EFFECTS OF TRAMPLING ON PLANT AND ANIMAL POPULATIONS ON ROCKY SHORES

We examined experimentally the effects of disturbances caused by pedestrain traffic on the animals and plants living on an intertidal rocky shore in south-eastern Australia. Individual footsteps damaged some animals, but the percentage of animals that was crushed was low. Molluscs, except large limpets, were more often dislodged than crushed, and some damage was sustained by barnacles and mussels. The survival of individuals of two gastropod species, Bembicium nanum and Austrocochlea constricta, was not affected by dislodgment, because they quickly righted themselves. The limpet Cellana tramoserica was not damaged by being kicked or stepped on. The dominant plant on these shores, the brown alga Hormosira banksii, was easily damaged, with ca. 20% of the biomass of individual plants being removed by a single footstep. The amount of tissue lost increased with the number of footsteps, with a maximum loss of approximately 75%. We investigated the effects of three levels of sustained trampling (33 d of trampling, spread over 4 mo) on the organisms in three habitats: Hormosira Mats, Coralline Algal Mats, and Bare Rock. The algal mats were most affected by trampling. Hormosira Mats were damaged severely by high- and low-intensity trampling. Upright coralline algae in the Coralline Algal Mat were damaged by high-intensity trampling. Numbers of the gastropod Turbo undulatus in both of these habitats were reduced. The Bare Rock habitat was not affected significantly by trampling. Five months after the end of trampling, heavily trampled and control strips of coralline Algal Mats were not distinguishable, as all treatments underwent pronounced seasonal changes. Within the Hormosira Mats, the low-intensity treatments and controls were not different, but algal cover in the heavily-trampled areas remained < 50% of pre-trampling levels. Within this habitat, the Hormosira had not recovered and there were substantial increases in the abundances of some grazing molluscs, particularly limpets. Even after > 400 d of recovery, Hormosira cover was only 60%, three quarters of the value for control areas. The indirect effect of increased mollusc abundance was still apparent. Trampling by visitors could result in replacement of the Hormosira mat species assemblage with 'bare' rock and grazing molluscs, and trampling should be considered as a disturbance capable of directly and indirectly influencing intertidal populations on rocky shores.