Do dense layers of invasive plants elevate the foraging intensity of small mammals in temperate deciduous forests? A case study from Pennsylvania, USA

Monospecific stands of invasive plants can dramatically restructure habitat for fauna, thereby elevating population densities or promoting foraging of consumer species who benefit in the altered habitat. For example, dense stands of invasive plants may protect small mammals from predators, which in turn could increase foraging pressure on seeds that small mammals feed upon. We used a before-after, control-impact experimental design to test whether small mammal capture rates were higher and giving-up densities (GUDs) lower beneath dense stands of Berberis thunbergii, an invasive shrub with a rapidly expanding range throughout eastern North America. Our experimental design included three plot categories: 1) plots heavily invaded by B. thunbergii, 2) control plots lacking invasive shrub cover, and 3) invaded plots where we eradicated B. thunbergii midway through the study. Although our overall small mammal capture rate was low, small mammal captures were 65% higher in B. thunbergii invaded habitat relative to control plots and eradication lowered capture rates by 77%. GUDs were also 26% higher within B. thunbergii relative to control plots and eradication decreased GUDs by 65%. Our findings suggest that small mammals perceive dense stands of B. thunbergii as relatively safe foraging habitat. Prior surveys within our study locations revealed dramatically depressed tree seedling densities under B. thunbergii, thus invasive plants may promote intensive foraging by small mammals and reduce recruitment for species with foraged seeds or seedlings.