Annual losses of weed seeds due to predation in organic cereal fields.
Post-dispersal seed losses in annual arable weed species are poorly quantified, but may be of significance for natural population control, especially if they can be manipulated. We hypothesized that weed seed predation on the soil surface was significant, so we measured rates in the field to estimate annual seed losses due to predation. Temporal patterns of weed seed losses due to predation ('demand') as well as weed seed production ('supply') were measured from May to June until harvest in August 1999 and 2000 during 2-week exposure periods in four organic cereal fields in the Netherlands. The proportion of weed seeds lost to predators Mi (number of seeds consumed per number of seeds exposed per 14 days) was measured, using cards containing seeds of Stellaria media, Chenopodium album or Avena fatua. Seed production, Yi (number of seeds per m2 per 14 days), was measured in 2000, using seed traps. Annual seed loss due to predation, M (number of seeds consumed per number of seeds produced per year), was calculated based on Mi and the exposure period of seeds to predators, starting with seed shed and ending with seed burial. The importance of the length of the exposure period on total seed loss was explored using a model. The temporal trend in Mi was consistent among farms and years: high in June and early July, lower in the second half of July and negligible in August and after harvest. Total seed production varied considerably among fields, i.e. 800-16 000 seeds per m2 per year. The timing of peak seed production also varied substantially. Calculated M ranged from 32% to 70%, when assuming continuous exposure of seeds to predators from seed shed till crop harvest. When exposure was limited to 2 or 4 weeks after seed shed, M decreased to 18-57% or 28-67%, respectively. Differences between fields and weed species were mainly due to differences in the timing of seed shed. Synthesis and applications. Our results suggest that seed predation in organic cereal fields is an important factor shaping the population dynamics of arable weeds. A combination of environmental conditions (hot and dry weather) and agricultural practices (an early crop sowing) can advance weed phenology and postpone seed burial, resulting in higher proportions of weed seed loss to predation in cereals.