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Towards a multi-stressor theory for coral reefs in a changing world

Coral reefs are facing a constant barrage of human impacts, including eutrophication, overharvesting and climate change. However, research and management are just beginning to depart from a single-dominant-stressor paradigm and a holistic ecosystem-based understanding of these systems is still in its infancy. We expand on a well-known theoretical model to motivate an integrated multi-stressor framework for coral reefs by incorporating empirical evidence of multi-stressor impacts (overfishing, eutrophication and climate-driven mortality) as well as general ecological and theoretical concepts. We show that: i) the geometry of a simple, empirically-motivated model suggests nutrients and harvesting can operate similarly in driving shifts from coral- to algae-dominated reefs; ii) these impacts increase nonlinearly when acting concurrently, resulting in clear context-dependent management implications; and iii) this same geometry suggests climate-driven coral mortality (temperature-stress, cyclonic storms) can drive the presence of long transients and climate-driven alternate states, even in moderately-impacted ecosystems. These results imply that reefs that appear to be in a “safe space” may in fact be in danger of being pushed into a degraded algae-dominated state as storms and bleaching events are increasing in frequency and magnitude. Altogether, we find that responses in benthic composition as “signatures of change” to multi-stressors allows us to develop a predictive multi-stressor framework for coral reefs. In line with this theory, we detail empirical evidence from Barbados that highlights the context-dependent nature of coral reefs in a changing world. Our results present novel and generalizable insights into the functioning of coral reefs, that draw from classic theoretical and ecological concepts such as keystone predation theory, ecological succession and life history theory, as well as the emerging fields of long transients and early warning signals. By bridging coral reef ecology and general ecological concepts, we can better understand ecosystem functioning and resilience in these important yet highly threatened systems. Open Research Statement: Data used in this manuscript are already published and publicly available. Sources can be found within the manuscript and in Supplement B. Mathematical analyses and simulations were performed in Wolfram Mathematica (version 12.1.0.0); code is available at: https://github.com/carlingbieg/corals