Making sense of a prairie butterfly paradox: The effects of grazing, time since fire, and sampling period on regal fritillary abundance

Although grasslands often require periodic disturbance to prevent woody plant invasion, some grassland-obligate butterfly species respond negatively to disturbance agents such as fire and grazing. We examined the effects of time since fire, grazing and sampling period on abundance of a grassland-obligate butterfly, the regal fritillary (Speyeria idalia), at four grassland sites in Missouri, USA. Each site consisted of two pastures: one managed with patch-burn grazing (rotational burning plus cattle), and one managed with rotational burning but lacking cattle. Population density was assessed via line transect distance sampling in three periods of 2006 and 2007. Floral resource availability was measured in 2007. Grazing and time since fire interacted in their effects on regal fritillary density in 2006, as grazing reduced density in patches burned earlier that year but not in patches burned 1 or 2 years prior. In 2007, regal fritillary density was a function of a three-way interaction. In recently burned patches, grazing reduced regal fritillary density and flowering stem density of two preferred nectar sources in early June and late July. In ungrazed pastures, recent fire increased density of regal fritillary and the same preferred nectar sources. Densities of regal fritillary and Echinacea pallida were positively correlated in early June, as were densities of regal fritillary and Liatris pycnostachya in late July, suggesting that regal fritillary adults recolonized recently burned prairie to use those nectar sources. Our research illustrates that prescribed fire can be compatible with grassland butterfly conservation.