Demographic insights for coral restoration

Abstract

O_LICoral 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: supplemental approaches, which increase population numbers through the repeated addition of recruits, fragments, or adults, without fundamentally altering long-term population dynamics; and structural approaches, which enhance vital rates-growth, survival, or reproduction- through changes that modify the demographic processes governing population growth on a sustained basis, either extrinsically (e.g., herbivory, habitat protection) or intrinsically (e.g., assisted evolution). Some interventions, such as supplemental feeding, may temporarily improve vital rates but still function as supplemental approaches because benefits persist only while interventions continue. C_LIO_LIWe synthesized 28 coral matrix population models spanning morphologically diverse coral species from the Caribbean, Hawaii, and the Great Barrier Reef to quantify supplemental and structural changes needed to increase population growth. C_LIO_LIResults 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 structural means produced a 5% increase in population growth rate across population on average, assuming the whole population was affected. By contrast through supplemental means, the same increase 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 adult and 104 recruit outplants per year--levels rarely logistically or economically feasible. C_LIO_LIThese findings yield two key implications. First, supplemental interventions are inefficient for large populations and demand sustained, large-scale effort, even when they temporarily enhance growth or survival. Second, strategies enhancing vital rates across broad geographic areas represent the most effective means of boosting coral abundance, including habitat protection, alleviation of environmental stressors, and interventions which promote long-term survival and recruitment. Our demographic framework underscores that if the restoration goal is sustained increases in coral cover, success depends less on repeated supplements and more on interventions that produce lasting improvements in vital rates. C_LI