Many native prairie butterfly species are highly at-risk due to habitat loss from agriculture. In western Washington, less than three percent of original prairie remains, with much of the former prairie grazed by cattle. This study aimed to understand how cattle grazing management strategies influence butterfly behavior and diffusion rates. We conducted behavioral experiments with two species of common native butterflies (silvery blue butterflies and ochre ringlets) at two native prairies with no grazing (high butterfly resource availability); two pastures grazed according to “conventional” management (low resource availability); and two pastures grazed according to “conservation” management (intermediate resource availability). We followed individuals for up to 60 minutes and recorded their behavior and location every 15 seconds. We recorded the nectar and host plant abundance at four randomly chosen points in each flight path. Using the turning angles and length of steps in the flight paths, we calculated diffusion rates for each individual. We assessed the effects of cattle grazing management on butterfly diffusion rates and compared rates using a generalized linear model with management type and sex as explanatory variables. We assessed the effects of path-level habitat characteristics on butterfly diffusion rates using two different a priori generalized linear models. The first model compared the effects of host species encountered along a flight path on butterfly diffusion rates. The second model compared the effects of total nectar, total host volume, and vegetation height on butterfly diffusion rates.
We found that silvery blue butterfly diffusion rates differed by sex and management type. Female silvery blue butterfly diffusion rates were highest in conventional grazing, lowest in native prairie, and intermediate in conservation grazing. Male silvery blue diffusion rates did not differ by management type. We were unable to obtain enough female ochre ringlet observations to run the full analysis, but male ochre ringlet diffusion rates also did not differ between management types. Silvery blue butterfly diffusion rates were strongly associated with the amount of their main host plants, which are large perennial species of lupine (Lupinus albicaulis or Lupinus oreganus), present along their flight path. We did not observe an effect of nectar amount or species or vegetation height on silvery blue diffusion rates.
Our research suggests that conservation grazing does have the potential to add habitat value for butterflies in agricultural landscapes if their host plants are present. Grazing management should focus on promoting a diverse native plant community, especially host plants for focal species, while still providing forage for cattle. Native prairie refugia should be maintained in the landscape, as we focused only on common generalist species and are unable to extrapolate to more sensitive species. ($19,600)