Divergent responses of butterflies and bees to burning and grazing management in tallgrass prairies

Abstract

Butterflies and bees contribute significantly to grassland biodiversity and play important roles as pollinators and herbivores. Grassland conservation and management must be seen through the lens of insect conservation and management if these species are to thrive. In North America, grasslands are a product of climate and natural disturbances such as fire and grazing. These natural disturbances have changed considerably since European colonization and subsequent landscape fragmentation. The aim of this study was to better understand the impacts of fire and grazing management on butterfly and bee communities in tallgrass prairie, enabling land managers and conservationists to better protect and manage remnant prairie. We examined butterfly and bee abundance, species richness, and diversity in Minnesota tallgrass prairies managed by grazing or fire. In 2016 and 2017, we surveyed butterflies, bees, vegetation, and surrounding land use at 20 remnant prairies (10 burned and 10 grazed) with known management histories. Butterfly and bee abundance at our study sites were significantly negatively correlated. Butterfly abundance, but not species richness, was higher in burned than grazed prairies, and prairie‐associated grass‐feeding butterflies were more abundant at sites with higher plant species richness. Bee abundance was unrelated to management type but was higher at sites with sandier soils; bee species richness was positively associated with forb frequency. These findings highlight the challenges of designing management plans tailored to wide groups of pollinators and the potential pitfalls of using one group of pollinators as indicators for another. They also point to the importance of a mosaic of management practices across the prairie landscape.

Prescribed fire increases the number of ground‐nesting bee nests in tallgrass prairie remnants

“…While Buckles and Harmon‐Threatt (2019) found a negative association between grazing and bee nesting, the two sites in our study with a history of grazing had the highest numbers of active nests in both burned and unburned areas. Thus, more work is needed to document whether prescribed burning confers fitness benefits to the ground‐nesting bee community and how other management techniques like grazing or haying may influence nest site choices (Leone et al, 2022).…”

“…Many studies across a diversity of ecosystems have found positive effects of prescribed fire on bee abundance and diversity (Brown et al, 2017; Carbone et al, 2019; Mason et al, 2021; Smith DiCarlo et al, 2019; Ulyshen et al, 2021; Williams et al, 2010, but see Griffin et al, 2021 and Leone et al, 2022). The underlying mechanisms for bees' positive responses to fire are generally considered to be altered flowering phenology and floral density, with fire leading to higher floral densities (Mola & Williams, 2018; Ponisio, 2020).…”

Fewer bowl traps and more hand netting can increase effective number of bee species and reduce excessive captures

“…Data for the wheatgrass sites can be found at Larson et al (2018) and Larson and Larson (2023) and for the tallgrass sites at Leone et al (2022a) Summaries of the data used in the analyses can be found in Tables A1 and A2. Code used for the analyses is in Supporting Information.…”

“…Both bowl traps and hand netting are often used in either inventory (Meiners et al., 2019 ) or hypothesis‐driven research (Lasway et al., 2022 ; Leone et al., 2022b ; Watson et al., 2011 ) because each method will capture somewhat different species. Common species have been found to be equally well represented by both methods, but rarer or more specialized species may be best captured by only one of the methods (Grundel, Frohnapple, et al., 2011 ).…”

Fire regime shapes butterfly communities through changes in nectar resources in an Australian tropical savanna

“…To our knowledge, although many studies have examined the effects of fire on butterfly communities in other grassy biomes such as North American prairie (e.g., Leone et al, 2022; Moranz et al, 2012; Swengel, 1996; Vogel et al, 2010), only one published study has examined the effects of fire on butterfly communities in tropical savanna: in an African mesic savanna, Gaget et al (2020) found that butterfly abundance and species composition varied with fire frequency. However, the mechanisms behind these responses were not identified.…”