Experimental analyses of the influences of topography of the substratum on movements and density of an intertidal snail, Littorina unifasciata

Abstract Topographic complexity of the substratum influences the movements of the intertidal snail Littorina unifasciata. To determine the relationships between movements and local density and dispersion, an objective quantitative measure of topography was evaluated for use in the field. Heights of 100 vertical pins placed a few cm apart in a grid were measured to determine a topographic index (TI), defined as the circular variance of vectors normal to the triangular planes formed by every triangle produced by three adjacent pins in the grid. Snails moved further on less complex surfaces (those with smaller TI). Their movements were more directional on simpler surfacesand, in most such sites, movements tended to be towards the sea. On more complex surfaces, movements were generally random, or, if directional, in any direction. There was a negative correlation between the distance moved and the topographic index. Snails on simpler surfaces also tended to move in the same direction on successive days; those on surfaces with a larger TI moved in different directions on successive days. There was no relationship between TI and mean density, nor with an index of dispersion. Small-scale spatial variance was negatively correlated with TI but long-term temporal variance of numbers in 30 populations was positively correlated with TI. The lack of relationship between density and TI, coupled with greater movements on less complex surfaces suggested that turnover of individuals in local populations should be negatively correlated with TI (i.e., more rapid turnover where surfaces were simple). This hypothesis was corroborated in short-term experimental manipulations. Emigration and immigration were both greater where surfaces were simple (i.e., surfaces with small values of TI). Implications of this for the population dynamics of L. unifasciata are discussed.

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