Dynamics of an Entire Population of the Subtidal Snail Umbonium costatum: The Importance of Annual Recruitment Fluctuation

Abstract

The dynamics of a population of the subtidal snail Umbonium costatum (Kiener), unaffected by larval dispersal from other populations, was studied in Hakodate Bay, northern Japan, during 1982-88 and 1992. During the study periods, the recruitment densities of U. costatum were highly variable from year to year (85-fold, minimum to maximum). In contrast, adult (i.e. age 1 + year) densities were not highly variable from year to year (3.2-fold, minimum to maximum), since the adult population is composed of many cohorts, with various densities resulting from fluctuating levels of recruitment. During the study period, the age structure of the U. costatum population was always dominated by a few year-classes, a result of sporadic, high recruitment success occurring once every few years. During the study period, density-dependent mortality and growth were not detected in this population. Assuming that only the annual recruitment rate was subject to environmental variability, while other demographic traits (i.e. age-specific survival rates and fecundities) were constant, we made eight independent Leslie matrices and simulated the long-term population dynamics of U. costatum by combining the eight matrices. The results show that the persistence of the U. costatum population in Hakodate Bay is secured by sporadic recruitment occurring once every few years. To our knowledge, this study is the first documentation of recruitment limitation in an entire population from the marine benthos.