Potential for omnivory and apparent intraguild predation in rocky intertidal herbivore assemblages from northern Chile

Abstract

We assessed the ingestion of animal items by 29 herbivore species of the most common invertebrates on rocky intertidal shores of northern Chile (21 to 30° S). Data were obtained from 4 communities sampled seasonally from winter 2004 to spring 2005. Gastric contents of 2671 individuals were categorized into 143 food items, 42.7% of them corresponding to animal prey. All herbivores were polyphagous generalists, showing a moderate to high dietary overlap and suggesting the potential for exploitation competition. Diet width and the proportion of animal items ingested per species were positively related with body mass, revealing a high potential for true omnivory among larger herbivores. Barnacles were the dominant prey item, followed by a suite of common intertidal animals, including herbivore species. The consumption of herbivores was defined as apparent intraguild (IG) predation, a framework that we used by analogy to describe pairwise interactions at a regional and community level. We recorded 29 IG predator–IG prey interactions, all of them asymmetrical (no reciprocal predation), and directed exclusively towards heterospecific IG prey, which in most cases were juvenile individuals that were taxonomically unrelated. All IG predators were large herbivore species, and they appeared to avoid consuming conspecifics. The high incidence of polyphagy and apparent IG predation may not be simply an epiphenomenon of grazing, nor a response to limited algal resources, and we discuss the nature of herbivores as consumers and the implications of potential omnivory for the connectedness, looping, and chain length of intertidal food webs.