Interaction motifs variability in a Mediterranean palm under environmental disturbances: the mutualism-antagonism continuum

The nature and the strength of plant-frugivore interactions often vary along an antagonism-mutualism continuum and are highly influenced by the local ecological context (e.g. level of environmental disturbances). However, little is known concerning how the local ecological setting where plant-frugivore interactions take place affects the seed dispersal effectiveness (SDE) and, eventually, plant recruitment. This knowledge gap relates to the scarcity of empirical investigations on individual-based plant-frugivore networks. We assessed whether the assembly of interaction modes (antagonist and mutualist) of the dwarf palm Chamaerops humilis (Arecaeae) affect the individual and the population level SDE in two Mediterranean sites differing in perturbation levels. We analysed the frequency distribution of interaction typologies and test its relationship with variation among individual palms in SDE. Additionally, we document how variation in interaction motif frequency (e.g. overrepresented interaction typologies) relates to changes in the SDE landscape of both disturbed study sites. We found that the interaction typologies of individual palms and its frugivores did not occur randomly. In a more complex landscape, interactions were more diverse and less dominated by simpler typologies, which may increase the stability of the plant-disperser interaction over the long term. We demonstrate that plants that interact with a more complex assemblage of frugivores, including both mutualistic and antagonistic partners, had the highest probability of recruitment. We found that the highly variable SDE among individual palms resulted from a few interaction modes occurring at higher than expected frequencies. We suggest this may be a reasonably frequent situation in diversified frugivore assemblages with a high heterogeneity of functional effects among mutualistic and antagonistic partners.