TOP-DOWN HERBIVORY AND BOTTOM-UP EL NIÑO EFFECTS ON GALÁPAGOS ROCKY-SHORE COMMUNITIES

Abstract

We evaluated the effects of marine iguanas, sally lightfoot crabs, and fish on rocky-shore sessile organisms at two sites at Santa Cruz Island, Galapagos Islands, Ecuador, for 3–5 years during and after the 1997–1998 El Nino, 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 Nino in the following way. During an early phase, crustose Gymnogongrus and/or red algal turf were dominant. In the heart of El Nino, 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-s...