Predation and interaction strength of octopuses and sea stars on different functional groups of the rocky intertidal shores of the Patagonian coast

Abstract

Predation is one of the main structuring forces in rocky intertidal communities, though with an effect depending on the environmental context and conditions. On the stressful Patagonia shores of Argentina, experimental studies concluded that predation does not play a significant ecological role. In the biogeographic transition zone of northern Patagonia, where medium-sized predators (like octopuses and sea stars) coexist and are relatively abundant, the effect of predators in the rocky intertidal zone was never tested. The present study evaluated the effect of two intertidal predators, the small octopus Octopus tehuelchus (d’Orbigny, 1834) and the sea star Anasterias antarctica (Lütken, 1857), on the abundance of prey and the interaction strength of these predators on different functional groups in a mid-intertidal rocky community of northern Patagonia. To do so, we conducted a short-term-exclusion experiment in the mid-intertidal area and estimated the interaction strength through dynamic indices. Our experimental exclusion of octopuses and sea stars had no positive effect on the abundance of prey in a short-term period, adding evidence that predation remains weak even in north Patagonia (i.e., under ameliorated physical stress), even with a relatively high abundance of predators. The combined interaction strength of those consumers on prey in the different functional groups was weak, with the dynamic indices being positive for mobile grazers and scavengers and negative for sessile filter feeders. These results emphasize the significance and deepen the understanding of the role of consumers, food web linkages, and community function in the biogeographic transition zone of north Patagonia.