Distance-dependent distribution patterns of ground beetles from non-crop habitats: Contrasting ranges of seed predation intensity and activity-density in grain fields

Non-crop habitats can serve as reservoirs for beneficial ground beetles (Coleoptera: Carabidae), potentially shaping their distribution and community compositions in adjacent croplands. These edge-related changes in carabid communities can influence multiple ecosystem services, including weed seed predation. The extent to which proximity to these habitats shapes carabid distributions and seed predation services within large fields and prairie ecosystems remains relatively understudied. Within the carabid communities, the foraging and movement response of smaller granivores to larger omnivores may further influence edge-related distribution patterns and weed seed predation services. In this study, we deployed seed cards with pitfall traps at sampling stations across 10 fields to investigate these relationships in Alberta, Canada. We found edge effect over a long-distance gradient for seed predation intensity, and for omnivorous carabids, but not for granivorous species. The activity-density of omnivorous carabids stayed elevated across an intermediate zone near non-crop habitats and declined only at greater distance, while seed predation intensity decreased sharply beyond the field edge. Moreover, omnivore and granivore activity-densities were positively associated, especially in field interiors and later in the growing season. Our results indicate that omnivorous carabid activity-density was higher near existing field edges and declined only at greater distances into field interiors. In contrast, seed predation intensity declined sharply within the first 41 m from the edge, suggesting that the spatial extent of measured seed predation was more restricted than the spatial pattern of omnivorous carabid activity-density, potentially because seed consumption depends not only on carabid occurrence, but also on other seed predators, local foraging behaviour, seed accessibility, and the availability of alternative food. Further, it emphasizes the importance of reducing effective field size and improving the spatial configuration of field-edge or within-field habitat features to help extend weed seed biocontrol across large crop fields.