The impact of exotic shrub invasions on the trophic composition of deciduous forest litter communities

Background/Question/Methods

Biological invasions may impact the multi-trophic interactions of ecosystems. Exotic species invading deciduous forest can change leaf litter inputs, which can impact detrital food webs. Microbial decomposer communities may differ between exotic and native leaves due to differences in physical and biochemical properties. Effects on this foundational trophic level could have cascading effects the food web. This study investigated the effects of invasive shrubs (Lonicera maackii and Rhamnus davurica) and an invasive tree (Ailanthus altissima) on the litter-dwelling communities of a hardwood forest in Virginia. We placed sets of litter bags containing native, invasive, or a mixture of leaves in five locations in each of the invaded or native habitats. After four months, we retrieved sets of litter bags from each habitat at 10-day intervals. Samples (1 g) were taken from each bag for microbial and fungal abundance using acridine orange direct counting. We placed the remaining litter in Berlese-Tullgren funnels for one week to collect the arthropods. To test for differences in the arthropod communities, we identified each organism to the lowest taxon to categorize trophic function. We also calculated total abundance, richness, and diversity of arthropods found in each habitat and litter mixture.

Results/Conclusions

We found significantly more bacteria in litter from all three invasives compared to native, but no differences among native and invaded habitats. Lonicera litter supported significantly more fungi than native litter, but there were no other significant effects of litter or habitat. Native habitat contained marginally significantly greater arthropod abundance than Lonicera, but no other differences were found among other habitats or litter. Simpson’s diversity of arthropods did not differ significantly between native and invasive habitats or litter. However, removing ants from the analysis revealed significantly greater diversity in native compared to Rhamnus habitat and marginally significantly greater diversity in native litter than Rhamnus. Total richness was not significantly different for any habitat or litter mixture. The richness of five trophic groups (predators, detritovores, herbivores, fungivores, and omnivores) was significantly greater in native compared to Ailanthus and Lonicera habitat in a MANOVA. In native habitat a MANOVA revealed that Lonicera and Ailanthus litter had significantly greater trophic group richness relative to native. The five trophic groups showed significantly greater abundance in native compared to both Lonicera and Rhamnus litter. These results show that by altering the nutrient base exotic plant invasions can impact the multi-trophic interactions in a forest ecosystem.