Simplified structure or fewer arthropods to eat? Disentangling the impacts of an invasive plant on breeding bird diversity in agricultural hedgerows

In agricultural landscapes, hedgerows provide critical habitat for songbirds. Himalayan Blackberry (Rubus armeniacus; HBB) is a widespread invasive species in the Pacific Northwest that has been linked to lower breeding songbird diversity. My study explored two possible explanatory mechanisms: reduced structural complexity and lower arthropod abundance as a food source. I conducted avian point counts in 51 hedgerow segments at 2 locations in the Lower Mainland of British Columbia. In these segments, I quantified vegetation structure using a Foliage Height Diversity (FHD) metric derived from LiDAR data. I sampled arthropod abundance on the foliage of woody understory vegetation. I used multiple regression to identify best fit generalized linear models. Songbird diversity decreased with HBB % cover and increased with FHD. However, arthropod abundance was unrelated to bird metrics, and similar between HBB and other native shrubs. This suggests that hedgerows should be managed to control HBB and maximize vegetation structure.