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, 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.

Interactive effects of grazing and environmental stress on macroalgal biomass in subtropical rocky shores: Modulation of bottom-up inputs by wave action

“…This high susceptibility to herbivory has been usually associated with the lower biomass of ephemeral macroalgae observed on many temperate shores (Coleman et al, 2006;Hawkins, 1981;Nielsen and Navarrete, 2004;Poore et al, 2012;Steneck and Dethier, 1994) and on a few tropical intertidal shores as well (Kennish et al, 1996;Menge et al, 1985;Vinueza et al, 2006). However, the extent to which grazers control algal biomass may vary substantially at different spatial scales, even within the same shoreline, independently of the general climatic region (e.g.…”

“…Algal blooms at the lower intertidal zone on Hong Kong shores extend longer into the summer, probably because of reduced heat stress at this level (Williams, 1993). In many cases, however, the effect of seasonal intensification of heat stress can be confounded with variation in nutrient supply, especially in nutrient-poor waters (Vinueza et al, 2006). Ormond and Banaimoon (1994) showed that macroalgal abundance peaked during late summer and autumn at intertidal rocky shores along the Hadramout Coast, southern Yemen, coinciding to the seasonal upwelling of nutrient-rich waters and not with air temperature minima.…”

Species-specific defense strategies of vegetative versus reproductive blades of the Pacific kelps Lessonia nigrescens and Macrocystis integrifolia

“…Herbivores can inXuence the structure of benthic algal communities through the consumption of large amounts of algal biomass (Lubchenco and Gaines 1981;Carpenter 1986;Vinueza et al 2006;Jormalainen and Honkanen 2008). In response to this herbivore pressure, macroalgae have developed diVerent strategies, one of which is the defense of tissues in ways that makes them less palatable for potential consumers (reviewed in DuVy and Hay 1990;Cronin 2001).…”

Variability and community organization in moderately exposed tropical rocky shore algal communities as influenced by different consumer groups

“…Reports (Vinueza et al 2006) of crabs such as the sally light-foot crabs (G. grapsus) in the Galapagos show preference to red algal turfs and Ulva and some amounts of filamentous greens but these crabs avoided encrusting algae, including filamentous brown algae.…”