Do Managed Burns in Grasslands Benefit Butterflies?
By Brooke Karasch
Pollinators are in global decline. Because they provide a variety of important ecosystem services—namely, pollination of both native plants and crops—it is important to understand and try to mitigate the causes of pollinator loss. A number of factors are driving declines, including pesticide overuse, climate change, and habitat degradation. In the North American Great Plains, habitat degradation is an important factor in many species’ declines.
Most of the Great Plains are what we might refer to as a “working landscape.” A majority of the native prairies have been converted to row cropland, and what’s left is often used as grazing land for cattle. Luckily, the Great Plains evolved with large herbivores, and studies have shown that grazing differences between native bison and non-native cattle are minimal (see Allred et al. 2011). However, the Great Plains also evolved with another disturbance that people aren’t always so fond of: fire.
Grassland obligate species—those that need grasslands for their entire life cycles—depend on the habitat created by the interaction of fire and grazing. One way to reinstate this interaction is through a system of patch-burn grazing, wherein managers burn one part of a pasture at a time and leave the rest, while also allowing cattle free access to the entire pasture. The Range Science program at North Dakota State University has begun a study that examines two versions of this system. In the first, we apply one 40-acre prescribed burn to a different patch of the landscape each spring. In the other treatment, we apply one 20-acre prescribed burn in the spring and a second 20-acre burn in late summer to an adjacent patch. We’re conducting this study at NDSU’s Central Grasslands Research Extension Center in Streeter, North Dakota.
To help determine the success of this type of system as conservation, my particular study will measure the butterfly community response to both patch-burn grazing and season-long grazing. The season-long treatment is meant to mimic more traditional grazing practices and excludes fire. We predict that the patch-burn grazing system will provide better habitat for butterflies, because it will help clear areas of the landscape from woody encroachment and invasive grasses, both of which can crowd out the floral resources adult butterflies rely on.
We have chosen to use butterflies for this study for a few reasons, including their important status as providers of pollination services. Additionally, butterflies can be considered an “indicator species” because of their complex life cycles and relatively short generation times. Many species also require a variety of habitat variables throughout their lives, which also contributes to their value as indicators. Some species need a single species of food plant for their larvae, as well as a variety of floral nectar resources as adults, and sometimes even deep layers of dead plant material for hibernation in the winter.
In North Dakota, the butterfly flight season is considerably shorter than it might be in more southern locations. We begin our surveys in June and conduct three rounds of surveys by mid-August. We’re collecting butterfly data via line-transect distance sampling, so we have transects set up in each pasture. We walk each transect three times each summer and count every adult butterfly we see, identifying them to the species level. We also record their distance perpendicular to the transect. All of this data is processed in a free program called Distance, which gives us density estimates for each species in each treatment.
Although we have only collected one year of data so far, it appears that overall butterfly density, as well as density of grassland obligate species, is highest in the patch-burn grazing system (see chart above). This makes sense, as the patch-burn grazing system offers more habitat heterogeneity. Because the cattle typically graze the highly nutritious forage found in the most recently burned patch, the rest of the pasture can accumulate more litter and allow more floral growth.
We still have two summers of data collection left on the project, so we can’t say anything for sure yet. We’re lucky to have a site with such high butterfly diversity (a total of 39 species in the first year!), and we’re hoping to see total numbers of butterflies grow as we continue restoring natural disturbance and greater habitat variability to the area.
“The role of herbivores in Great Plains conservation: comparative ecology of bison and cattle,” Allred et al 2011, Ecosphere
“Improving our science: the evolution of butterfly sampling and surveying methods over time,” Kral et al 2018, Journal of Insect Conservation
Brooke Karasch is a graduate student in the Range Science program at North Dakota State University’s School of Natural Resource Sciences. Email: firstname.lastname@example.org