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.