Effects of native megaherbivore Bison bison grazing and wallowing activities on lepidopteran species and community dynamics in a complex savanna-barrens grassland over a 6-year study

Evaluating land management practices to best return landuse health and resilience is a growing interest in the face of rapidly declining worldwide invertebrate populations. American bison (Bison bison), a native megaherbivore, is well understood to benefit the natural cycle of vegetation growth and soil on the landscape. Activities of these native grazers has also been found to benefit specifically lepidopteran populations, but additional evaluation is needed. To assess this, we conducted lepidopteran and groundcover surveys within an oak savanna landscape located at the Sandhill Wildlife Area from 2014-2019, during spring and summer seasons. Bison grazing pastureland was delineated into four cover types including open grassland areas (other), shrubs and trees (shrubs), areas of exposed mineral soil (disturbance), and wallows resulting from disturbance as generated directly by American bison wallowing activities (wallows). A total of 60 plots (15 per cover type) were assessed for percent groundcover categories, recent presence of bison (indicated through bison chip counts), and observations of all lepidopteran species. Bison wallowing activities increased over the course of the study, with more wallows visible during spring season. Bison chips were highly separated from wallows, suggesting that bison graze and wallow in different areas. Lepidopteran family groups (Lycaenidae, Nymphalidae, Pieridae, Hesperiidae, Papilionidae, and Riodinidae, as well as grouped "moths" that were dominated by Geometridae) differed significantly across these areas of high and low disturbance. A total of 17 groups were observed, 13 of which were seen at least 3 times over the 10 surveys. Environmental predictors bison wallows, forbs, grass, nectar, and shrub cover were significantly associated with distribution of individual taxa. Bison wallows were significantly associated with lepidopteran family communities, while chips shaped trends in 6 groups identified to species, and vegetation types further significantly structured the community. Temporal trends captured significantly different community assemblages 2014-2019 and across seasons, with summer showing greater variation in families. Overall, this suggests that bison activities and resulting changes in groundcover impact habitat suitability and thus influence the abundance and distribution of lepidopteran family groups. A further assessment of bison influenced habitat features and breakdown of lepidopteran species units is needed to further understand how temporal bison activities may influence the habitat structure and subsequently the reliant lepidopterans within this ecotype and will be assessed in subsequent studies.