Effects of riparian and grassland habitats on ground beetle (Coleoptera : Carabidae) assemblages in adjacent wheat fields

Natural habitats surrounding agricultural fields provide a source of natura l enemies to assist in pest control. The boundaries among landscape: elemen ts filter some organisms attempting to cross them, resulting in differing c ommunities within the landscape elements, Ground beetles are numerous and g enerally disperse by walking. These dualities make them excellent organisms for the study of boundary dynamics. Our goal was to determine if natural h abitats adjacent to wheat fields affected the species composition of ground beetles within the wheat fields. We captured ground beetles from autumn th rough spring 1996-1997 at two sites using directional pitfall traps placed in wheat fields and adjacent grasslands and riparian zones. Ground beetle a bundance reached two peaks, one in autumn and the other in spring. Species composition was most strongly related to these seasons. Axis 1 of a canonic al correspondence analysis separated spring active beetles front autumn act ive beetles. Axis 2 separated winter active beetles. With the effects of se ason and sites removed, axes 1 and 2 of a partial canonical correspondence analysis separated beetles with respect to habitat. Axis 1 separated beetle s into wheat and natural habitat assemblages. Axis 2 further distinguished assemblages in wheat fields as those adjacent to grasslands and those adjac ent to riparian habitats. Axis 2 also separated grassland, grassland edge, and riparian edge assemblages from riparian assemblages. Net dispersal of b eetles across the boundaries showed no consistent pattern during autumn, wi nter, ol spring. However, mark-recapture studies showed that several specie s routinely cross boundaries, which resulted in different community structu res and an increase in abundance of beetles in the wheat interiors during s pring.