The influences of patch shape and boundary contrast on insect response to fragmentation in California grasslands

Abstract

Landscape ecologists typically identify boundaries to demarcate habitatpatches. The boundary between two habitat types may be abrupt, such as thetransition between a grassland and a parking lot, or more gradual, such as theshift between successional forest stages. Two key aspects of landscapeboundaries, their shape and contrast, are predicted to influence movement ofmaterials, plants, and animals. Ecological theory suggests that a patch’sperimeter-to-area ratio should strongly influence animal emigration when patchboundaries are relatively permeable, but not when boundaries are more severe.Weinvestigated the interactive effects of patch shape and boundary contrast onmovement of ground-dwelling beetles (Carabidae and Tenebrionidae) in nativegrassland habitat at Jepson Prairie, Solano County, California, USA. Weconducted a field experiment with two patch shape treatments, square andrectangle, that held patch area constant, and two boundary contrast treatmentscreated by mowing grass surrounding each plot at two different heights. Wemonitored the number of beetles leaving each patch over a three-week periodfollowing treatment establishment. We observed a significant effect of boundarycontrast on net movement of beetles, with low contrast boundaries exhibitingnetimmigration and high contrast boundaries experiencing net emigration. Moreover,the importance of patch shape appeared to be greater for low contrast versushigh contrast boundaries, consistent with theoretical expectations. Ourcombinedobservations indicate that these ground-dwelling beetles were more likely tomove into patches that were rectangular and surrounded by a low contrast matrixthan patches that were square or surrounded by a high contrast matrix. Weconclude that net movement of beetles across patch boundaries is stronglyinfluenced by boundary contrast and may be affected by patch shape whenboundarycontrast is low.