Relative contribution of top-down processes in rocky intertidal communities of central Peru

FERNANDO J. HIDALGO

Concepción, Chile

Simposio; CENSOR Midterm Symposium; 2006

Universidad de Concepción, Chile

Top down control refers to situations where the structure (abundance, distribution and/or diversity) of lower trophic levels depends directly or indirectly on trophic activities at higher trophic levels. Top down control by herbivores and predators has long been recognized as an important factor shaping many rocky intertidal communities around the world. Many experimental studies have shown that predation is a process capable of prevent competitive exclusion and of exerting an important influence on marine communities. In analogous way, herbivorous are able to determine the distribution and abundance of plants, influencing the species composition and diversity of the entire community. Despite the vast information gathered regarding consumers control on rocky shores, and the general truths about processes and mechanisms obtained, many parts of the world remain unstudied, hindering a better understanding of such mechanisms. The Peruvian coast is one of such unstudied regions. Most of the works on the rocky shores of this region have been descriptive or related to ecological aspects of some organisms; and many others fall inside the quite inaccessible “grey literature” (i.e. internal reports, thesis works, unreferated publications). However, there is a great gap of information about the processes that structure the intertidal community. The central coast of Peru has some characteristics that make it particularly important for the study of the relative contribution of the processes that shape intertidal communities, and for making generalizations on the importance of such processes. The Peruvian upwelling system makes this zone one of the most productive coastal areas of the world. Furthermore, the region is strongly affected by ENSO events, with important consequences for the marine communities. One important effects of El Niño is the reduction in the nutrient input to the coast. In this nutrient-unlimited region, it could be predicted that community structure shifts from being top-down controlled under “normal” conditions to be bottom-up controlled by nutrient scarcity under El Niño events. However, there is no baseline information on the magnitude of the factors that organize these communities. The aim of this study is to evaluate the contribution of top down processes in the organization of rocky intertidal communities of the Peruvian coast. In order to address this objective, surveys and manipulative experiments are being conducted to quantify the effects of those predators and grazers that given their abundance or their potential significant effects, is expected to be important in the community dynamics. These organisms are the littorinid snail Nodilittorina peruviana, the urchin Tetrapygus niger and the snail Tegula atra, the starfish Heliaster helianthus, and the oystercatcher Haematopus ater. Exclusion experiments indicate that Nodilittorina peruviana grazing can control the abundance of the epilithic biofilm of microalge and the survival of barnacle recruits in the mid-high intertidal. Higher in the shore, however, N. peruviana effects are less evident. In the low intertidal, manipulation of herbivores densities in inclusion cages indicates that the urchin Tetrapygus niger prevents the settlement and survival of sessile organisms; while the snail Tegula atra just delays the settlement. Moreover, it also suggests that the crustose alga depends on herbivorous activity to keep its surface free of macroalgae and other sessile organisms. The mussel Semimytilus algosus was the main space holder within these exclusions. In the mid intertidal, mortality of transplanted mussel clumps indicates that the abundance of adult S. algosus is controlled by predation of the starfish Heliaster helianthus. In the mid-low intertidal, estimates of predation rates and comparison of the abundance and size of mussels from surfaces accessible and non-accessible to predation by the oystercatcher Haematopus ater indicates that, despite its relatively low abundance, it is able to control the size frequency distribution of their main prey S. algosus. While preliminary, results indicate that consumers can exert an important top-down control in Peruvian rocky intertidal communities, and can contribute to understand the dynamic of the relative importance of the forces that control community processes under “normal” and El Niño conditions