Contagious seed dispersal and the spread of avian-dispersed exotic plants

Seed dispersal by avian frugivores has driven the invasive success of numerous exotic fleshy-fruited plant species around the world. Birds' movements are often directed toward food sources, producing seed shadows that are highly structured in space. The distribution of native fleshy-fruited plants in a landscape could thus form a spatial template for the contagious spread of exotic fleshy-fruited species in the early stages of invasion. We compared seed rain beneath fleshy-fruited and dry-fruited native-tree canopies in forested habitats and open fields in southeastern Michigan. We predicted that exotic seed rain would be highest beneath the canopies of fleshy-fruited plants, and that localities with higher densities of fruit-bearing fleshy-fruited plants would receive more exotic seed rain. Our results suggest that the seed shadows of exotic fleshy-fruited species are strongly influenced by the spatial distributions of native fleshy-fruited trees, and by the local density of fleshy-fruited plants. Over 92 % of exotic seeds were dispersed beneath fleshy-fruited trees, whereas less than 8 % of exotic seeds were dispersed beneath dry-fruited trees. Exotic seed rain was positively related to the local density of fleshy-fruited plants in forest, but not in open fields. Our study shows how shared dispersal syndromes and frugivore behavior influence the seed shadows of avian-dispersed exotic plants, enabling spatially explicit predictions of invasive spread in their novel ranges.