Biodiversity consequences of predation and host plant hybridization on an aphid–ant mutualism

We examined the hypothesis that mutualists, predators, and host plant quality act in concert to determine the distribution and abundance of a common herbivore. The aphid, Chaitophorus populicola, is found only in association with ants, which provide tending services and protection from predators. As a consequence, aphid abundance declined by 88% on host plants located ≥6 m from an ant mound. Differences in host plant quality resulted in aphid fecundity being greatest on narrowleaf cottonwoods, 7–22% less on backcross hybrids, 37–46% less on F₁ hybrids, and 57–61% less on Fremont cottonwoods. Due to the combined effects of these factors, we found that the realized aphid habitat was only 21% of their potential habitat. On trees where aphids and tending ants are present, aphids and ants greatly outnumber any other arthropod species; therefore, we examined the hypothesis that the factors affecting aphid density and distribution are also determinants of arthropod community structure. On a per‐tree basis, observational data showed that arthropod species richness was 51% greater and abundance was 67% greater on trees where aphid–ant mutualists were absent relative to trees where they were present. When aphids were experimentally removed and ants abandoned the tree, we found the same pattern. On a per‐tree basis, arthropod species richness increased by 57%, and abundance increased by 80% where aphid–ant mutualists were removed, relative to control trees. Overall, a total of 90 arthropod species were examined in this study: 56% were found only on trees without aphid–ant mutualists, 12% were found only on trees with aphid–ant mutualists, and 32% were common to both. Specific guilds were also affected; the aphid–ant mutualism had a negative effect on herbivores, generalist predators, and other species of tending ants, and a positive effect on specialist enemies of aphids. These results suggest that, by examining the factors responsible for the population dynamics of a common herbivore, we may also uncover mechanisms that determine arthropod community structure. Furthermore, studies that address the role of mutualism in shaping communities are underrepresented in the literature; these results emphasize the importance of mutualism in ecological communities.