Urbanization, Not Invasive Shrubs, Alters Tree Seed Fate by Modifying Rodent Activity

Changes in the global environment are widespread and may have unappreciated effects on the activity of animals and the strength of animal-mediated interactions. For example, urbanization and the spread of invasive species are aspects of global change that may lead to shifts in the activity of granivorous rodents, potentially leading to changes in the survival and establishment of seeds rodents consume. Importantly, these two aspects of global change could interact to affect rodent activity. We used a large-scale manipulation of common invasive shrubs (Rhamnus cathartica, Lonicera macckii) across a rural-to-urban forest gradient spanning southern Wisconsin in summer and autumn to examine the effects that urbanization, invasion, and seasonality had on modifying rodent activity and granivory. Using two 14-day sampling sessions, we recorded the activity of three granivorous rodents (Peromyscus leucopus, Sciurus carolinensis, and Tamias striatus) with motion-activated cameras and quantified seed removal using six tree species (Quercus rubra, Quercus alba, Acer saccharum, Prunus serotina, Pinus strobus, and Pinus resinosa) with seed depots to link animal activity with seed removal. Our findings reveal that Quercus seed removal was highest in urban sites, potentially linking hotspots of S. carolinensis activity in urban forests to decreased seed survival. In capturing a large number of P. leucopus photos during the autumn in R. cathartica removed plots, our findings suggest that P. leucopus may be responding to the provision of short-term resources. Our results underscore the primacy of seed mass in determining rodent seed choice: although urbanization and invasive shrubs had different effects on the activity of rodent species, the removal of seeds was always strongly dependent upon seed mass. Our findings help to illuminate potential hotspots of granivorous rodent activity along an urbanization gradient, the shifts in species-specific seed loss associated with this gradient, and the dominance of seed mass in contributing to rodent seed preference.