Resilience of Tropical Ecosystems to Ocean Deoxygenation.

The impacts of ocean deoxygenation on biodiversity and ecosystem function are well established in temperate regions, and here we illustrate how the study of hypoxia in tropical ecosystems can offer insights of general importance. We first describe how mechanisms of resilience have developed in response to naturally occurring hypoxia across three tropical ecosystems: coral reefs, seagrass beds, and mangrove forests. We then suggest that the vulnerability of these systems to deoxygenation lies in interactions with other stressors that are increasing rapidly in the Anthropocene. Finally, we advocate for the adoption of a broader community- and ecosystem-level perspective that incorporates mutualisms, feedbacks, and mechanisms of self-rescue and recovery to develop a better predictive understanding of the effects of deoxygenation in coastal ecosystems.

Density-dependence mediates coral assemblage structure.

Density dependence (DD) controls community recovery following widespread mortality, yet this principle rarely has been applied to coral assemblages. The reefs of Mo'orea, French Polynesia, provide the opportunity to study DD of coral population growth, because coral assemblages in this location responded to declines in abundance with high recruitment and an increase in cover during which recruitment of pocilloporid corals was inversely associated with density. This study tests for DD in this system, first, by describing the context within which it operates: coral cover changed from 46% in 2005, to <1% in 2010 following an outbreak of a corallivorous sea star and a cyclone, and then increased to 74% by 2017, in large part through inverse density-associated pocilloporid recruitment. Second, a test for DD of recruitment was conducted by decreasing Pocillopora spp. cover from 33% to 19%: one year later, the density of Pocillopora spp. recruits was 1.65-fold higher in the low vs. high cover treatment. Finally, the effects of DD were investigated by comparing simulated and empirical distributions of pocilloporid colonies: as predicted by DD, small colonies were randomly distributed, while large colonies were uniformly distributed. Together these results demonstrate DD of population regulation for Pocillopora spp. corals, thus revealing the potential importance of this ecological principle in determining the resilience of coral assemblages.