Macroinvertebrate communities on rocky shores: Impact due to human visitors

Rocky intertidal habitats harbor unique and diverse ecosystems that, when located in accessible area, result highly attractive to human visitors. It is known that many activities caused by humans, such as, rock overturning, trampling and organisms' collection harm rocky shore ecosystems, reducing the biodiversity, modifying abundance and altering normal ecosystem functioning. Despite that the collection of edible macroinvertebrates, such as clams, gastropods, sea urchins, anemones and crabs is a very common practice along the Spanish near shore coast, their effects on the ecosystem are poorly documented. This study aims to assess and quantify the human impact on the intertidal rocky shore macroinvertebrate community in two Spanish beaches located together but with very different grade of anthropization, one open to visitors (Santibariez beach) and other with restricted human access (Torregorda beach). A total of 345 intertidal boulders were sampled during one year on the upper and lower intertidal zones. The richness, density, diversity, and evenness of macroinvertebrates were determined and compared. A total of 21,329 individuals were recorded in both beaches belonging to 73 taxa (56% gastropods, 13% crustaceans, 11% bivalves, 8% echinoderms, 12% other groups). Univariate and multivariate statistical analyses revealed significant differences in macroinvertebrate assemblage composition and diversity between beaches, tidal zones and, to less extent, between visitor seasons. Although differences observed in the assemblages of the two beaches can be partially due to natural ecological differences, human impacts were also detected. The most obvious effect of the visitors was a significant reduction of some top predators, as the crabs Eriphia verrucosa and Xantho poressa, and an increased number of disturbed boulders at the visited beach (78.3%) versus the restricted beach (2.3%), suggesting that human recreational activities cause effects on target species and on the ecosystem structure. Moreover, restricted beach had lower number of species and densities than visited beach (56 and 64 taxa, 195.1 and 554.6 ind. m(-2), respectively), which could be partially due to the higher density of some top predators at the restricted beach. (C) 2017 Elsevier Ltd. All rights reserved.