Indirect Effects of Emerald Ash Borer-Induced Ash Mortality and Canopy Gap Formation on Epigaeic Beetles

Abstract

Exotic herbivorous insects have drastically and irreversibly altered forest structure and composition of North American forests. For example, emerald ash borer (Agrilus planipennis Fairmaire) from Asia has caused wide-scale mortality of ash trees (Fraxinus spp.) in eastern United States and Canada. We studied the effects of forest changes resulting from emerald ash borer invasion on epigaeic or ground beetles (Coleoptera: Carabidae) along a gradient of ash dieback and gap sizes in southeastern Michigan. Ground beetles were sampled in hydric, mesic, and xeric habitats in which black (Fraxinus nigra Marshall), green (Fraxinus pennsylvanica Marshall), and white (Fraxinus americana L.) ash were the most common species, respectively. During 2006-2007, we trapped 2,545 adult ground beetles comprising 52 species. There was a negative correlation between percent ash tree mortality in 2006 and catches of all beetles. Catches of Agonum melanarium Dejean (in 2006) and Pterostichus mutus (Say) (in 2006-2007) were negatively correlated with tree mortality and gap size, respectively. However, catches of Pterostichus corvinus Dejean were positively correlated with gap size in 2006. As ash mortality and average gap size increased from 2006 to 2007, catches of all beetles as well as P. mutus and Pterostichus stygicus (Say) increased (1.3-3.9 times), while species diversity decreased, especially in mesic and xeric stands. Cluster analysis revealed that beetle assemblages in hydric and mesic stand diverged (25 and 40%, respectively) in their composition from 2006 to 2007, and that hydric stands had the most unique beetle assemblages. Overall, epigaeic beetle assemblages were altered in ash stands impacted by emerald ash borer; however, these impacts may dissipate as canopy gaps close.

Review of Ecosystem Level Impacts of Emerald Ash Borer on Black Ash Wetlands: What Does the Future Hold?

“…One of the only published studies on the impacts of EAB on wildlife focused on the indirect effects of EAB-induced mortality to ground beetle community assemblages (Coleoptera: Carabidae) in infected ash stands in Michigan [37]. The results of this study indicated that species diversity decreased in ash stands in response to tree mortality and increasing canopy gap size [37].…”

“…The results of this study indicated that species diversity decreased in ash stands in response to tree mortality and increasing canopy gap size [37]. Hydric black ash wetlands had the most unique assemblage of ground beetles, with numerous beetle species only found in the hydric moisture category, but also had moderate turnover (25%) of species from 2006-2007 [37].…”

Responses of Ground-Dwelling Invertebrates to Gap Formation and Accumulation of Woody Debris from Invasive Species, Wind, and Salvage Logging

“…The speed, synchrony, and specificity of ash mortality differs from gap-phase dynamics caused by other forest insects. EAB-induced ash mortality increases both the frequency of canopy gaps as ash trees die ( Figure 1A) and the accumulation of ash CWD as trees fall ( Figure 1B) [78,89]. Our model predicts that this pattern of tree mortality has an inverse temporal relationship in the effect sizes of canopy gaps and accumulation of ash CWD on ground-dwelling invertebrate communities (Table 1), as the effects of canopy gaps diminish with canopy closure, while the effects of ash CWD are predicted to increase and change over time as trees fall and decompose [60].…”

“…During early stages of ash mortality, decreased invertebrate richness and diversity was reported in canopy gaps, owing to the decreased activity-abundances of harvestmen, scarab beetles, camel crickets, and springtails [60]. Activity-abundance of ground beetles initially was lower in canopy gaps, but these effects were ephemeral, suggesting some degree of resilience to small-scale canopy gaps [53,89]. During late stages of ash mortality, higher densities of ground-dwelling invertebrates were observed near ash CWD, including earthworms, spiders, harvestmen, isopods, millipedes, beetles, and springtails [78].…”

“…Based on this conceptual model, canopy gaps are predicted to have their greatest impact on ground-dwelling invertebrates soon after they form, with these effects dissipating over time as growth from suppressed understory trees and surrounding canopy trees close the gaps. Gaps are presumably at their maximum size soon after tree death, and gaps created by EAB-induced tree mortality averaged 18.8% (±1.8) to 26.5% (±2.0) canopy openness [53,89]. Growth from understory and canopy trees closed the gaps, decreasing their canopy openness to 1-10% during late stages of ash mortality [78,90].…”