Urbanization is increasing at a dramatic rate as the human population increases. While it is well-known that urbanization tends to decrease species diversity (i.e., biotic homogenization), it is not known how urbanization affects the frequency and efficiency of species interactions. Seed dispersal is a plant-animal interaction that depends on disperser feeding rate, disperser diversity, probability of seed dispersal and germination. How these factors are affected by urbanization however is unknown. In this study, we evaluate how urbanization alters these factors. Urban sites had 2x higher feeding rate and 3x higher number of disperser species. The probability of seed dispersal however was the same between natural and urban sites. Moreover, the probability of germination after dispersal was 20% lower in urban sites, leading to overall negative effects of urbanization on T. radicans seed dispersal. In this study we demonstrated that urbanization can affect species diversity, as well as their ecological functions.

The rate of global urbanization is increasing rapidly as the human population expands, leading to species loss and biotic homogenization. Less studied, however, is the effect of urbanization on the frequency and efficiency of species interactions. Animal-based seed dispersal interactions may be especially sensitive to urbanization because they depend on several factors: 1) the rate of seed dispersal interactions (feeding), 2) diversity of disperser species, 3) the probability of seed dispersal and 4) the probability of germination after seed dispersal. However, how urbanization disrupts species interactions, including seed-dispersal, is still poorly known. In this study, we evaluate differences in the frequency and efficiency of seed dispersal between urban and natural sites using Eastern Poison Ivy (Toxicodendron radicans) as the focal species. Individual T. radicans lianas within natural and urban sites were observed in twelve-minute intervals (total 185.8 hours) at urban and natural sites during which the number and identity of feeding avian species was recorded. A total of 9500 fruits between natural and urban sites were marked with a UV fluorescent dye. Undispersed marked fruits were recovered via seed traps to estimate probability of dispersal. Defecated fruits were collected from natural and urban sites to evaluate germination efficiency after dispersal. Feeding rate was twice as high in urban compared to natural sites (P=0.007). Additionally, seed disperser diversity was on average twice as high in urban sites and species composition was significantly different between natural and urban sites. However, probability of seed dispersal was not significantly different between urban and natural sites (P=0.3). Interestingly, germination rate was 20% higher in defecated seeds collected from natural sites compared to defecated seeds from urban sites (P=0.005). Our results suggest that while T. radicans attracts a higher number and greater diversity of seed dispersers in urban areas, overall dispersal success is the same or even greater at natural sites, as seeds have a higher chance of germinating after being consumed by dispersers at natural compared to urban sites. Species composition differences between sites may play an important role in germinability of seeds; differences in species’ feeding strategies result in differences in their ability to scarify seeds in their digestive systems, a necessary step for seeds that rely on animal seed dispersers. Urbanization can thus negatively affect seed dispersal interactions by altering the composition of disperser species. Other animal-based interactions may be similarly affected by urbanization, and thus we emphasize the need for further studies.

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.