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 34 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. Achieving a 5% increase in population growth rate through additive means required ∼100 recruit outplants or 5–10 adult outplants per 1,000 individuals in a population annually. For large populations typical of restoration targets (10 ⁵ –10 ⁷ individuals), this translates to 10 ³ –10 ⁴ outplants per year—efforts rarely logistically or economically feasible. By contrast, improving adult survival or recruitment rates by 20% (1.2 fold) produced equivalent population growth increases, depending on the fraction of the population affected. 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. Our demographic framework underscores that if the restoration goal is to enhance coral cover on a sustained basis, success depends less on adding individuals and more on interventions that will lead to long-term increases in vital rates.