The influence of sessile organisms on the population biology of three species of Acmaeid limpets

Abstract

Changes in population densities of Acmaea digitalis (Rathke), Acmaea paradigitalis Fritchman, and Acmaea scabra (Gould), on a series of intertidal wharf pilings were determined at ≈ 6 monthly intervals from December 1969 to November 1971. In addition, monthly censuses of these three species, together with estimates of the amount of piling substratum covered by barnacles and macroscopic algae were made from September 1970 to November 1971. Two patterns of numerical change are described. Variation in recruitment between years results in changes in mean density of these species over the full period. Differences in density and population structure of A. digitalis between pilings within a single year are also demonstrated. Occupation of settling and grazing substrata by barnacles produced significant recruitment and pre-reproductive mortality of A. digitalis. No effect of macroscopic algae was detected. The effect of barnacles apparently overrides the influence of adult A. digitalis on juvenile settlement patterns. High adult densities give lower growth rates. The combination of higher densities and increased barnacle cover may alter adult mortality patterns. This relationship is distorted by the influence of other species of limpets and changes in the percentage cover of barnacles. This results in variable and unstable demographic patterns in areas which are subject to very similar physical regimes. In A. digitalis high growth rate was associated with reduced barnacle cover and relatively low mortality of large individuals. Barnacle cover also modifies the demography of A. paradigitalis and A. scabra. In addition, interspecific competition between A. digitalis and A. paradigitais results in a higher mortality rate of large individuals of the latter and its confinement to lower intertidal levels. This was demonstrated by experimental manipulations. Evidence for interspecific competition between A. digitalis and A. scabra is slight. These species were separated by tidal level and their differing responses to barnacle cover.