Mutualism and predation have contrasting effects on pine canopy arthropod diversity

Predators are recognized to increase prey diversity by suppressing competitively dominant species, where mutualists are predicted to reduce diversity by promoting a competitively dominant partner. However this trend, and the effect of these interactions when they cooccur, remain underexplored. We investigated the effects of predation and mutualism on the diversity of pine-associated arthropods by excluding insectivorous birds and mutualist ants from branches of Pinus ponderosa and sampling arthropods during a 2-year period. We identified 92,549 arthropods to the species or morphological species level. In the absence of ants, birds had no effects on diversity while in the absence of birds, ants decreased Simpson diversity and Pieolu's evenness but did not affect species richness. However, in the presence of ants, birds increased diversity, evenness, and richness. Effects on arthropod composition mirrored diversity: birds alone had no effect on composition, ants alone increased aphid and aphid predator abundance, but in the presence of ants, birds reduced aphid and aphid predator abundance. In summary, we provide evidence that mutualists reduce diversity and alter community composition in pine-associated arthropods by promoting the dominance of partner species, and these interaction together are non-additive, with predator effects likely working through the disruption of the mutualism.