The effects of urbanization on avian seed dispersal success of Toxicodendron radicans (Anacardiaceae)

The rate of global urbanization is increasing at an alarming pace, as the human population has grown to over 7 billion people—from 1.6 billion people in the 1900s—, half of which reside in urban areas. This increase has necessitated the expansion of urban habitat and increased consumption of natural resources. While the effects of urbanization on species diversity is well-documented (increasing urbanization decreases species diversity), its effects on species interactions have been less studied. Plant-animal interactions, such as seed dispersal, may be especially sensitive to urbanization. For plants, animal-based seed dispersal depends on several aspects, including 1) the rate of interactions with seed dispersers, 2) the probability of seed dispersal from an interaction event, 3) the identity and the number of seed disperser species –especially regarding differential ability to scarify seeds through digestion, and 4) the probability of germination after seed dispersal. Urbanization may affect seed dispersal dynamics by altering the frequency of interactions and/or the identity and diversity of seed dispersers. Consequently, the probability of seed dispersal and the ability of seeds to germinate and survive after being dispersed may be negatively affected by urbanization. In this study we ask specifically: 1) Will birds visit T. radicans at a greater rate in urban or natural habitats? 2) Will the diversity of dispersers be higher in urban or natural habitats? 3) Do seeds from urban or natural sites have a greater probability of dispersal? 4) Will seeds from urban or natural habitat be more likely to germinate? To compare differences in rate of visitation and disperser diversity between urban and natural habitats, individual T. radicans plants in two urban and two natural sites were observed for interactions by birds. Dispersal probability was estimated by marking fruits with a UV fluorescent dye and estimating a proportion of dispersed seeds at the end of the season. Seeds dispersed = total fruits marked – number of recovered fruits. Germination success will be estimated by collecting defecated—thus scarified—seeds in natural and urban sites as well as collecting non-dispersed seeds (that will be treated with either water or sulfuric acid). Seeds will be cold stratified 90 days before planting in constant 28oC and 16:8 L:D conditions. Preliminary results indicate that the rate of visitation, species diversity, and probability of seed dispersal are all significantly higher in urban sites. This trend suggests that T. radicans in urban habitat may be more successful than in natural habitat, however further research is necessary to confirm this