Regime shifts on tropical coral reef ecosystems: future trajectories to animal-dominated states in response to anthropogenic stressors

Despite the global focus on the occurrence of regime shifts on shallow-water tropical coral reefs over the last two decades, most of this research continues to focus on changes to algal-dominated states. Here, we review recent reports (in approximately the last decade) of regime shifts to states dominated by animal groups other than zooxanthellate Scleractinian corals. We found that while there have been new reports of regime shifts to reefs dominated by Ascidacea, Porifera, Octocorallia, Zoantharia, Actiniaria and azooxanthellate Scleractinian corals, some of these changes occurred many decades ago, but have only just been reported in the literature. In most cases, these reports are over small to medium spatial scales (<4 × 10⁴ m² and 4 × 10⁴ to 2 × 10⁶ m², respectively). Importantly, from the few studies where we were able to collect information on the persistence of the regime shifts, we determined that these non-scleractinian states are generally unstable, with further changes since the original regime shift. However, these changes were not generally back to coral dominance. While there has been some research to understand how sponge- and octocoral-dominated systems may function, there is still limited information on what ecosystem services have been disrupted or lost as a result of these shifts. Given that many coral reefs across the world are on the edge of tipping points due to increasing anthropogenic stress, we urgently need to understand the consequences of non-algal coral reef regime shifts.