The spatio-temporal distribution of weed seed predation differs between conservation agriculture and conventional tillage

One potentially important ecosystem service in agricultural fields is the regulation of weeds by carabid beetles, but the effect of agricultural management on the level of regulation has so far been poorly documented. In this study, we monitored weed seed predation rates of Viola arvensis and Capsella bursa-pastoris and carabids from March to September using a grid sampling in two adjacent winter-wheat fields, one in conventional tillage (T) and the other converted to direct drilling with cover-crop for five years (DD). At the field level, weed seed predation was positively correlated to the activity of granivores in the tilled field and was marginally higher in DD than in T during wheat growth. After harvest, granivores and predation rates declined sharply in the cover crop of the DD system whereas they increased in the bare tilled field. This result suggests that the dense cover crop set up after harvest was not suitable for the local pool of autumn-breeding granivores. Spatial aggregations of carabid and predation variables were detected using the MAPCOMP software; these spatial patterns differed between the two management systems and were temporally unstable but consistent over large intervals of spatial resolutions. There were significant spatial associations between trophic guilds and predation rates, i.e. predation rates were positively or negatively associated either to granivores or to omnivores, depending on the time and on the management system. These results highlight the spatial and temporal heterogeneity in the level of interactions taking place during a crop cycle, a complexity which explains the important variability in the delivery of ecosystem services such as weed seed predation.