Seed dispersal facilitation and geographic consistency in bird-fruit abundance patterns

Avian seed dispersal mutualisms are characterized frequently by stochastic interactions between birds and fruits; however, many studies report coarse-scale correlations in annual abundances of birds and fruits at particular locales (i.e. 'phenological synchrony'). This study tested the geographical consistency of phenological synchrony in a meta-analysis of data from 14 biogeographic locations. Data from a single site in British Columbia, Canada, were then used to test the dispersal facilitation hypothesis, which postulates that synchronous bird-fruit abundance patterns result from deterministic seed dispersal processes (i.e. avian fruit consumption). Results showed that phenological synchrony is a geographically consistent pattern. However, fruit production occurred after peak periods of bird abundances in British Columbia. Although phenological patterns were asynchronous at this site, observational and experimental fruit removal patterns supported the dispersal facilitation hypothesis. Avian fruit consumption covaried with bird abundances, suggesting selection may favour earlier fruit production and increased phenological synchrony. Environmental data suggest that earlier fruit production is constrained by cold spring temperatures, which inhibit the activity of pollinators and earlier dates of fruit maturation. Overall, the results show that phenological synchrony is a geographically consistent pattern in seed dispersal mutualisms. However, decoupled bird-fruit abundance patterns may occur despite deterministic processes favouring phenological synchrony.