Experimental analysis of intraspecific composition in an intertidal gastropod, Littorina unifasciata

Abstract

L. unifasciata dominates the upper levels of rocky shores at Cape Banks, New South Wales. Peak densities of up to 9600 m-2 occur near the centre of the littorine's vertical zonation and are associated with small body sizes. Density declines, and mean size rises, further down and more particularly further up the shore. Largest animals occur at the top of the shore and in areas experiencing strong wave action, where densities are low. Field experiments showed that increase in density results in a decrease in body weight and an increase in mortality. However, even at a density four times that of the natural population, mortality remained remarkably low, and this is probably a key feature allowing L. unifasciata to penetrate high up the shore. L. unifasciata feeds mainly on lichens and food levels are low over most of the zone occupied by this littorine, rising above and below this zone and being particularly high in the supralittoral immediately above the range of L. unifasciata. Thus, food cannot be a factor limiting the height that L. unifasciata extends up the shore. Experimental caging shows that the standing stock of lichen is inversely related to the density of L. unifasciata. The zonation pattern and size gradient of L. unifasciata may be due to a combination of two factors: a decline of body size due to increasing intraspecific competition at higher densities, and the tendency of L. unifasciata to migrate (probably upwards) away from areas of low food availability. The latter was experimentally demonstrated. L. unifasciata suffers from intense intraspecific competition and is responsible for limiting the availability of its food. Its populations are seemingly not regulated by predators. It borders on a zone of high food availability in which there are no important competitors. These are all circumstances favouring range expansion of the species, to the limits of physiological tolerance, to allow the species to capitalize on the adjacent rich food source.