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.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
1. Introduction
Populations of organisms typically occupy patches of habitat scattered in a matrix of different habitats. Densities within these patches fluctuate because of local stochastic or
demographic processes and dispersal among patches, which might be via propagules, adults or both Den Boer, 1968; Levin, 1976; Hansson, 1991; Harrison, 1991.
Understanding the factors that maintain individuals within, or cause dispersal among patches of habitat is important in understanding how local populations interact.
Movement through patchy habitats determines spatial patterns of many species Wiens et al., 1995; Underwood and Chapman, 1996 and, ultimately, spatial variability in
assemblages and local patterns of biodiversity Underwood and Chapman, 1998.
Although many experimental studies have identified how individuals move with respect to broad-scale environmental features, e.g. height on intertidal shores McQuaid,
1981; Williams, 1995, small-scale features of habitat Underwood, 1977; Levings and Garrity, 1983; Underwood and Chapman, 1989, 1992; With, 1994; Wiens et al., 1995,
1997 or other organisms Erlandsson and Kostylev, 1995; Chapman, 1998, quantitative comparative studies of movements of the same species among habitats of very different
structure and of different species moving across the same range of habitats are not common but see With, 1994; Dempster et al., 1995; Wiens et al., 1995 for studies of
insects. Without such quantitative comparative information, there cannot be general understanding about the relative importance of the ecological characteristics of the
organisms themselves or the types of habitat they are moving through. This is particularly important for invertebrates where ‘lack of knowledge of dispersal by
endangered invertebrates is a major constraint for conservationists’ Thomas and Morris, 1994.
Intertidal shores are ideal environments for testing the generality of patterns of movement because they contain different, easily-accessible habitats which are isolated to
different degrees and which are occupied by many taxa. These habitats contain closely-related species which forage on different food sources in different ways, but, in
addition, different taxa exploit the same food sources. Many intertidal animals can be easily handled, marked, manipulated and relocated Chapman, 1986; Chapman and
Underwood, 1992 and their habitats altered or moved to test specific hypotheses Petraitis, 1982; Worthington and Fairweather, 1989; Chapman and Underwood, 1994;
Crowe, 1996. Techniques for monitoring movement, the necessary controls for experimental manipulation of species and their habitats and the levels of replication
necessary for quantifying short- and long-term movement of gastropods have been documented Mackay and Underwood, 1977; Underwood, 1977, 1988; Chapman, 1986,
1999; Chapman and Underwood, 1992; Crowe, 1996. Finally, the basic ecology and patterns of distribution and abundance of many of the organisms are understood and
therefore comparisons among species are relatively easy to interpret.
Most such experimental studies have measured movement over a specific period of time, determined by the hypothesis being examined. For example, Underwood 1977
examined movements of Austrocochlea porcata A. Adams, Nerita atramentosa Reeve and Bembicium nanum Lamarck over 24 h because he was specifically testing
hypotheses about short-term movements and use of habitat during subsequent low tides.
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
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Chapman 1999 similarly examined movement of Littorina unifasciata Gray over 24-h periods in specific tests of variability in foraging during different conditions of the sea
and weather. There have been relatively few comparative studies of movements of the same species in the same habitat over different periods of time. Yet relationships
between short-term and long-term patterns of movement are essential to understanding the ecological consequences of different patterns of dispersal. Baur and Baur 1993
examined dispersal of the land snail Arianta arbustorum L. in a forest clearing and linear patch of habitat and compared daily movements to movement over 10 months.
Although daily movement in each habitat was random in distance and direction, long-term movement varied between the two habitats. Similarly, daily Hamilton, 1977a
and long-term Hamilton, 1977b movements of the intertidal snail Littorina irrorata Say showed that one could not generalize from movement measured in the short-term to
explain long-term movement.
Ecologists are becoming increasingly concerned about the need for and difficulties of generalizing from single experiments for improving our understanding of ecological
patterns and processes Underwood et al., 1983; Menge and Farrell, 1989; Underwood and Petraitis, 1993; Crowe, 1999. A number of studies of movements of intertidal
animals in different habitats have shown variability in the details of patterns on movement in different experiments, but some generality of pattern when the different
experiments were examined together Underwood and Chapman, 1992; Chapman and Underwood, 1994; Chapman, 1999; Crowe, 1999.
In this study, which is part of a larger study of comparative movement of a suite of intertidal animals across natural and experimentally manipulated habitat, movement of
three species of gastropods N . atramentosa, B. nanum and A. porcata was compared in
a number of replicated experiments. These three species were chosen because they each graze micro-algae and live in the same area of the shore. Local densities of these species
have previously been shown to be correlated with cover of water or algae Underwood, 1975, 1976. Nevertheless, despite these strong associations with particular habitats,
short-term movement over 24 h was random with respect to distances and directions moved in all three species Underwood, 1977. Although movement of these species has
not been previously examined with respect to topographic complexity of the rock- surface, topographic complexity can have a strong influence on patterns of distribution,
abundance and size-structure of populations Emsen and Faller-Fritsch, 1976; Raffaelli and Hughes, 1978; Underwood and Chapman, 1992, patterns of dispersion Chapman,
1994 and rates and directionality of movement in intertidal gastropods Levings and Garrity, 1983; Fairweather, 1988; Underwood and Chapman, 1989; Chapman and
Underwood, 1994.
Here, patterns of movement were compared among A . porcata, N. atramentosa and B.
nanum in a number of sites with different physical structure to test the hypotheses that i movement will vary consistently among species across sites of different topographic
and other physical complexity, ii movement will vary consistently across sites of different complexity in the same way for the different species or iii movement will
vary interactively among species across habitats of different complexity. Movements were measured in three different experiments to test the hypothesis that they would be
similar from time to time and therefore results could be generalized from single
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.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
experiments. Finally, movement was measured over three periods of time 1 day, 1 week and 2 weeks to test the hypothesis that differences in patterns of movement among
species or among habitats are consistent, irrespective of the period over which they are measured.
2. Materials and methods
