Among-individual variation in a host’s interactions with mutualists and parasites is often explored in the context of characteristics intrinsic to the participants (e.g., reward production, immuno-competence). Variability in these interactions can also be shaped by neighbors – whether conspecifics, confamilals, or more distantly related species – and understanding the effects of particular species and local biodiversity is particularly salient in an era of biodiversity redistribution. Working with plants in the Sonoran Desert in 2015-2017, I quantified the incidence of mutualists (pugnacious ants attracted to extrafloral nectar) on two species of legume (Cercidium microphylla and Acacia constricta) and both mutualists and sucking parasites (Chelidinea spp) on three species of cactus (Cylindropuntia versicolor, C. Leptocaulis, Opuntia engelmanni), and contrasted these interactions on plants near and far from Ferocactus wislizeni (<2m and > 10m, respectively). This comparison was motivated by the observation that ant tending at Ferocactus far exceeds most other Sonoran Desert plants and that, as a result, this plant may modify interactions with adjacent plants. I also test a second hypothesis: that interactions between Ferocactus and mutualists, parasites, and a virus (cactus potexvirus) change with the composition of the local plant community.
Interactions with mutualists – quantified here as incidence of on-plant foraging by ants – were more frequent for Acacia, Cercidium, and Cylindropuntia spp within 2m of Ferocactus; the average ratio, relative to more distant plants, was 1.51 ± 0.34 (mean and SD). Inter-seasonal variation in interactions with ants was also less pronounced (CV decreased by 0.23 ± 0.19) in plants adjacent to Ferocactus. The opposite pattern was observed for Opuntia engelmanni; here, ants were observed marginally less often (overall) and less consistently (among seasons) when in close proximity to Ferocactus. The incidence of sucking insects (parasites) was twice a great on C. versicolor and O. engelmanni with neighboring Ferocactus (relative to more distant individuals), and these parasites were rare on C. leptocaulis irrespective of neighborhood. The frequency of interactions between Ferocactus and ants and/or sucking parasites was relatively insensitive to the composition of the neighboring plant community. Last, likelihood of deformation of Ferocactus plants (a symptom of cactus potexvirus) increased with richness of confamilials within 2m and decreased with richness of more distantly related species within 2m after controlling for plant age. These results support the interpretation that interactions with parasites and mutualists are influenced by the proximity of functionally similar individuals.