Invasive plants can potentially impact multiple assemblages and ecosystem services, especially when the species establishes homogeneous coverage over large areas. One particularly successful invasive shrub, Berberis thunbergii, is spreading rapidly throughout North America and growing into recalcitrant layers in deciduous forest understories. Thick patches of B. thunbergii can greatly reduce light penetration and fundamentally alter soil chemical properties. Consequently, native flora and fauna may be significantly impacted when the species spreads. We quantified associations with B. thunbergii patches in western Pennsylvania forests by surveying flora, terrestrial snails, and small mammals in forests where the species is currently spreading. Additionally, we experimentally removed patches of B. thunbergii to test how small mammal activity and flora responded in immediate aftermath. Our hypotheses included the following: 1) native flora would be negatively impacted by B. thunbergii and respond favorably to control, 2) small mammals use B. thunbergii as shelter and exhibit elevated densities and foraging behavior, which would be reduced if controlled, and 3) native snails would be relatively reduced under B. thunbergii patches due to adverse effects on the soil.
Results/Conclusions
Results show idiosyncratic associations with B. thunbergii among assemblages. As expected, native flora is less biodiverse in invaded patches, particularly with respect to native tree seedlings. Removing B. thunbergii increased the density of other flora in the first growing season following removal, but the majority of taxa that benefitted from removal were other nonnative species. In contrast to our expectations, native snails were significantly more biodiverse and abundant under B. thunbergii patches. Such findings may reflect more favorable habitat for snails created by the recalcitrant layer with respect to temperature, humidity, and soil pH. Small mammals appear to be associated with B. thunbergii patches in the manner that we hypothesized: catch rates in Sherman traps were elevated within patches and giving up densities (as quantified by seed trays) were dramatically reduced when B. thunbergii was removed. Thus, the nature of how an assemblage responds to B. thunbergii invasion appears to depend on how the relevant habitat is affected. Although assemblages such as terrestrial snails may positively associate with B. thunbergii, our findings include an alarming reduction in native tree recruitment that could be associated with elevated small mammal abundance and activity. Consequently, B. thunbergii management to reduce the extent of invasion should be a prioritized in managed forests where it is invading.