MEPS 181:13-23 (1999)  -  doi:10.3354/meps181013

ABSTRACT: This study examined the effects of removing dominant organisms from different types of patches at different heights on a rocky shore in the northwest Mediterranean. It was proposed that pre-emption of the substratum was a key process in maintaining differences among patches low on the shore, while physical factors were potentially more important at higher tidal levels. In a first experiment we removed barnacles and turf-forming/encrusting algae from areas of substratum just above and below the upper and lower limits of distribution of the red alga, Rissoella verruculosa, respectively. The experimental plots were either completely cleared, partially cleared (organisms were removed from 50% of the substratum) or left undisturbed (controls). Treatments were replicated in 4 sites (stretches of coastline of 8 to 10 m) above, and 4 sites below, the limits of distribution of Rissoella. This experiment was initiated twice, in July 1994 and January 1995. In a second experiment we manipulated the coverage of Rissoella in correspondence to its upper and lower margins of distribution. The design of this experiment was similar to the previous one. In this case the experimental units consisted of plots where Rissoella was either completely removed or where only the erect fronds were removed while the encrusting base was left in place, and controls. This experiment was done only once (from July 1994 to July 1996). The results of the 2 experiments indicated that when dominant organisms were cleared from the substratum, other species could either extend their vertical range of distribution occupying areas where they did not occur before, as in the case of Rissoella, or change their local abundance as in the case of encrusting algae, Rivularia spp., limpets and the recruits of Chthamalus stellatus. With the exception of Rivularia, clearing areas of substratum had similar effects at different heights on the shore. The model that pre-emption of the substratum maintained differences among patches low, but not high on the shore, was not supported by the experimental results. These indicated that pre-emption of the substratum operated independently from other physical and/or biological processes in influencing vertical patterns of distribution. The large variability among sites documented in this study suggested that models of the distribution of plants and animals should also consider horizontal scales of variation.

KEY WORDS: Algae · Boundaries · Invertebrates · Natural variability · Patchiness · Rocky shores · Spatial heterogeneity