Effects of marine iguanas, fish, and crabs on the diversity of Galapagos rocky shore communities at different levels of productivity

The rocky shores of the Galapagos Islands harbor one of the most unique flora and fauna of the world, including the Galapagos marine iguanas, sea turtles, sea lions, Galapagos penguins and flightless cormorants. However, the structure and function of this unique ecosystem is relatively unknown and is seriously threatened by fishing, ENSO and human induced climate change. Here we report the results of a series of experimental exclusions  to evaluate the effect of marine iguanas, fish and sally light foot crabs on the diversity of rocky shore benthic communities at three Islands that experienced different levels of productivity (Fernandina = high, Santa Cruz = mid and Genovesa = low) between January 2006 and April 2007. We found that at the most productive location (Fernandina Island) the presence of grazers increased diversity by removing competitively superior species of algae.  In the absence of marine iguanas and sally light foot crabs, encrusting and upright coralline algae dominated primary substrate, reducing the diversity of sessile organisms dramatically. At the low productive site (Genovesa), any combination of consumers reduced the diversity of marine algae to encrusting forms while the absence of consumers increased it. Our results suggest that the effect of consumers is modulated by bottom up effects (nutrients and productivity) and illustrate the variable effects that the extinction of grazers will have on the community structure and function of this unique system.