Pre-emption of the substratum and the maintenance of spatial pattern on a rocky shore in the northwest Mediterranean

This study examined the effects of removing dominant organisms from differe nt types of patches at different heights on a rocky shore in the northwest Mediterranean. It was proposed that preemption of the substratum was a key process in maintaining differences among patches low on the shore, while ph ysical factors were potentially more important at higher tidal levels. In a first experiment we removed barnacles and turf-forming/encrusting algae fr om areas of substratum just above and below the upper and lower limits of d istribution of the red alga, Rissoella verruculosa, respectively. The exper imental plots were either completely cleared, partially cleared (organisms were removed from 50% of the substratum) or left undisturbed (controls). Tr eatments were replicated in 4 sites (stretches of coastline of 8 to 10 m) a bove, and 4 sites below, the limits of distribution of Rissoella. This expe riment was initiated twice, in July 1994 and January 1995. In a second expe riment we manipulated the coverage of Rissoella in correspondence to its up per and lower margins of distribution. The design of this experiment was si milar to the previous one. In this case the experimental units consisted of plots where Rissoella was either completely removed or where only the erec t fronds were removed while the encrusting base was left in place, and cont rols. This experiment was done only once (from July 1994 to July 1996). The results of the 2 experiments indicated that when dominant organisms were c leared from the substratum, other species could either extend their vertica l 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 o f encrusting algae, Rivularia spp., Limpets and the recruits of Chthamalus stellatus. With the exception of Rivularia, clearing areas of substratum ha d similar effects at different heights on the shore. The model that pre-emp tion of the substratum maintained differences among patches low, but not hi gh on the shore, was not supported by the experimental results. These indic ated that pre-emption of the substratum operated independently from other p hysical and/or biological processes in influencing vertical patterns of dis tribution. The large variability among sites documented in this study sugge sted that models of the distribution of plants and animals should also cons ider horizontal scales of variation.