Northeastern forests are experiencing a constantly rising level of invasion by nonnative species, which is associated with the increased probability of positive interactions within the nonnative component of the biota and subsequent negative impacts on forest ecosystems. Facilitation of nonnative plants by nonnative insects has been repeatedly hypothesized, yet, it has not been explicitly tested. This study aimed at understanding and comparing the patterns and assessing the intensities of two distinct types of insect-plant interactions: (1) between a nonnative insect and nonnative plants; and (2) between a native insect and nonnative plants in northeastern forests. The study system comprised the invasive nonnative defoliator, gypsy moth (Lymantria dispar), the native defoliator, forest tent caterpillar moth (Malacosoma disstria), and nonnative plant species. We coupled insect disturbance data available through the Forest Health Monitoring Program of the USDA Forest Service with data on nonnative plant species collected within the Forest Inventory and Analysis Program of the USDA Forest Service. The study region included 13 eastern states in United States that have the history of the gypsy moth and/or forest tent caterpillar moth outbreaks. The treatment plots were FIA plots for which (1) both data on defoliation and nonnative plants were available and complete; (2) stand age was ≥ 40 years old; and (3) defoliation event(s) occurred at least one year before the FIA survey was completed. Each treatment plot was paired with a reference plot of the similar forest type and age as a treatment plot. We fitted regression models to data on nonnative plant species presence, abundance, and density. To compare species richness among the treatments, we computed sample-based rarefaction curves for each treatment category.
Results/Conclusions
The site selection analysis resulted in over 400 and 300 treatment-reference plot pairs for the gypsy moth and forest tent caterpillar moth disturbance, respectively. The gypsy moth and forest tent caterpillar moth plots were spatially segregated within the study region, which allowed us to exclude the possibility of competition among the two species and analyze the two datasets independently. The comparison between the native and nonnative insect species with similar biological and ecological characteristics served as a robust test of the hypothesis on interspecific facilitation among nonnative organisms. Further results will be presented at the meeting.