10.1371/journal.pone.0171569.
Where do you go when you’re ravenously hungry? Probably some fast-food joint, sandwich shop, grocery store—the ready to go meal section—or snack shop, right? Basically, you go where the food is plentiful and quickly available. As it turns out, the crown-of-thorns sea stars (Acanthaster spp.), abbreviated COTS, do the same thing. However, they do so en masse. Imagine an entire hungry football team traveling through the middle of nowhere, miles and miles without exits or signs of food, finally descending upon a small town all-you-can-eat buffet. They clean the place out! So too could the COTS on small coral-rich Marine Protected Areas.
Let’s start at the beginning. Marine habitats are globally degrading due to anthropogenic impacts. To combat this loss and protect ecosystem inhabitants and services, there has been an increase in the establishment of Marine Protected Areas (MPAs), generally to protect foundation species (those upon which other species depend on for habitat, such as coral, seagrass, mangroves, kelp). Surrounding areas will then benefit from the spillover of larvae and dependent species. In general, MPAs have been successful. However, MPA design and management varies and in some cases, they seem to hasten decline of the habitat they were meant to protect. Sometimes, small MPAs are surrounded by heavily degraded and exploited habitat. There is great debate about the best design for MPAs, but predators of the foundation species or ecosystem engineers are not normally discussed. They probably should be though.
Enter our predator, the COTS. They are one of the major factors in the decline of coral on reefs in the Pacific, including the Great Barrier Reef. They experience population outbreaks with the potential to collapse reef systems. And there are many hypotheses as to why these outbreaks occur such as: 1) reduction in population control via predation, disease, etc. or 2) aggregation of adults who cued in on the same signals from preferred corals. The cause remains unknown, though studies are underway. Regardless, they may pose a risk to MPAs, especially small ones.
Researchers Clements and Hay assessed the impact of COTS on three small no-take MPAs on reef flats in Fiji, each paired with a fished reef flat area nearby. The MPAs showed high coral cover (38-56%), low macroalgal cover (1-3%), and higher and more diverse populations of fishes while the fished areas had low coral cover (4-16%), high macroalgal cover (49-91%), and not surprisingly fewer and less diverse fishes. They surveyed the abundance of COTS in each of these areas. Then they tagged 120 COTS (20 from each of the 3 MPAs and 3 fished areas). Then 40 from each area were released along the MPA border and their movements were monitored daily for 4-8 days. Benthic (seafloor) surveys of the border were also conducted to assess the habitats inside and outside of the MPA.
The density of COTS was 2-3.4 times higher in MPAs than fished areas. And these densities were approximately twice as high as what is considered outbreak density.
In the release study, COTS showed a preference for the MPA: 73% of tagged individuals move to the MPA, a pattern that remained consistent even when accounting for whether they were found in the MPA or fished area. (Fig. 2 from article).
The difference in benthic habitat immediately on each side of the border was dramatic. Coral cover only 20m inside the border was already 80-440% higher than 20 m outside in the fished area. Macroalgal cover was 20-610% greater outside the MPA than inside it. Those differences increased in the centers of the areas. As coral cover increased, the COTS rate of displacement declined. That means that they quickly moved from areas with low coral cover toward higher coral cover and as the coral increased, they slowed down.
What that means
Unlike previous studies that have shown lower COTS densities in large MPAs, the increased coral cover within these small MPAs seems to attract the corals’ predator, the COTS, so much so that they reach outbreak densities. This may be a terrible and unanticipated result of small MPAs, especially those with such stark differences in coral within and outside the boundary. The greater perimeter to area ratio of small MPAs may also enhance the movement of COTS into the MPAs. Finally, the reefs in this study were on shallow flats unlike previously studied reefs and therefore may have different processes at work. Finally, it may also be that predators of the COTS are not supported by smaller MPAs, however little is known about the critical predators of COTS or the lifestage at which they are most vulnerable. We do know some creatures deter them though. Clearly though, COTS actively migrate to coral-rich habitats and aggregate there in high densities. The coral cover is still high in these Fiji MPAs, possibly due to the COTS outbreak being recent or possibly due to coral growth rates outstripping the consumption of the COTS. However, their densities are still concerning and warrant further monitoring.
This study adds to evidence that MPAs can attract consumers of the foundation species they seek to protect. Other examples include sea turtles overgrazing MPAs with high seagrass cover and elephants causing plant community shifts in African reserves. It will be important to consider the susceptibility of an area to COTS outbreaks and the potential for MPA success to increase that susceptibility when designing new and managing current MPAs. Small MPAs are a popular choice in the Pacific where ownership and governance of marine resources are local. And they have been immensely successful where enforced, but with this success may come a risk of attracting coral consumers who could flip that success into failure. Plans need to be made for managing predators like the COTS, such as removal or injections. Otherwise, small coral MPAs could become a series of tiny all-you-can-eat buffet diners put out of business by the ravenous team of crown-of-thorns sea stars.
What do you think? Should small MPAs still be used as a conservation tool? What other examples of unintended consequences of conservation methods do you know about? Sound off in the Comments!