Award: OCE-1061475

Sound management of the oceans requires understanding the roles that key species and biodiversity plays in ecosystem functioning. Since we cannot investigate all marine ecosystems in enough detail to extract this information, we rely on theories to predict when and where strong ecological interactions from key species will control ecosystem functioning, such as primary production of algae on the sea floor. One of these theories predicts that strong controlling interactions from top predators will be less common in diverse food chains (webs) containing large numbers of species than in food webs with few species. The basis for this is the notion that the many interactions among species that arise in diverse food webs, such as competition for food or aggressive behaviors, could reduce the ability of top predators to consume species on lower levels. However, realistic field experiments are seldom applied to test this theory in diverse marine ecosystems, such as the tropics. Consequently, we used the diverse food webs of the Galapagos Islands to test the theory, predicting that by feeding on herbivorous sea urchins, predatory fish could indirectly increase the abundance of primary producers (bottom dwelling algae) that the sea urchins typically control. To mimic conditions of the natural subtidal ecosystem as much as possible, we developed a novel "open" experimental design where unconfined predatory fish could move in and out of the treatments to attack the sea urchins, and interact with each other and top predators (sharks, sea lions) while foraging. Since the week- long experiments were monitored by high frequency (1 second) time-lapse underwater photography, it was possible to identify both feeding and non-feeding (behavioral) effects of predators, as well as to document the species involved in predation on sea urchins. The results were striking; pencil urchins (Eucidaris galapgensis) were eaten by blunthead triggerfish (Pseudobalistes naufragium) within 21 hours, releasing benthic algae from urchin grazing, which establishes a strong ecological interaction called a trophic cascade, as triggerfish reduce sea urchin abundance, which indirectly increases algae. Finescale trigger fish (Balistes polylepis) were also identified as major predators of pencil urchins. When unchecked by predators in predator exclusion cages, the pencil urchins removed 10% of algae in a week. Spanish hogfish, previously considered a top predator of sea urchins, were instead revealed as "hitchhikers" on the triggerfish, often circling them closely and reducing their feeding efficiency on sea urchins. Sea lions swam rapidly over the experiments in search of prey, disturbing feeding triggerfish and causing them to drop urchin prey on occasion. The high frequency database on species interactions enabled us to construct a mathematical model of triggerfish foraging on sea urchins. The model revealed that the presence of both top predators (sharks, sea lions) and hogfish reduce triggerfish feeding rates. Surprisingly, these results were highly contingent on the species of sea urchins involved. For example, predators (triggerfish, hogfish) virtually ignored green sea urchins (Lytechinus semituberculatus) when they were used in the experiments instead of pencil urchins. The green urchins consumed 3 x more algae than the pencil urchins. Counter to theory, our results indicate that that strong predatory interactions do occur in diverse food webs, and can cascade down to transform the abundance of bottom dwelling algae, and indicator of ecosystem functioning. The Galapagos marine food web can be characterized as one in which strong predatory interactions are embedded in a diffuse web of behavioral interactions. Biodiversity matters, as certain species such as two species of triggerfish and sea urchins play unique roles in the functioning of the marine ecosystem. Whether or not a trophic cascade occurs depends of the species of sea urchins involved and the prey...