Abstract
Outbreaks of predatory crown-of-thorns seastars (COTS) can devastate coral reef ecosystems, yet some corals possess mutualistic guardian crabs that defend against COTS attacks. However, guarded corals do not always survive COTS outbreaks, with the ecological mechanisms sealing the fate of these corals during COTS infestations remaining unknown. In August 2008 in Moorea (17.539° S, 149.830° W), French Polynesia, an unusually dense multi-species aggregation of predators was observed feeding upon guarded corals following widespread coral decline due to COTS predation. Concurrent assaults from these amplified, mixed-species predator guilds likely overwhelm mutualistic crab defense, ultimately leading to the fall of guarded corals. Our observations indicate that guarded corals can sustain devastating COTS attacks for an extended duration, but eventually concede to intensifying assaults from diverse predators that aggregate in high numbers as alternative prey decays. The fall of guarded corals is therefore suggested to be ultimately driven by an indirect trophic cascade that leads to amplified attacks from diverse starving predators following prey decline, rather than COTS assaults alone.
Highlights
Identifying ecological processes that drive species trajectories is a prerequisite for ecosystem management
Trapeziid crabs and alpheid shrimps inhabiting large pocilloporids are known for their ability to effectively defend their host corals from crown-of-thorns seastar (COTS) assaults (Glynn, 2013; McKeon & Moore, 2014), guarded pocilloporids do not always survive COTS outbreaks (Leray et al, 2012; see Figure 1)
While butterflyfishes were increasingly observed to gather around guarded corals as the COTS outbreak progressed around the island, the aggregation of 9 butterflyfishes within a single square-meter (9 fish.m-2), as captured in Figure 2, was surprising
Summary
Any reports and responses or comments on the article can be found at the end of the article. Keywords Predator outbreak, Acanthaster, Mutualistic defense, Guardian crab, Trapezia, Mixed-species predator guild, Trophic cascade, Density dependence. We have updated our manuscript following judicious comments provided by the referees. The new version notably includes further information, as well as an extended discussion, on the reported ecological process and its prevalence in our study system. The new version provides more details on coral mutualist crustaceans and clearer Figure 1, Figure 2, and Supplementary Image 1. Following comments from the referees, we have added arrows to the figures, in order to clarify which parts we are referring to. We are thankful to the referees for their constructive remarks
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