Background/Question/Methods

How herbivory and productivity affect patterns of community structure is a matter of intense ecological research due to the overexploitation or introduction of herbivores and their predators and the alteration of nutrient cycles by humans and large scale environmental perturbations. Few studies conducted at tropical rocky shores have characterized them as strongly driven by top down effects, however, the spatial and temporal scale of those studies are still very few and limit our ability to formulate conservation strategies. The Galapagos Islands are an ideal model system for studies of climate change. Tropical, subtropical and temperate currents, equatorial upwelling and the upwelling of the equatorial undercurrent sustain an outstanding and diverse array of species. Changes in upwelling regimes due to large scale environmental perturbations can shut down the supply of nutrients that sustains its flora and fauna. Here we present the results of a series of manipulative experiments and observations to understand how herbivory, productivity and environmental disturbance interact to affect the diversity of sessile communities.

Results/Conclusions

First, the 1997-1998 ENSO event, one of the strongest on record shut down primary productivity, at that time herbivores exacerbated the impact of ENSO by overgrazing foliose and filamentous algae and by facilitating the invasion of warm adapted species of algae. Further experiments from 2006 to 2009 revealed that at sites of low productivity, herbivores had a consistent top down effect during both the warm and cold phase of ENSO, reducing the diversity to few grazing resistant forms of algae. At sites of mid productivity, during the warm phase of ENSO herbivores had a similar effect to that observed at sites of low productivity. However, during the cool phase, different mechanisms occurred; herbivores reduced diversity by facilitating the monopolization of Ulva sp. one of its main food items. In their absence diversity increased due to the colonization of shade tolerant species. At sites of high productivity, no impact of herbivores was apparent during the warm phase, however, during the cold phase, herbivores increased diversity by opening space to other species. In their absence articulated coralline algae monopolized space, decreasing diversity. Our results suggest that top-down and bottom-up effects interact to shape primary producer communities on Galapagos Islands rocky shore communities, and that the relative magnitudes of these effects are sensitive to large-scale climatic variation. Therefore conservation actions should be focused on both effects.