Reduced herbivory on exotic buckthorn (Rhamnus cathartica) compared to native tree species in an Illinois woodland

European buckthorn (Rhamnus cathartica), an ornamental and fencerow shrub native to Eurasia, is a prolific invader throughout woodlands of the northeastern and midwestern United States and southern Canada. The spread of R. cathartica may result in the replacement of native species and the alteration of ecosystem processes. We present evidence of a potential mechanism underlying the success of R. cathartica release from herbivore pressures. We compared levels of foliar herbivory between R. cathartica and eight co-occurring native tree species in northeastern Illinois: sugar maple (Acer saccharum), shagbark hickory (Carya ovata), ironwood (Ostrya virginiana), black cherry (Prunus serotina), bur oak (Quercus macrocarpa), red oak (Quercus rubrum), basswood (Tilia americana) and witchhazel (Hamamelis virginiana). Herbivory on native species (ranging from 4.0% to 6.8% of leaf area consumed) was five to nine times higher than on Rhamnus cathartica (0.8%). These results, combined with studies documenting the evident herbivory in the native range of R. cathartica, suggest that exotic North American populations have experienced a release from herbivore pressure.