Transient Grazing and the Dynamics of an Unanticipated Coral–Algal Phase Shift

Abstract

Extreme disturbances often lead to community reorganisations, yet sometimes ecosystems unexpectedly fail to recover. Such surprising outcomes may pinpoint important yet overlooked mechanisms that drive ecosystems into undesirable states. Using long-term field observations, experimental manipulations and mechanistic modelling, we document the drivers of an unexpected phase shift from coral to macroalgal dominance following typhoon disturbance on reefs in Palau (Micronesia). After extensive coral mortality, an ephemeral bloom of a canopy-forming macroalga (Liagora sp.) provided physical refuge from herbivore grazing, resulting in the establishment of a secondary, understory macroalga (Lobophora spp.). After disappearance of Liagora canopies and resulting loss of grazing refuge, the Lobophora patches continued to expand and led to a macroalgal (Lobophora-) dominated state that has persisted for more than 2 years. We developed a mechanistic model of Lobophora patch dynamics parameterised with rates of growth measured in situ to simulate the observed proliferation of Lobophora under variable grazing refuges in space and time. Model simulations showed that short-term escapes from grazing were pivotal in allowing establishment of patches of Lobophora. Ephemeral grazing refuges created an opportunity to reach a cover above which Lobophora growth exceeds grazing, so that Lobophora could expand after disappearance of Liagora canopies. Critically, in the absence of grazing refuge, herbivore biomass was sufficient to prevent the establishment of Lobophora patches. Our model demonstrates that with rapid algal growth and low grazing, a relatively minor grazing refuge (6 month) is sufficient to escape herbivore control after extensive coral mortality, leading to unexpected recovery failure. Transient fluctuations in the intensity of control mechanisms, such as herbivore grazing, can have disproportionate and long-lasting effects on community structure. Overall, this study stresses that our perception of reef dynamics must integrate the time scales at which reefs can be sensitive to transient changes in mechanisms promoting coral dominance.