Rising cover amid population density decline: the unstable demography of a reef-building coral

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Abstract
Monitoring habitat-forming species, such as scleractinian corals, is crucial for managing ecosystems and biodiversity. Yet for assessing population health, challenges remain in reconciling conventional areal coverage surveys with individual-based demographic techniques. Here, we explore both monitoring approaches to characterize the population dynamics of the reef-building coral, Acropora cf. digitifera , on a West Pacific outer reef using photogrammetry reef mapping data spanning 5 years. Tracking 906 coral colonies showed that those exceeding approximately 10 cm diameter exhibit the fastest planar growth but also suffer excess mortality, possibly owing to increased structural vulnerability to dislodgement by wave energy or to other factors, such as disease or predation, that may be exacerbated by senescence. Area-controlled orthomosaic subsampling paired to integral projection modelling revealed a consistent decline in population density, as recruitment was insufficient to balance losses of larger colonies. Yet, total areal coverage and median colony size increased over the study, suggesting this disequilibrium population is on a recovery trajectory from past disturbance and heavily reliant on sporadic recruitment pulses not detected in our study. We find that conventional areal monitoring and demographic approaches can yield contrasting conclusions about population dynamics. Reconciling these differences for disequilibrium populations requires long-term demographic data over periods long enough to detect infrequent yet critical demographic events such as large post-disturbance recruitment events.
Funding Information
  • European Research Council
  • Royal Geographical Society
  • Natural Environment Research Council
  • Reef Restoration and Adaptation Program

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