Effect of variable recruitment and post‐recruitment herbivory on local abundance of a marine alga

For marine algae, the relative importance of recruitment and post‐recruitment processes in determining population abundance is not well understood. We documented patterns of recruitment and post‐recruitment mortality for a common, clonally reproducing subtidal red alga from southeastern Australia, Delisea pulchra, and used a combination of empirical analyses and population simulations to assess the relative importance of the two processes to local population abundance. Using a hierarchical sampling design consisting of two depths and two spatial scales (quadrats and sites) within each depth, we followed monthly cohorts from recruitment to adulthood (6 mo of age). Recruitment of D. pulchra varied significantly in space and time and was generally decoupled from local adult abundance. Mortality was consistently high, particularly in the deep sites where only 1 of 291 plants survived >5 mo, and in the shallow sites when high densities of grazing sea urchins were present. Both early (1 mo) and late (6 mo) post‐recruitment mortality of monthly cohorts were density independent at both spatial scales examined. Despite this pattern of mortality, variation in recruitment explained a significant amount of the subsequent variation in adult abundance in only 3 out of 10 cohorts at both spatial scales. This result indicates that high density‐independent mortality was often important in determining adult abundance for individual cohorts. However, cumulative (across cohorts) recruitment into sites and quadrats over the entire study period explained a significant amount of the variation in the total number of plants that survived to reproduce there. Simulation models based on the measured demography of D. pulchra examined how the relative importance of recruitment and survivorship to population growth changed under two categories of sea urchin density: low (<80 urchins/m²) and high (>80 urchins/m²). These two categories represented the densities of sea urchins above and below a transition point where the mortality of D. pulchra went from being relatively low (17–33%/mo) to very high (63%/mo). When the effects of high densities of sea urchins were simulated, they caused a 95% decline in the population abundance of D. pulchra, compared to only a 31% decline when there were low densities of urchins. Significantly, the relative importance of recruitment and survivorship varied with sea urchin density: when there were high densities of sea urchins, population growth was more sensitive to increases in recruitment, whereas when there were low densities of urchins, survivorship was more important. Our findings demonstrate that the local population abundance of D. pulchra is determined by a combination of recruitment and density‐independent mortality following recruitment, but it appears that the relative importance of the two processes varies in time and space.