Range Expansion of Obligate Antagonists but Not Mutualists in a Desert Fig (Ficus petiolaris)

Species may respond to climate change by shifting habitats. For species that engage in obligate interactions, however, such range shifts are only possible if the interaction can occur in the extended range, either via dispersal of the original interaction partner or by acquisition of a novel interaction partner in the extended range. To explore how obligate interactions may persist by parallel dispersal of partners, we investigated the community of obligate fig wasps associated with an isolated fig tree (Ficus petiolaris) in Arizona, USA, found 200 km north of its native range. We expected that if the fig wasps (the single obligate pollinator as well, perhaps, as species-specific non-pollinating wasp associates) were occasionally acquired via dispersal from the native range, then (a) there would be months in which fig wasps were absent, and (b) fig wasps would not be genetically differentiated from those in the native range. Conversely, if fig wasps had formed persistent populations in Arizona on the single tree, we expected that (a) the tree would exhibit high reproductive asynchrony, a trait necessary for long-term pollinator persistence, and (b) the pollinator species would be present throughout the study. We collected and identified fig wasps from this tree for 20 months to determine the composition of the community as well as presence/absence patterns. We also used existing genetic data from two species of non-pollinating fig wasps from Arizona to determine their geographic origin and the extent of genetic differentiation from the same species in the native range. We found a persistent fig wasp community, but one consisting almost exclusively of at least two non-pollinating species in the genus Idarnes; the pollinator species was virtually absent. Our results suggest that previously unknown aspects of the natural history of Idarnes have allowed it to follow its fig tree host out of their shared range. These results point to several puzzles about range extension, as well as a testable hypothesis that the two Idarnes species groups can only persist without pollinators when they occur in tandem. Future ecological and genomic studies are needed to resolve these issues.