Effects of Riparian and Grassland Habitats on Ground Beetle (Coleoptera: Carabidae) Assemblages in Adjacent Wheat Fields

Natural habitats surrounding agricultural Þelds provide a source of natural enemies to assist in pest control. The boundaries among landscape elements Þlter some organisms attempting to cross them, resulting in differing communities within the landscape elements. Ground beetles are numerous and generally disperse by walking. These qualities make them excellent organisms for the study of boundary dynamics. Our goal was to determine if natural habitats adjacent to wheat Þelds affected the species composition of ground beetles within the wheat Þelds. We captured ground beetles from autumn through spring 1996Ð1997 at two sites using directional pitfall traps placed in wheat Þelds and adjacent grasslands and riparian zones. Ground beetle abundance reached two peaks, one in autumn and the other in spring. Species composition was most strongly related to these seasons. Axis 1 of a canonical correspondence analysis separated spring active beetles from autumn active beetles. Axis 2 separated winter active beetles. With the effects of season and sites removed, axes 1 and 2 of a partial canonical correspondence analysis separated beetles with respect to habitat. Axis 1 separated beetles into wheat and natural habitat assemblages. Axis 2 further distinguished assemblages in wheat Þelds as those adjacent to grasslands and those adjacent to riparian habitats. Axis 2 also separated grassland, grassland edge, and riparian edge assemblages from riparian assemblages. Net dispersal of beetles across the boundaries showed no consistent pattern during autumn, winter, or spring. However, markÐrecapture studies showed that several species routinely cross boundaries, which resulted in different community structures and an increase in abundance of beetles in the wheat interiors during spring.