Invasive fruiting plants with bird-dispersed seeds compete with native plants for birds dispersal services. An understanding of frugivore preferences as well as dispersal success can inform how the invasion of non-native species impacts the diversity of native species. In a four-year study, we compared the seeds moving via perching birds into an oldfield adjacent to a second-growth mesic forest in Indiana. Up to eight fecal traps were placed each year through the midline of the field and bird dispersed material was collected weekly from early September through early April. Seeds of fruiting species were separated and identified to species using a known collection of local seeds. Two surveys along the edges of the oldfield were made in early and late Autumn each year using a stratified random sampling protocol with cylindrical quadrats to determine the proportion of each fruit type available in the landscape near the fecal traps.
We collected a total of 5991 seeds of fruiting plants in fecal traps during fall and winter 2013-2017. Non-native species accounted for over half of the seeds traveling into the focal field via perching birds (57.5%). The bulk of the seeds of invasive species were dispersed during winter months December-March. A total of 4040 fruits were counted in four years of landscape surveys and a large proportion of those (67.2%) were of non-native species. An early Autumn comparison of fruits available in the landscape to fruit equivalents contained in the fecal material in seed traps revealed a strong preference for native Virginia creeper (Parthenocissus quinquifolia) (average across four years of 65.2% of all fruits in birds, 0.8% of fruits available in landscape), while non-native oriental bittersweet (Celastrus orbiculatus) was dispersed less than expected (1.8% in birds, 30.32% in landscape) given its availability. However, later in the fall, oriental bittersweet seeds became more prevalent in birds but still remained lower than expected (26.8% in birds, 54.9% in landscape). Non-native species dominated in new recruitment of woody fruiting species in the focal field, making up 86% of seedling fruiting species in sample quadrats. Thus, we found that native fruiting species were preferred by birds when available but non-native species still travelled in abundant quantities in perching birds in this habitat. Recruitment success of non-native species into the field was greater than expected given the percentages of seeds dispersed by perching birds, suggesting further factors are involved in the invasion success of these non-native fruiting plants.