Tissue loss rather than colony size determines the demographic fate of the branching coral Acropora cervicornis

Partial mortality is a common process affecting coral colonies. Yet, the impact of tissue loss on the demography of the threatened reef-building coral Acropora cervicornis has been poorly investigated. This limits our understanding of how this species will fare under unfavorable environmental conditions. In this study, we examined the growth and survival of colonies with varying degrees of partial mortality, indicated by tissue loss, for 2 yr at 2 reefs in Puerto Rico. We found that irrespective of colony size, rates of coral growth and survival declined significantly once the proportion of dead tissue exceeded 20% of the total colony size. Projections of state matrix population models indicated that partial mortality could also have a negative impact at the population level. For instance, a 25% increase in the number of colonies with >20% tissue loss would reduce the time in which 75% of the population is lost by 3 to 4 yr. Our results provide a new perspective on the effect of partial mortality on the demography and population dynamics of A. cervicornis. First, 20% of tissue loss can be considered a threshold value in which colony fate and population growth are compromised. Second, colony size is not the most important determinant of a colony's demographic performance; instead, the surface area lost to partial mortality is a better predictor of colony growth and survivorship. Taking into consideration the relationship between partial mortality and the demographic fate of A. cervicornis can aid in the development of stronger conservation and restoration programs.