Effects of periodic disturbances from trampling on rocky intertidal algal beds

We investigated the ability of an assemblage of animals and plants on rocky shores in southeastern Australia to resist and/or recover from repeated pulse disturbances in the form of trampling. Disturbances of four different intensities were applied experimentally over six summers, with no human access at other times of the year. The dominant intertidal plant, the brown alga Hormosira banksii, was affected by trampling, but the effects were heterogeneous between sites. At two sites, a series of pulse disturbances produced a series of pulse responses, although the effect of a given pulse varied among years, possibly related to the severity of summer desiccating conditions each year. At the third site, pulse disturbances produced a press response; at high levels of trampling, Hormosira was almost eliminated within 2 yr, and at two intermediate levels of trampling, cover was reduced from >90 to 60-70%, where it remained for 4 yr. Effects of trampling showed little small-scale spatial variation. Untrampled areas did fluctuate through time, often as a result of summer burnoff of algae. Natural disturbances occurred irregularly through the study, and their effects varied on very small spatial scales (among plots <30 m apart). Trampling enhanced the densities of a range of herbivorous mollusks, especially limpets, and reduced the abundance of articulated coralline algae, which were abundant in the understory of Hormosira mats. These effects varied among sites but showed much less variation on smaller spatial scales. The reductions in coralline algae may be a direct effect of trampling, but increases in mollusk abundance occurred some time after changes to Hormosira cover, and those changes may be an indirect effect of trampling. We compared the effects of trampling on areas of the shore that had been trampled for two and four summers, to test whether a past history of disturbance influenced the effect of a new disturbance. No significant effects were found on algae or mobile animals, although a mild summer may have made our test of history relatively weak. Hormosira banksii fits the definition of a keystone species or engineer and, as such, is an appropriate focus for management and as an indicator. Spatially heterogeneous effects of a constant physical perturbation, however, mean that management of these rocky shores requires more complex models and indicate that caution should be used in adopting this species as a uniform indicator of environmental change.