Variability of foraging in highshore habitats: dealing with unpredictability

Intertidal areas are habitats at the border of two very different environme nts: the marine environment and the terrestrial environment. In contrast to many habitats at borders, intertidal areas are very variable in space and time. They have upshore and alongshore gradients of environmental condition s, which change through time in predictable (due to changing tides) and unp redictable (due to changing weather) ways. Because most animals and plants on rocky shores are marine in origin, extreme highshore levels are generall y considered more harsh and unpredictable environments than are mid- or low shore levels. In this study, the linear distances and directions dispersed by the littorinid Littorina unifasciata while foraging were compared across randomly-chosen mid- and highshore replicated sites to test the hypothesis that movement during foraging was more variable from place to place in com plex midshore habitats. Experiments were repeated on different days to test the hypothesis that temporal variability in movement was greater at high- than at midshore levels because environmental conditions necessary for fora ging were more variable high on the shore. Finally, the data were used to t est models about differences in variability among individuals in the same p atch of habitat according to their recent history of submersion/emersion. I n contrast to expectations, dispersal was more variable in space and time w ithin and among highshore sites. The implications of such variable behaviou r are discussed with respect to the generalization of patterns of behaviour from sparse data and the levels of replication needed in the design of exp eriments to investigate behaviour of intertidal animals.