Demographic insights for coral restoration

Coral reef decline has prompted a global surge in reef restoration initiatives. The success of initiatives that aim to sustain coral populations or assemblages will depend on demographic principles. Restoration strategies generally follow two demographic pathways: additive approaches, which increase population numbers by introducing recruits, fragments, or adults without altering vital rates; and multiplicative approaches, which enhance vital rates-growth, survival, or reproduction-either extrinsically (e.g., herbivory, habitat protection) or intrinsically (e.g., assisted evolution). We synthesized 28 coral matrix population models spanning morphologically diverse coral species from the Caribbean, Hawaii, and the Great Barrier Reef to quantify additive and multiplicative changes needed to increase population growth. Results highlight two consistent demographic leverage points for which population growth was most sensitive:(1) survival of reproductive adults and (2) successful recruitment. Improving adult survival or recruitment by 20% through multiplicative means produced a 5% increase in population growth rate, depending on the fraction of the population affected. By contrast through additive means, the same population growth increases required [~]100 recruit outplants or 5-10 adult outplants per 1,000 individuals in a population annually. For large populations typical of restoration targets (105-107 individuals), this translates to 103-104 outplants per year--levels rarely logistically or economically feasible. These findings yield two key implications. First, additive interventions are inefficient for large populations and demand sustained, large-scale effort. Second, strategies enhancing vital rates across broad population segments represent the most effective means of boosting coral abundance, including habitat protection, the alleviation of environmental stressors, and interventions which promote long-term survival and recruitment.