Seed predation by rodents suppresses recruitment of a bird-dispersed tree at its upper range limit

Abiotic factors have traditionally been considered the primary determinants of species' range limits and responses to climate change. However, growing evidence shows that biotic interactions, such as seed predation and dispersal, also play crucial roles in shaping plant distributions and influencing range shifts. We investigated how scatterhoarding rodents affect the recruitment of Swiss stone pine (Pinus cembra) at its upper altitudinal limits in the Swiss Alps. We focused on interactions between the pine and the rodent community, exploring how these interactions can potentially affect the effectiveness of seed dispersal by the spotted nutcracker (Nucifraga caryocatactes). We conducted seed crop estimation, rodent live-trapping, seed tracking experiments and seedling emergence trials across an altitudinal gradient over 3 years. We tested three hypotheses: (1) rodents cache a considerable portion of removed seeds, including those pilfered from nutcracker caches, in favourable microsites; (2) rodents consume seeds from nutcracker caches, impeding pine recruitment; and (3) the role of rodents varies with seed availability and rodent abundance, shifting between mutualism and antagonism. Rodents acted consistently as seed predators and cache pilferers. In seed tracking experiments, they consumed 97% of the seeds they removed and cached only 3%. In cache pilferage experiments, rodents reduced seedling emergence from simulated caches by two-thirds. These findings support hypothesis (2), indicating that rodents suppress Swiss stone pine recruitment by reducing the effectiveness of nutcracker-mediated seed dispersal. The negative impact was most pronounced when rodent populations were high and seed availability was low, aligning with the concept of predator satiation. Our study suggests that climate warming, which is expected to increase rodent abundances at higher altitudes, may intensify the negative effects of rodents on pine recruitment, impeding the upward migration of Swiss stone pine. These findings stress the importance of including biotic interactions, particularly seed predation and interspecific seed pilferage, into ecological models to improve predictions of ecosystem responses to environmental changes. Understanding these dynamics is crucial for developing effective conservation strategies in the face of climate change.