We evaluated the effects of marine iguanas, sally lightfoot crabs, and fish on rocky‐shore sessile organisms at two sites at Santa Cruz Island, Galápagos Islands, Ecuador, for 3–5 years during and after the 1997–1998 El Niño, using exclusion cages to separate the effects. Plots exposed to natural grazing were dominated either by encrusting algae or by red algal turf and articulated corallines. Algae fluctuated in response to El Niño in the following way. During an early phase, crustose Gymnogongrus and/or red algal turf were dominant. In the heart of El Niño, grazers had limited effects on algal cover but influenced algal sizes substantially. Most algae (particularly edible forms) were scarce or declined, although warm‐water ephemeral species (notably Giffordia mitchelliae) flourished, increasing diversity and overgrowing crusts. Iguana mortalities were high, and crab densities low. When normal conditions returned, warm‐water ephemerals declined, crab densities rose, and grazers had significant but site‐specific effects on algae. At one site, any combination of grazers diminished most erect species, reducing diversity and restoring dominance of competitively inferior grazer‐resistant crusts. At a second site, only the combined effect of all grazers had this effect. Laboratory experiments confirmed that crabs could control erect algae and promote crustose forms, and crustose Gymnogongrus developed into an erect form in the absence of crabs. Differences between sites and large‐scale temporal changes associated with El Niño indicate that tropical shores are not all as constant in time and space as previously suggested. Mobile grazers did affect algal communities, but over the period of our observations far greater effects were attributable to intersite differences and temporal shifts in oceanographic conditions. El Niño events reduce nutrients, intensify wave action, and raise sea levels, affecting food availability for intertidal herbivores and their influence on benthic algae. Thus, the dramatic transformations of communities during El Niño presage the impacts of global climate change.

The crab Carcinus maenas (L.) has an endemic distribution along the European Atlantic coastline but has been accidentally introduced into intertidal habitats in several parts of the world. Its discovery al the Cape Peninsula in South Africa is recorded here. The crab is well established in Table Bay harbour and the population is at least seven years old. From here it has spread 117 km north and 10 km south. Populations on the open coast are more recently established. The crab feeds on a wide range of intertidal animals, particularly the isopod Paridotea ungulaw, limpets and polychaetes, but it is too early to make quantitative assessments of its impact on local intertidal communities.Initial predation experiments suggest that the barnacle Notomegabalanus algieola and the whelks o.l)'stele I'l/riegata and O. tigrina have no refuge in size or zonation from the crab. O. sinensis, of similar size to O. tigrina, is less vulnerable by virtue of its stronger shell. Its critical size (above which it is too large to be attacked) is correlated with crab size. Larger species, such as the mussels Chorom)'lilus meridionalis and Aulacomya ateI' and the periwinkle Turbo sarmatieus, clearly achieve a refuge in size, but predation on their recruits may influence their abundance and size structure. C. maenas rejected only the urchin Pareehinus angulosus as a source of food. While it is conceivable that shifts in community structure, selection for thicker shells in molluscs and localized extinction of certain species may occur at high crab densities, it is predicted that C. maenas will only have a significant impact in areas protected from wave action.Die krap Carcinus maenas (L.) kom endemics langs die Atlantiese kuslyn van Europa voor, maar is per ongeluk in lussengetyse habitatte van etlike werelddele ingebring. Sy ontdekking langs dic Kaapse Skiereiland in Suid-Afrika word hiermee geboekstaaf. Die krap is goed gevestig in Tafelbaai, en die bevolking is ten minste sewe jaar oud. Hiervandaan het dit 117 km noord en 10 km suid versprei. Bevolkings aan die onbeskutte kus is meer onlangs gevestig. Die krap teer op 'n wye verskeidenheid tussengelyse diere, veral die isopode ParidOiea ungulata, klipmossels en borselwurms, maar dit is te vroeg om nou al kwantitatiewe ramings van sy invloed op plaaslike tussengetyse gemeenskappe te maak. Voorlopige voedingstoetse dui daarop dat die scepok NOlomegabalanlls algicola en die wulke Ox)'slele variegara en O. ligrina geen ontvlugting in Of grootte of sonering van die krap vind nie. O. sinensis, van soortgelyke grootle as O. ligrina, is minder kwesbaar van wee sy sterker skulp. Sy kritieke grootte (waarbo hy te groot is om aangevalte word) hou verband met krapgrootte. Groter spesies, soos die mossels Choromyti/llS meridionalis en Aulacom)'a aler en die alikreukel Turbo sarmalieus, verwerf wei deeglik beskerming wanneer hul groot word, maar roofbedrywighede op hul rekrute kan die talrykheid en groottesamestelling be'invloed. C. maenas het net die seekastaiing Parechinl/s angulosus as 'n voedse...

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The influence of physical factors on the trophic structure of rocky intertidal communities in the Cape of Good Hope was investigated by examining species composition and biomass on shores subject to different conditions of substratum, sea temperature and wave exposure. Biomass of herbivores on shores with an unstable substratum was particularly high while the sessile filter-feeders and algae were impoverished, but actual rock type did not affect trophic structure. Sea temperature influenced species composition but not trophic structure which was most strongly affected by the degree of wave exposure. Exposure influenced both vertical distribution of biomass and trophic composition of total biomass. Total biomass showed a simple decrease upshore on sheltered shores but the pattern was more complex with greater exposure. Filter-feeders, carnivores and omnivores all exhibited significantly higher biornass under exposed conditions. Consequently the balance between consumers and primary producers shifted, implying alterations in the net balance between importation and exportation of production between these communities and the inshore marine system. The addition of huge filter-feeder components to the intertidal system resulted in significantly higher total biomass under exposed conditions. This allows the importation of production from the water column to a community in which consumers exhibit considerably higher standing crops than primary producers. Benthic carnivores exhibited a positive correlation with filter-feeder biomass. It is therefore suggested that domination of exposed shores by sessile filter-feeders and of sheltered shores by algae and mobile herbivores may explain divergent views in the literature on the relative importance of competition and disturbance/predation to community structure on rocky shores.

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