Field density and distribution of weeds are associated with spatial dynamics of omnivorous ground beetles (Coleoptera: Carabidae)

Abstract

Soil seedbanks contribute to development of weed communities in agroecosystems and thus, understanding their regulation contributes to weed management strategies. In this context, the impacts of beneficial seed-consuming arthropods like carabids may contribute to sustainable long-term weed management. We were interested in assessing the spatial relationship between carabid beetles, weeds, and weed seeds in canola (Brassica napus L.), one of the most widely grown crops in Canada. To do this, we collected spatially referenced count data representing the activity-density of seed predatory carabid species and data regarding weed density and weed seed density. We used Spatial Analysis by Distance Indices (SADIE) to explore spatial distributions of carabids in four commercial canola agroecosystems in western Canada. Carabids and weed populations were highly clustered. Moreover, there was significant spatial overlap between activity-density of carabids and patches of high weed density, with an association index, X, ranging between 0.40 and 0.71. Patterns of spatial association also became generally stronger with more advanced crop development. Our results suggest that presence of weedy patches of vegetation in cropped areas contributes to biodiversity by conserving populations of carabids, which also increases rates of weed seed predation.