Competition hierarchies among ants and predation by birds jointly determine the strength of multi-species ant-aphid mutualisms

Abstract

Protection mutualisms often involve multiple species of protector that vary in quality as mutualists. Because protectors may compete for access to mutualists, concordance between competitive ability and degree of benefi t will determine the overall strength of multi-species mutualisms. We compared the abilities of two similarly sized congener ants as competitors for, and mutualists of pine-feeding aphids, and how insectivorous birds aff ected each ant–aphid mutualism. Formica planipilis and F. podzolica were indistinguishable in forager abundance, but the former was 13-fold more abundant at competition baits and provided 11-fold more benefi ts to aphids. Th ese results highlight how, in a single environment, a great ecological distance can exist between two congener ants of similar size. Insectivorous birds disrupted the two mutualisms to a similar extent, reducing aphid and ant abundance by 91% and 39% respectively. Nevertheless, birds had an important infl uence on the relative benefi ts of the two ants to aphids: where F. planipilis consistently benefi ted aphids, F. podzolica only did so in the absence of birds. Consequently, the presence of insectivorous birds and ant species identity jointly determined whether ant–aphid mutualisms occurred in pine canopies and the strength of such interactions. Our study highlights the inter-relatedness of competition, predation and mutualism, and how competition can serve to strengthen mutualism by fi ltering inferior mutualists.