Proximity to roads disrupts rodents' contributions to seed dispersal services and subsequent recruitment dynamics

Seed dispersal and subsequent recruitment dynamics play a crucially important role in regulating species coexistence and structuring tree diversity in diverse forests. Wildlife, which can dually shape the tree recruitment process by simultaneously functioning as natural enemies and seed dispersers, are undergoing widespread changes in population and behaviour due to the detrimental effects of expanding global road networks. However, the impact of these changes on recruitment dynamics through the alteration of seed dispersal processes remains understudied. Here, we assessed how roads affect animal-mediated recruitment dynamics from the seed to adult stages of trees using a ubiquitous vertebrate-seed model: rodents and acorns. To quantify the degree to which proximity to a road alters seed dispersal and regulates subsequent recruitment dynamics, we conducted seed dispersal and predation experiments, and investigated the natural recruitment of a dominant tree species (Quercus aliena) from September 2009 to July 2012 in a subtropical forest of central China. Roads caused a decreased seed dispersal distance and increasing larder hoarding, demonstrating a weakened contribution of rodents to seed dispersal services. These patterns were stronger in the masting year than in the non-masting year. Correspondingly, seedlings, saplings and adult trees had higher densities near roads than far from roads. Near roads, recruit transition rates (i.e. seedling-to-sapling and sapling-to-juvenile) were low, and the influential strength of conspecific adult density was weakened over these recruit transitions, indicating that space limitation and lottery competition, not conspecific negative density-dependent effects, approximately determined the near-road recruitment process. Furthermore, most adults near roads were young, and their ages matched the road ages. Synthesis. Roads diminished animal-mediated seed dispersal services and disrupted subsequent recruitment dynamics in Quercus aliena populations, thus weakening the key process promoting diversity in forest ecosystems. Given that these animal-regulated roles in recruitment dynamics are widespread stabilizing forces for tree coexistence, our findings suggest that the ubiquity of roads and their continued expansion will contribute to the increase in forests dominated by a relatively low number of species. Consequently, the growing expansion of road networks will ultimately cause profound changes in community composition and structure across diverse forests worldwide.