Project Report: September 2007

Since January 2006, a series of manipulative experiments were set up to evaluate the interactive effect of herbivory (marine iguanas, fish and crabs), temperature and nutrients on the diversity of marine sessile organisms (i.e. algae and sessile invertebrates). These experiments were conducted on three islands located at three biogeographic regions characterized by different levels of temperature and productivity: Genovesa (North, low productivity), Santa Cruz (Center, mid productivity), and Fernandina (West, high productivity). Experiments will continue to be monitored until June 2008. Initial results show that at Northern, low productive locations any combination of herbivores will reduce the diversity of marine algae to few grazing resistant forms (mainly crustose algae). In areas of intermediate productivity the system was dominated by two types of algae, crustose and Ulva sp., and herbivores also tended to decrease diversity. At sites of high productivity, consumers had the opposite effects to those observed at less productive locations. Herbivores increased the diversity and maintained the system on an early successional and productive stage. Temperature and consumers also had an important effect on biomass, species richness and diversity of the flora and fauna associated with it. Thermal amelioration experienced in white plates facilitated the settlement of algae. Biomass was also higher on white plates.

Figure Pictures of experimental plots taken during June and September 2007. Upper panel represents closed treatments; lower panel represents open treatments of high grazing intensity. Left pictures correspond to areas of low productivity, middle photograph corresponds to areas of mid productivity, and right pictures correspond to sites of high productivity.

Conclusions:
The effect of consumers depends on the productivity of the system. At sites of low productivity, grazers generally decrease diversity; whereas the opposite pattern was observed at sites of mid and high productivity. At these locations, grazers eliminated competitively superior species that otherwise will monopolize the primary substrate. It also appears that at sites of mid and high productivity grazers maintain a system at an early succesional stage, favoring the productivity of the most important items of their diets such as Ulva sp. and red algal turf. At sites of mid productivity, the fronds of Ulva sp. could reach up to 20 cm on closed plots, the canopy of Ulva more likely limited the access of light to other understory species.

At all sites grazers modified the canopy structure of Ulva sp. by reducing it to a low algal matt. By keeping the algal matt at a uniform low level, herbivores prevent the settlement of more complex but unpalatable forms of algae that appear to displace palatable forms of algae when grazers are absent. Both grazing and temperature have important effects on community structure. White plots ameliorated thermal stress, below what was experienced by the black plates. Thermal amelioration experienced on white plates favored the settlement of algae. White plates also had a higher biomass of algae, and a higher richness and diversity of fauna associated to the algal canopy. To my knowledge, the use of black and white plates is a new technique that has been developed and refined during the present study. This technique has already called the attention of world experts on marine climate change studies (Dr. Christopher Harley, University of British Columbia) and has a lot of potential for future research on different ecological aspects.

Several students including local Galapagueños, Ecuadorians, and students from other countries, including the United States, Germany and Brazil participated in the project since January 2006. Two students from the United States are doing their thesis honors project using some of the information collected in the present study. Some of the local students are now applying to their own projects the techniques that they have learned. This international and diverse group of students has fostered friendship, technical and cultural exchange, and future collaboration among them.