Premise of the study: Plants and animals may experience reproductive Allee effects in fragmented populations, and obligate pollination mutualisms may be especially sensitive to extinction risk via this density-dependent process. In this study we examine how a shift from within-crown reproductive synchrony to asynchrony influences reproductive assurance through contributions to selfing and outcrossing in small, spatially isolated populations of Ficus.
Methods: The research focuses on the monoecious fig F. petiolaris and consists of phenological analyses and genetic assessments of selfing and outcrossing for populations located in Baja California's Sonoran Desert.
Key results: Phenological censuses of eight populations revealed within-crown asynchrony in 44% of reproducing trees, with 16% having sufficient overlap of male and female flowering phases to permit selfing via the cycling of pollinating fig wasps within natal trees. In mating system analyses of two of these populations, however, multilocus outcrossing rates (t(m)) were indistinguishable from 1. This result, combined with low levels of inbreeding, indicates selfing to be absent or at best a minor contributor to reproductive assurance.
Conclusions: The results indicate that the fitness benefits of within-crown asynchrony lie not with selfing, as commonly asserted, but with increased opportunities for outcross pollen transmission and receipt, changing our understanding of the mechanisms by which reproduction is facilitated and extinction risk minimized in naturally fragmented Ficus populations. Given the role of fig fruit as a keystone food resource in many tropical environments, trait variation leading to reproductive assurance in figs, such as within-crown asynchrony, has broader ecosystem-level implications.