Mutualisms play a central role in structuring populations and maintaining biodiversity. However, although most mutualisms involve networks of interacting species, we have a limited understanding of the importance of species diversity within mutualisms. If mutualists provide complementary benefits, species may benefit from increased mutualist partner diversity; however, species also often compete with each other for access to mutualists, which may interfere with the services that they provide. Because few studies have documented the demographic consequences of associating with multiple mutualist partners or the mechanisms underlying such effects, our understanding of the role of biodiversity in mutualisms is limited. To investigate this topic, we evaluated the effects of three mutualist ant species on the demography of the aphid Aphis helianthi and examined the competitive interactions between these ants. We hypothesized that aphids would receive the strongest benefits when tended by aggressive, competitively dominant ants or when tended by multiple ant species if they provide complementary benefits. To test these hypotheses, we evaluated ant competitive interactions in experimental baits and constructed demographic models parameterized with empirical data to determine how each ant species affected the population growth (λ) of aphid colonies.
Results/Conclusions
Compared to when aphid colonies were untended (λ = 0.91), each ant enhanced aphid colony population growth rates: Formica rufa, the most competitively dominant ant (wins 82% of aggression tests), provided the strongest benefit (λ = 1.06), followed by F. podzolica (λ = 1.03; wins 18% of aggression tests) and T. sessile (λ = 1.02; wins 15% of aggression tests), respectively. However, when multiple ants simultaneously tended the same aphid colony, they provided no benefit (λ = 0.91), indicating that competition among ants may interfere with the services that they provide. Future work will investigate the mechanisms driving differences in ant effects on aphid population dynamics as well as the effects of diversity in ant partners across time. Elucidating the effects of multiple mutualists on the demography of their partners and how such effects are mediated by competition is necessary for determining the importance of biodiversity in mutualisms and how such interactions may respond to species invasions and extinctions.