Relative importance of vertebrates and invertebrates in epigeaic weed seed predation in organic cereal fields

Exclosure trials were conducted in four organic cereal fields in The Netherlands in 1999 and 2000 to determine the relative importance of vertebrates and invertebrates in weed seed predation. The trials showed that seed predation by vertebrates was rather consistent and predictable, occurring on all fields and both years, being low early in the season, increasing towards mid-June and decreasing thereafter. The occurrence and level of seed predation by invertebrates were unreliable and unpredictable over time. Predation by both vertebrates and invertebrates showed no apparent pattern related to field margin. Vertebrates, presumably mice, accounted for the larger part of weed seed consumption (30–88% per fortnight and farm). Invertebrates, probably granivorous ground beetles, accounted for the smaller part of seed consumption (4–38%). They were the dominant seed predators in only one out of eight cases in July 1999 (74%), and overall contributed little to variability in seed predation.