Factors regulating population dynamics of the marine bivalve Gemma gemma: intraspecific competition and salinity

A population of Gemma gemma (Totten) was investigated for 6 yr (1978–1983) in Connecticut, USA to determine the relationship between population fluctuations, life tables of successive cohorts and factors affecting life table parameters. Seasonal cycles occurred from 1978–1981. Although density was not abnormally low in summer of 1982, the population declined rapidly thereafter because individuals present during the reproductive season were smaller than the minimal size for reproduction. Forecasts of population size from a time series model based on past densities were compared to forecasts from the intrinsic rates of growth from four separate clam life tables. The latter approach was less precise but more consistent at successfully predicting temporal trends in population abundance. Two factors were identified that potentially controlled population density and structure during the study. Correlational evidence indicated that intraspecific competition between juveniles and adults was intense in 1978 but was reduced during subsequent years. This interaction reduced the size and subsequent fecundity of the young-of-the-year. Correlational evidence also indicated that low salinity in spring of 1978, 1979, 1982 and 1983 reduced juvenile survival, but not that of adults. Qualitative evidence indicated that a third factor, predation, might also have affected G. gemma's population size and structure.