Locally unusual fruit compositions drive rare-biased seed dispersal

Frugivore preferences for rare fruits are a powerful mechanism for plant diversity maintenance. Nevertheless, the drivers of frugivore preferences and avoidances remain unknown. Using 12 years of bimonthly data on plant-frugivore interactions and fruit counts collected in Portugal, we assessed the contribution of fruit nutritional and energetic composition and heterospecific density-dependent effects to frugivore preferences and avoidances. We found that preferences were higher and avoidances lower for rare fruits with more distinctive fruit compositions. Moreover, we detected negative effects of neighboring fruit density on frugivore avoidances, but not on preferences, suggesting that neighboring plants facilitate interactions of the surrounding plant community by attracting birds to the area. Our results provide the first empirical evidence of the diet-complementarity hypothesis as a driver of rare-biased seed dispersal, likely contributing to global plant biodiversity maintenance.