The importance of intraspecific competition in a Littorina littorea population in the Wadden Sea

Abstract

Two field experiments were carried out to test whether effects of intraspecific competition in a Littorina littorea population can be detected in a short-term investigation. Different size classes of L. littorea showed no significant difference in preferences when offered four kinds of either possible food or substrata (Fucus vesiculosus, Ulva lactuca, Carcinus maenas, brick). Large and medium winkles preferred Fucus vesiculosus, followed by Ulva lactuca. Dead shore crabs (Carcinus maenas) were the least preferred objects for all size classes. On the first day of the experiment bricks were more attractive to small littorines than to larger ones. Considering all four days, the same ranking occurred for all size classes: Fucus vesiculosus> Ulva lactuca>brick > Carcinus maenas. The reaction of juveniles to increased densities was examined using an in situ caging experiment on a mussel bed. Mesh size of the cages allowed adult densities to be increased while juveniles could escape by passing through the meshes. However, there was no significant emigration of small winkles even from cages with 10 to 20 times natural density of large individuals. Of greater importance was the original number of winkles at the site. The available resources on the mussel beds appear to be sufficient to maintain a high population density. Intraspecific competition does not seem to play a major role in this L. littorea-population.