Secondary invasion? Emerald ash borer (Agrilus planipennis) induced ash (Fraxinus spp.) mortality interacts with ecological integrity to facilitate European buckthorn (Rhamnus cathartica)

Invasive insects facilitate secondary invasive species by altering forest structure and function. Specifically, invasive insect herbivores may promote the establishment and growth of invasive plants by creating canopy gaps. Such secondary invasions may be influenced by ecological integrity - the degree to which ecosystem composition, structure, and function deviate from their natural or historical range of variation. Here we investigate (i) whether emerald ash borer (Agrilus planipennis; EAB) induced ash (Fraxinus spp.) mortality facilitates European buckthorn (Rhamnus cathartica) - an invasive, shade-tolerant shrub, and (ii) the role of ecological integrity in this relationship. We use a principal component analysis (PCA) to calculate an index of ecological integrity and a zero-altered negative binomial generalized linear mixed model (GLMM) to describe European buckthorn occurrence and abundance. European buckthorn occurrence is influenced by canopy gaps, independent of EAB-induced gap formation. Ecological integrity and EAB-induced ash mortality interact to control European buckthorn abundance, with high ecological integrity limiting EAB-facilitated buckthorn invasion. This is the first evidence for EAB-facilitated buckthorn invasion and for an interaction between a secondary invasion and ecological integrity. Thus, ecological integrity plays an important role in the EAB-buckthorn system and may be used to manage the impacts of secondary invasions.