Does having multiple partners weaken the benefits of facultative mutualism? A test with cacti and cactus-tending ants

Interspecific facultative mutualisms typically involve guilds of interacting species, and species within a guild can vary in the quality of services they provide. For plants that secrete extrafloral nectar (EFN), visitation by multiple ant species that vary in anti-herbivore abilities may result in reduced benefits, relative to an exclusive association with a high-quality mutualist. This raises the intriguing problem of how facultative ant-plant mutualisms persist, given that EFN is costly to produce, yet may confer diminishing returns as partner diversity increases. I tested the prediction that association with two ant partners (Crematogaster opuntiae and Liometopum apiculatum) weakens benefits to the EFN-producing tree cholla cactus (Opuntia imbricata). I found that only one ant (L. apiculatum) provided protection against herbivores and seed predators. However, this species associated with cacti more frequently than Crematogaster across multiple temporal scales. Within years, Liometopum showed greater constancy on plants they occupied, more frequently colonized vacant plants, and replaced but were never replaced by Crematogaster. Across years of plant development, Liometopum was more abundant on reproductive plants and showed greater overlap with cactus enemies. Simulations of cactus lifetime reproductive output indicated that associating with high- and low-quality mutualists did not significantly reduce plant benefits relative to an exclusive L. apiculatum-O. imbricata association. The results suggest that non-random interaction frequencies, possibly driven by competition, may contribute to the maintenance of facultative mutualisms involving multiple, qualitatively different partners.