Urbanization increases seed dispersal interaction diversity but decreases dispersal success in Toxicodendron radicans

Urbanization is increasing at a rapid pace with negative consequences for native biodiversity. While it is well-known that urbanization can lead to biotic homogenization (dominance of a few competitive species), effects of urbanization on ecological functions that rely on the frequency and efficiency of species interactions are less understood. Seed dispersal success depends on seed disperser feeding rate and diversity, which can affect the probability of fruit removal (i.e. seed dispersal) and germination rates. However, how these factors are affected by urbanization is unknown. In this study, we evaluate the effects of urbanization on factors that contribute to seed dispersal success (seed disperser diversity, frequency, probability of fruit removal and germination rate using Toxicodendron radicans and its seed disperser community as a model system. We found that urban sites had three times more disperser species and two times higher feeding rate compared to natural sites. However, the probability of individual fruit removal did not differ between natural and urban sites. Moreover, germination rate after dispersal was 20% lower in urban sites, leading to overall negative effects of urbanization on T. radicans seed dispersal. We propose differences in seed germination rate are driven by changes in seed disperser species composition and their differences in seed gut transit time. This in turn affects disperser species' ability to successfully scarify seeds. Overall, our results highlight the need to evaluate urbanization's effects on functional ecological processes, in addition to biotic homogenization effects, in order better understand and mitigate its negative impacts on biodiversity. (C) 2020 The Authors. Published by Elsevier B.V.