Role of Planktonic and Benthic Factors in the Population Dynamics of the Bryozoan Membranipora Membranacea

The bryozoan Membranipora membranacea possesses a long—lived planktonic larva and occurs in encrusting colonies in the Macrocystis pyrifera kelp canopies on southern California. Seasonally Membranipora populations varied from >1000 to less than one colony per kelp blade in late winter and late summer, respectively. Populations often varied by more than an order of magnitude in 2 wk. Several planktonic and benthic factors play important roles in the population dynamics of Membranipora. Upwelling conditions and the presence of two nudibranch predators accounted for 55% of the variability in larval abundance relative to adult population size. In turn, low recruitment rates relative to larval abundance corresponded with periods of high surface temperatures. The combined effect of larval abundance and surface temperature accounted for 79% of the Vaccinium in recruitment. Although a field transplant experiment demonstrated that predation plays a major role in the survival of colonies, recruitment was a major factor underlying population fluctuations and accounted for 75% of the variability in reproduction as indexed by the production of zooids by Membranipora colonies. A 12—fold density—dependent effect on reproduction attributable to space limination and to the density—dependent component of predation was observed over the 1000—fold range in recruitment resulting from density—independent factors. A population model based on field—derived relationships accounted for 64% of the observed variability in colony density. Temperature, as it affects recruitment, was the major factor controlling the seasonal fluctuations in Membranipora populations. Over longer, multiseasonal time scales the small density—dependent effect was capable of maintaining model populations within the range of values observed in the field and therefore was sufficient to regulate Membranipora populations. The results of this study emphasize the importance of small density—dependent effects in the regulation of single—species populations.