Suppression of weed communities by granivores over time in an agroecosystem

Invertebrate granivore communities can consume numerous weed seeds in cropland, but how this granivory influences weed recruitment over time in continuous no-till systems is unknown. Weed and surface-active granivore communities were determined in soybeans (Glycine max) over 3 years in eastern South Dakota. We examined seed removal rates of sentinel redroot pigweed, green foxtail, and lambsquarters (Chenopodium album) seeds and applied gut content analysis of granivores to determine which species consume immuno-marked seeds of green foxtail (Setaria viridis). Weed stands with low diversity and high biomass production were associated with granivore community complexity. In turn, granivore community complexity was associated with weed communities with low diversity and high biomass in the subsequent growing season. Fall seed consumption, as measured by seed removal rates and gut content analysis of foxtail seed markers, was positively correlated with weed recruitment in the subsequent growing season. Seed specialists behaved differently toward focal weed species than the granivore community at large. Gryllus pennsylvanicus abundance was correlated with higher green foxtail consumption and removal rates. None of the other dominant green foxtail consumers revealed a similar set of responses. We propose a feedback mechanism whereby granivore community complexity in cropland leads to larger but fewer weeds over multiple years. Weed communities with these characteristics then lead to more complex granivore communities. If these relationships persist over time with minimal disturbance in continuous no-till cropping systems without pesticides, this feedback loop could reduce weed cover.