Seasonality of Carabid Beetles on an Organic Agricultural Field and Its Effect on Foraging Use

Ground beetle species from marginal areas invade organically farmed fields in a higher abundance and species richness than conventionally farmed fields. Seasonal invasion into organic fields was studied at Ritzerau Manor, converted to organic farming 18 years ago. Carabid species were explored with 123 pitfall traps within the field and in marginal near-natural habitats over the 18 years after conversion. For 56 species, seasonality could be studied in a distance gradient from the field margin to the field center. The results revealed that ground beetles from marginal habitats can use the fields differently depending on their seasonal activity. Early and fast-moving species can reach the center of the field at a 240 m distance from margin; late and slowly moving species only reach the 120 to 60 m distance level. The foraging effect of species, thus, depends on the seasonality and duration of activity. Overall, marginal species make up to 35% of the total foraging of ground beetles. Thus, organic farming not only supports a closer interaction between farmland and the adjacent near-natural landscape, but also benefits from higher biological pest control by immigrating marginal species.