American bison influences on lepidopteran and wild blue lupine distribution in an oak savanna landscape

The natural influences of American bison have been virtually eliminated from the North American landscape, making it difficult to study native grassland ecosystems. The Sandhill Wildlife Area provides a unique opportunity to study this once-natural phenomenon in an oak savanna landscape. This area is home to the largest known population of the federally endangered Karner blue butterfly. We evaluated lepidopteran and lupine distribution relative to cover types and generated statistical models to predict occupancy and abundance relative to in situ groundcover variables as well as American bison related habitat variables. Overall variance of lepidopteran populations was best explained by increasing nectar and forb cover (29.4 % occurrence explained), while lepidopterans excluding Karner blues were further associated with decreasing cover of shrubs/trees (25.8 % occurrence, 59.8 % abundance explained) and were more likely to occur in areas of disturbance cover type. Karner blue occurrence was best explained (41.6 %) by increasing nectar and forb cover and an increasing number of bison chips (indicative of the recent presence of bison). Karner blue females, specifically, were more likely to be present in areas of wallow cover type and were best explained by increasing nectar plant cover (33.4 % occurrence, 60.1 % abundance explained) and increasing shrub cover. Females were also associated with an increasing number of bison chips, number of wallows and total size of wallows. Karner blue male occupancy were best explained (40.7 %) by increasing nectar cover and increasing number of bison chips. Lupine was significantly less likely to occur in areas of shrubs/trees cover. Lupine was explained (55.7 % occupancy and 88.2 % abundance), by the decreasing cover of shrubs/trees, and by the cover of nectar, forbs, and grass, and total size of wallows. We conclude that the activities caused by bison reduce the overall woody growth and produce improved habitat for nectar plants and especially for disturbance-dependent lupine, and subsequently enhance habitat for lepidopterans. Karner blues were significantly related to bison activities, suggesting that improved management techniques should consider mimicking megaherbivore activities through increased disturbance frequency and increase mineral-soil disturbance.