Effects of environmental and biotic factors on the fluctuations of abundance of Littorina obtusata (Gastropoda: Littorinidae)

To explain periodic fluctuations in abundance of an intertidal flat periwinkle Littorina obtusata, we tested the potential impacts of several key environmental and biotic factors on the population density of this mollusc. The analysis showed that 79% of density variation was due to the changes of the recruitment of the periwinkles and likely caused by intraspecific competition for resources (algae Fucus vesiculosus) between the recruits and the older molluscs under the conditions of depleted habitat capacity. The cyclical changes in L. obtusata density result from the alternating periods of high and low recruitment. High recruitment is observed when the abundance of mature periwinkles is relatively low and enough resources are available to insure high survival of the recruits. The duration of high recruitment periods (approximately 3 years) is determined by the maturation time of the periwinkles. As the recruits mature, the density of adult periwinkles increases and the subsequent recruitment decreases. The duration of the low recruitment phases (an average of 3–4 years) appear linked to the longevity of mature L. obtusata. As the adults age and die out, their density declines and the release of resources leads to an increase of recruitment heralding the beginning of the next cycle.