Abstract: According to the Exploitative Ecosystems Hypothesis (EEH), the number of trophic levels in a
system predicts the intensity of herbivory on the plant community. We applied this hypothesis to the Río Claro in Corcovado National Park, Costa Rica, where marine predators at high tide may represent an added trophic level. We also hypothesized that herbivorous fish would seek refuge upriver. We measured percent algal cover, substrate, and herbivorous fish abundances in 1 km transects moving upriver from the mouth. Algal cover declined with increasing distance from the mouth within the tidal zone, and was higher above the tidal zone. Large herbivores were highest in abundance at the upriver edge of the tidal zone, while small herbivores were found exclusively above the tidal zone. The results support the existence of a shift in trophic interactions predicted by the EEH as well as the predicted movement of herbivorous fish upriver, further reducing algal cover there.
http://dartmouth.edu/dujs/wp-content/old_articles/2006F/carreras.pdf – Download PDF