Mutualisms play a central role in structuring populations and maintaining biodiversity. Most mutualisms consist of networks of interacting species, but our understanding of the effects of partner identity and diversity on the fitness of species within these networks is limited. If mutualists provide complementary benefits, associating with a greater diversity of mutualist partners may be beneficial; however, species also often compete with each other for access to mutualists, which may interfere with the services that they provide. To investigate the consequences of associating with multiple mutualist partners, we compared the effects of three mutualist ant species on the demography of the aphid Aphis asclepiadis and examined the competitive interactions among these ants. We hypothesized that aphids would receive the strongest benefits when tended by aggressive, competitively dominant ants, since they would likely provide stronger protection against predators. In addition, we hypothesized that aphids would benefit more from associating with a greater diversity of ant species, since different ants could provide complementary benefits. To test these hypotheses, we evaluated ant competitive interactions in experimental baits and compared the effects of ants on aphid colony population growth (λ) by constructing demographic models parameterized with data collected across two years.
Results/Conclusions
While aphids performed worse overall in 2018 than in 2017, the relative effects of ants were consistent across years. In both 2017 and 2018 (respectively), each ant species increased aphid population growth (relative to λ = 0.85 and 0.80 for untended aphids), and the best mutualist (Formica rufa; λ = 1.04 and 1.00) was also the best competitor (won 82% of aggression tests). Thus, competition among ants may benefit aphids if it limits their associations with poorer mutualist ant species (Tapinoma sessile, λ = 1.00 and 0.96; Formica podzolica, λ = 0.97 and 0.94). However, when multiple ants simultaneously tended the same aphid colony, aphids received weaker benefits than when they were tended by any of the three ant species individually (λ = 0.91 and 0.86 in 2017 and 2018, respectively), indicating that competition among ants may interfere with the services that they provide. In summary, our results highlight that although competition for mutualists may be beneficial if it limits associations with poorer mutualists, competition may also make mutualists less effective and thus decrease the benefits of enhanced partner diversity. This work illustrates that in order to predict the outcomes of mutualisms, we must study them in a multi-species context.