Invasive exotic shrub (Rhamnus cathartica) alters the timing and magnitude of post-dispersal seed predation of native and exotic species

Aims: Invasive exotic plants may generate deleterious indirect effects on native species, such as increasing consumer pressure on native plants (i.e. apparent competition). Although rarely examined, invasive shrubs may create novel pulses of consumer pressure because their leaves senesce later, providing temporal increases in cover. Apparent competition generated by invasive species may have a strong temporal signal due to extended phenologies of invasive plants. Using an experimental manipulation of the invasive shrub, Rhamnus cathartica, we evaluated whether seed prédation of multiple species is higher in invaded areas, and if the novel extended phenology of R. cathartica leads to temporal changes in seed predation. Location: University of Wisconsin-Madison Arboretum, Madison, WI, US. Methods: We removed R. cathartica from half of 16 invaded plots. Using three 17-d sessions over two seasons, we quantified seed prédation on R. cathartica and four native species: Acer ruhrum, A. saccharum, Prunus serótina and Quercus rubra. Studies included autumn sampling to evaluate whether extended phenology of R. cathartica led to changes in seed prédation. We quantified habitat characteristics and small mammal activity. Results: Rhamnus cathartica removal led to decreased seed removal for A. rubrum, A. saccharum and R. cathartica, but magnitude depended upon season. For R. cathartica and A. rubrum, autumn seed removal was higher in R. cathartica invaded areas, when these areas have substantial leaf cover. Predation of A. saccharum seeds was highest in areas with intact R. cathartica during the summer. R. cathartica removal led to significantly increased light levels at the forest floor, increased leaf litter depth and lower total small mammal captures. Conclusions: Presence of R. cathartica altered seed removal of native and invasive species, and the unique phenology of R. cathartica provided cover that generated novel, previously undocumented temporal shifts in consumer pressure. Although our work illustrates how mature R. cathartica can increase seed removal, we conclude that apparent competition between seeds of R. cathartica and native species is unlikely because higher seed removal caused by R. cathartica was seen for R. cathartica seeds. Our work highlights how predicting and ameliorating effects of invasive plants may require considering temporal dynamics that are currently poorly understood.