Experiments on effects of sampling biota under intertidal and shallow subtidal boulders

Abstract

Diverse assemblages of sessile and mobile fauna live under intertidal and subtidal boulders. When these biota are sampled, it is necessary temporarily to overturn the boulders. Many mobile species respond to overturning by moving onto the other side of the boulder or moving off the boulder into the surrounding area. Overturning without replacing a boulder has been shown to affect abundances and diversity of its sessile and mobile organisms. Overturning when sampling has, however, not previously been examined as a potential source of disturbance, because replacement of the boulders has been assumed not to disturb the organisms. These experiments were designed to test the hypothesis that sampling is a measurable disturbance to the mobile fauna and groups of sessile biota living under boulders. The sessile organisms were mainly encrusting algae, encrusting tubeworms and bryozoans, with smaller amounts of foliose algae, other colonial animals, bivalves and cnidarians. The mobile fauna were primarily large (> 5 mm long) chitons, gastropods and echinoderms. Assemblages on boulders that had been previously sampled were compared to those on undisturbed boulders. Effects of sampling on temporal changes in abundances were also compared to similar changes on sets of undisturbed boulders. There were no or very few differences in assemblages, mean densities of selected species or the proportions of boulders occupied by selected species among undisturbed boulders and boulders subjected to different frequencies of prior sampling when there were 3–4 month intervals between samples. There were, however, short-term responses to disturbance associated with sampling. These were measured as differences in densities and the proportion of boulders occupied by some species of molluscs and echinoderms and differences in the molluscan assemblage between undisturbed and disturbed, i.e. previously sampled, boulders. These differences disappeared during the following month. These data suggest that organisms on boulders are disturbed by careful sampling, but that they rapidly recover during the following few weeks. Thus, sampling is possible without altering the abundances of the target species if there is long enough between samples for the densities under the disturbed boulders to recover.