Climatic variability and neighboring plants mediate plant-insect mutualism in the Sonoran Desert

Background/Question/Methods

Arid landscapes are a model setting for parsing the roles of competition, facilitation and microclimate in shaping the distribution of plants.  Here, I explore the influence of climate and neighboring plants on the incidence of ants tending Ferocactus wislizeni, an extrafloral nectary-bearing cactus common in the Sonoran Desert of the southwestern United States, and explore how the phylogenetic proximity of neighboring plants influences the susceptibility of F. wislizeni to attack by insect herbivores (Narnia and Chelinidea sp.).  I paired 1) records of the presence/absence of herbivores on >400 F. wislizeni plants monitored in long term plots from 2003 to 2013 with 2) repeated inspections of 70 plants at one site in 20 climates (intersection of ambient temperature and humidity) and 3) descriptions of the woody plant communities within 2m of each of the >400 plants. Here, I test two hypotheses: 1) F. wislizeni plants that are adjacent to other plants with extrafloral nectaries (conspecifics, Carnegiea gigantea, Cylindropuntia versicolor, Acacia constricta) are tended more frequently than expected (based on knowledge of plant size and climate), and 2) herbivores are more commonly found on F. wislizeni plants adjacent to conspecifics and confamilials.

Results/Conclusions

Descriptive logistic models were created to predict the incidence of ant tending based on plant size and climate at the time of inspection.  Tending incidence increased with plant size (a surrogate for nectary abundance), temperature and humidity.  After accounting for these effects, F. wislizeni plants adjacent to Carnegiea gigantea were tended by ants with significantly greater frequency.  Effects of adjacencies to Cylindropuntia versicolor, Acacia constricta and other species (Cercidium microphyllum, Larrea tridentata and the invasive grass Pennisetum ciliare) were more modest.   Herbivorous insects were significantly more common (likely to be detected) on F. wislizeni within 2m of conspecifics or confamilials.   This work illustrates the role of indirect interactions (positive and negative) between adjacent plants, and demonstrates that facilitation in arid environments is not limited to interactions between nurse plants and seeds/seedlings.  Specifically, adjacent nectary-bearing plants can “buffer” interactions with prospective mutualists in the face of climatic variation.