Variation in neighbourhood context shapes frugivore‐mediated facilitation and competition among co‐dispersed plant species

Abstract

Summary Co‐occurring and simultaneously fruiting plant species may either compete for dispersal by shared frugivores or enhance each other's dispersal through joint attraction of frugivores. While competitive plant–plant interactions are expected to cause the evolutionary divergence of fruit phenologies, facilitative interactions are assumed to promote their convergence. To which extent competitive and facilitative interactions among plant species with similar phenological niches are controlled by spatial variation in their local abundance and co‐occurrence is poorly understood. Here, we test the hypotheses that when a plant species fruits in high densities, large phenological overlap with other plant species causes competition for seed dispersers owing to frugivore satiation. Conversely, we expect large phenological overlap to enhance the dispersal of a plant species fruiting in low densities through attraction of frugivores by other species in its local neighbourhood. We test these predictions on plant–frugivore networks based on seed removal from 15 woody, fleshy‐fruited plant species by 30 avian and 4 mammalian frugivore species across 13 study sites in Białowieża Forest, Poland. A null model indicated that fruit phenologies of the regional plant assemblage were more differentiated than expected by chance. In the local networks, the tendency of plants to share frugivores increased with phenological overlap. High phenological overlap reduced the seed removal rates, interaction strength (proportion of interactions) and the number of partners of plant species fruiting in high densities. Conversely, plant species fruiting in low densities mainly profited from high phenological overlap with other species. Importantly, the sharing of mutualistic partners among co‐fruiting plant species was also reflected in their co‐occurrence. Synthesis. Our study highlights that, in spite of the overall signal of competition, frugivore‐mediated interactions among cofruiting plant species may consistently promote the establishment and persistence of rare species through facilitation. In addition, our results suggest that, among other factors, indirect coupling of species through shared mutualistic partners might be an important determinant of plant community assembly. The coupling through shared mutualists may cause the formation of associations among co‐dispersed plant species and might contribute to the coexistence of species in plant–animal mutualistic communities.