Population models reveal synergistic fitness effects of climate change and urbanization on poison ivy ( Toxicodendron radicans ) via disruption of seed dispersal interactions

Two of the most important human disturbances on natural ecosystems are urbanization and climate change. While their individual effects are well-documented, how the combined effect of urbanization and climate change can synergistically affect species’ success as well as ecosystem function is less understood. Advancing our understanding of their potential synergistic effects is crucial for mitigating their consequences on biodiversity and ecosystem stability. Here, we developed a stock and flow model to quantify the effects of urbanization and climate change on avian seed dispersal of poison ivy fruits using parameters estimated in the field. We simulated fruit fate in urban and natural sites and calculated overall seed germination success as our estimation of plant fitness. Our model predicted the general dynamics of fruit ripening, fruit loss (fall to ground), and fruit consumption by birds and forecasted lower germination success at urban sites that generally have higher fruit production. Different climate change scenarios predicted different degrees of phenological mismatch between plants and the seed dispersing birds resulting in significant decreases in fitness (germination success). More importantly, the negative effects of climate change are predicted to be more detrimental in urban (i.e., synergistic effects) compared to natural areas. Our study emphasizes the need to evaluate synergistic effects of human disturbances in order to better predict and mitigate its effects on species interactions and ecosystem function.