Experimental support for alternative attractors on coral reefs

ABSTRACT: Significance Ecological theory predicts that under the same environmental conditions, an ecosystem could have more than one community state that is maintained by reinforcing feedbacks. If so, a sufficiently large disturbance can flip the system to a less desired community that is difficult to reverse. Here, we demonstrate that a coral reef can become trapped in a seaweed-dominated state in the same conditions under which corals thrive. The implications are profound, particularly in light of the increasing occurrence of shifts to seaweed on coral reefs worldwide. Our results indicate that anticipatory management strategies that lessen the chance of a switch to seaweeds will be more effective than those aimed at restoring the coral community after a shift. Ecological theory predicts that ecosystems with multiple basins of attraction can get locked in an undesired state, which has profound ecological and management implications. Despite their significance, alternative attractors have proven to be challenging to detect and characterize in natural communities. On coral reefs, it has been hypothesized that persistent coral-to-macroalgae “phase shifts” that can result from overfishing of herbivores and/or nutrient enrichment may reflect a regime shift to an alternate attractor, but, to date, the evidence has been equivocal. Our field experiments in Moorea, French Polynesia, revealed the following: (i) hysteresis existed in the herbivory–macroalgae relationship, creating the potential for coral–macroalgae bistability at some levels of herbivory, and (ii) macroalgae were an alternative attractor under prevailing conditions in the lagoon but not on the fore reef, where ambient herbivory fell outside the experimentally delineated region of hysteresis. These findings help explain the different community responses to disturbances between lagoon and fore reef habitats of Moorea over the past several decades and reinforce the idea that reversing an undesired shift on coral reefs can be difficult. Our experimental framework represents a powerful diagnostic tool to probe for multiple attractors in ecological systems and, as such, can inform management strategies needed to maintain critical ecosystem functions in the face of escalating stresses.

SUBMITTER: Schmitt R 

PROVIDER: S-EPMC6410839 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature