First year demography of the foundation species, Ascophyllum nodosum, and its community implications

Abstract

Long‐lived species of large size can play critical roles as ecosystem engineers but the demography of early life stages is poorly known for many such species. For example, the fucoid alga, Ascophyllum nodosum, serves as the foundation species for many sheltered rocky shore communities of the North Atlantic Ocean, yet its early demography is incompletely characterized. Here we provide data on age‐specific survivorship throughout the first year. We outplanted zygotes generated from artificial fertilizations and followed their survivorship under different conditions that tested the following hypotheses: (1) survivorship is inversely related to clearing size, (2) germlings survive better in clearings in conspecific stands than in clearings in mussel beds, (3) survivorship in microhabitat refugia (grooves of artificial surfaces) is greater than that on flat surfaces, and (4) protection from grazing and physical factors, like canopy sweeping and desiccation, enhances germling survival. Germlings were censused on days 2, 9, 13, 17, 64, 102, 127 and 399 following outplanting. Germling mortality was consistently higher on flats than in grooves of artificial surfaces, and was greatest within the first two days of life. Germlings protected by cages survived better during the day 0–2 interval, but after 2 days caging did not matter. Interval‐specific mortalities after day 2 showed that survivorship was inversely related to clearing size. The type of clearing, in conspecific stands or mussel beds, did not affect survivorship. Ascophyllum nodosum shows a strong type III survivorship curve with mortality during year 1 exceeding 99.9%. The poor survivorship of first year germlings accounts for the paucity of newly recruited juveniles often observed in mature stands of A. nodosum. The lack of juveniles and the scale dependent survivorship of germlings renders populations of A. nodosum, and the associated community that depends on it, vulnerable to large scale perturbations.