Litter quality predicts ground beetle communities more than tree diversity across eastern temperate forests

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Janey R. Lienau and Meghan Midgley, Center for Tree Science, The Morton Arboretum, Lisle, IL, Robert W. Buchkowski, Department of Biology, Western University, London, ON, Canada

Background/Question/Methods
Human life relies on insects to maintain our ecosystems through complex food web interactions. However, environmental changes are resulting in rapid insect diversity loss. Given that fine-scale litter properties often better predict decomposition dynamics than large-scale climate properties, characterizing the relationships between litter properties and soil insect communities may be key to mitigating the effects of environmental change. While high litter diversity and density may foster high insect diversity and density, litter quality may also shape insect communities. We explored relationships between soil insect communities and forest tree communities using data from four National Ecological Observatory Network (NEON) sites and ground beetles (Carabidae) as an indicator family. We used this data to address our leading question: Do litter quality, quantity, and diversity drive insect community composition, diversity, and density patterns in forest systems? We selected four temperate NEON forested sites that contained representative evergreen, mixed, and deciduous 40 m × 40 m sampling plots from which ground beetle and tree vegetation data was collected since 2013. Using a series of linear mixed effects models and PERMANOVAs, we evaluated the effects of forest stand properties on ground beetle diversity, density, and community composition.
Results/Conclusions
Using a total of 7331 collected ground beetles from 41 plots and tree stand metrics as indices of litter diversity, density, and quality, we found that the relative abundance of evergreen trees better predicted beetle diversity and density than tree stand diversity and density. Specifically, evergreen plots (as defined by NLCD classifications) hosted less dense beetle communities than deciduous forests (P =0.021). Similarly, ground beetle Shannon diversity (P =0.068) and beetle density (P=0.004) decreased as the relative abundance of evergreen trees increased, while tree diversity had no significant effects on beetle diversity (P=0.68), and tree density had no significant effects on beetle density (P=0.93). Finally, we observed shifts in ground beetle community composition as the relative abundance of evergreen trees increased (P=0.04). Although further study is needed to explicitly link litter quality with soil insect communities, resource quality drives variation in ground beetle diversity and density. More than tree diversity or density, litter quality mediates observed variation in soil insect communities across eastern temperate forests. The expanding evergreen forest range due to climate change may have a drastic effect on soil insect communities and the services they provide for human life.