Exploring trophic effects of spotted knapweed (Centaurea stoebe L.) on arthropod diversity using DNA metabarcoding

Abstract Biological invasions are of particular concern in grasslands, as these systems are highly susceptible to changes in ecosystem energy flows following invasions by exotic plants. Spotted knapweed (Centaurea stoebe L.), a Eurasian, perennial forb, is considered one of the most ecologically harmful invasive plants in North American grasslands and may lead to changes in trophic dynamics, particularly within arthropod communities, which depend on plants for food and habitat. Using DNA metabarcoding to assess community dynamics of arthropods collected from pitfall traps and sweep nets, we explored the effects of C. stoebe density on the alpha, beta and functional diversity of arthropods in a semi-arid grassland in British Columbia, Canada. We used trait-based approaches to investigate the functional responses of terrestrial arthropod communities to better understand the effects of C. stoebe on trophic dynamics. Our study found seasonal differences in the beta-diversity of arthropods, but no differences in arthropod alpha-diversity in knapweed populations. However, our study found a significant reduction in detritivore relative abundance coupled with increases in predator relative abundance, indicating that knapweed density altered the detrital food web. Conversely, herbivores were unaffected by knapweed density, suggesting evidence for greater stability of the grazing food web. Further, predator:prey ratios were highest under high knapweed density. These ratios suggest that top-down effects are likely stronger than the bottom-up effects of C. stoebe invasion. DNA metabarcoding provides the tools to develop detailed surveys of species diversity across a range of environments and trophic levels, which could be a useful guide for planning restoration.