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

Abstract

Invasive insects have been shown to 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 1) whether emerald ash borer (Agrilus planipennis; EAB) induced ash (Fraxinus spp.) mortality facilitates European buckthorn (Rhamnus cathartica)—an invasive shade-tolerant shrub; 2) 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.

Positive association between emerald ash borer residence time and accumulation of invasive plants

“…Despite the lack of a clear signal between EAB residence time and forest canopy metrics, we nonetheless detected an effect of EAB and its primary host, ash, on the accumulation and spread of invasive plants—evidenced by both our regression model and randomized plot comparisons. This finding is largely consistent with other work at smaller scales that has shown EAB and other invasive pests, can enhance the spread, coverage, and growth of invasive plants (Baron & Rubin, 2021; Eschtruth et al, 2011; Hoven et al, 2017). The lack of association between EAB and forest structure metrics, discussed above, complicates the interpretation somewhat, but could indicate that EAB can facilitate the spread of invasive plants even without large systematic shifts in overall forest structure, perhaps due to the low ash abundance.…”

“…invasive species accelerates the accumulation and impact of other invasive species) are poorly documented (Simberloff, 2006), growing evidence suggests pest-induced tree mortality can increase the growth, abundance, and number of non-native plants (e.g., Baron & Rubin, 2021). However, it remains uncertain whether these effects are strong enough to be evident at broad spatial scales given the constraints of other landscape features affecting non-native plant accumulation.…”

“…Mortality and damage caused by non‐native pests can also facilitate the spread of non‐native plants by opening forest canopy gaps and promoting environmental conditions conducive to non‐native plant establishment (Gandhi & Herms, 2010). While pest‐induced “invasion meltdowns” (whereby the occurrence of an invasive species accelerates the accumulation and impact of other invasive species) are poorly documented (Simberloff, 2006), growing evidence suggests pest‐induced tree mortality can increase the growth, abundance, and number of non‐native plants (e.g., Baron & Rubin, 2021). However, it remains uncertain whether these effects are strong enough to be evident at broad spatial scales given the constraints of other landscape features affecting non‐native plant accumulation.…”