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

Abstract

Summary1. 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. 2. 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. 3. 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 Białowie_ za Forest, Poland. 4. 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 cofruiting plant species was also reflected in their co-occurrence. 5. 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.

Trait‐associated loss of frugivores in fragmented forest does not affect seed removal rates

“…In the study region, the fruiting period of the focal plant species ranges from the end of June until mid‐October (Albrecht et al . ). During this period, the focal plant species represent critical resources for a diverse array of frugivorous birds and mammals (Table S3).…”

“…Forest fragment sites were located in small ash–alder forest remnants entirely surrounded by meadows (for details on characteristics of the study sites, see Table S1; Albrecht et al . , ). Thus, our study was conducted on a total of 17 sites scattered over c .…”

“…In ash–alder floodplain forests, these species occur in high densities and often form the dominant aspect of the mid‐ and understorey (Albrecht et al . , , ). Across study sites, the number of species varied between 7 and 14 (10 ± 1.7 species per site, mean ± SD).…”

Connecting Resource Tracking by Frugivores to Temporal Variation in Seed Dispersal Networks

“…Additionally, fruit removal of Actaea spicata was highest in sites where fruiting densities were highest, regardless of neighboring species (von Zeipel and Eriksson, 2007). The facilitation hypothesis is also one possible mechanism explaining maintenance of rare plant species in a Polish forest; rare species having greater phenological overlap with more abundant species benefit from the increase in potential dispersers attracted to a neighborhood by the high density of the fruit of the abundant species (Albrecht et al, 2015).…”

Complementary Effects of Species Abundances and Ecological Neighborhood on the Occurrence of Fruit-Frugivore Interactions

“…Although both mechanisms are not mutually exclusive, neutral processes are usually expected from systems dominated by generalist species (e.g., González-Castro et al, 2015), whereas niche-driven processes are expected to be more relevant in those systems where high biodiversity involves a large trait variation among species (e.g., in tropical contexts, Bender et al, 2017). Unfortunately, a focus restricted to the dichotomy between these mechanisms ignores other drivers related to the occurrence or abundance of species other than the interacting pair in the community (e.g., context dependency driven by competition or facilitation by other species) (Perea et al, 2013;Martínez et al, 2014;Albrecht et al, 2015). These context dependencies, also termed neighborhood effects, may be inferred from the occurrence of third species comprising the spatial or temporal biotic context surrounding pairwise interactions.…”