Bison at the Sandhill Wildlife Area wallowing in dust surrounded by grassland flowers.
Bison and butterflies
A "Beauty and the Beast" tale.
Anna Hess, Bob Hess, Joy Hess, Bev Paulan and Julie Hess
The Sandhill Wildlife Area is a small 9,150–acre wildlife area and research facility in central Wisconsin located five miles south of Pittsville. Dotted by a mix of marshes and sandy uplands, the facility is completely enclosed by a fence to prohibit the movement of large animals, enabling wildlife biologists to make observations on animal populations like deer and wolves.
The Sandhill Wildlife Area is especially well set up for those seeking a day excursion to watch marsh and grassland wildlife. A system of roadways and hiking trails intertwine the oak and pine forests and the extensive network of marshes and dikes. Old drainage ditches, left over from early–1900s attempts to drain the central Wisconsin marshes for farming, crisscross the landscape. These ditches are now filled with water and provide habitat for waterfowl, muskrats, beavers and other marsh dwellers. Several lookout towers are located along the trails, including one at the edge of the bison pasture, where visitors may see bison, as well as some of the dozens of butterflies and moths that profit from the presence of this native herbivore.
A unique feature of this wildlife area is the small herd of bison that help to maintain the native oak savanna ecosystem, one of the few controlled areas of this kind in the world.
The bison pasture — 200 acres of savanna habitat — is a remnant of the savannas that used to dominate central and southern Wisconsin. Oak savannas were historically maintained through low intensity disturbance. This included infrequent fires; the activities of bison, elk and deer; and droughts. The sandy soils, left behind by Glacial Lake Wisconsin, create low–nutrient soil quality and make these areas what are essentially the deserts of the Midwest. Many of the plants that adapted to the savanna ecosystem are exceptionally drought–resistant and often require scarification or fire to properly reproduce.
This area provides a significant opportunity to study how the bison interact with other important grassland creatures, such as butterflies. For the last three years, a small team of researchers has observed the butterflies and moths in this pasture and their connections to the bison herd.
They have found through several separate studies, and one recently published in the Journal of Insect Conservation, that the bison produce superior habitat conditions for butterflies and moths, in comparison to the surrounding areas where the bison are not allowed to roam. These studies have observed all areas of the bison pasture and many adjacent fields, including the grassier flowered areas, the wallowing areas where bison dust themselves and the areas of heavy shrubs and trees where the bison do not frequently pass through. To be certain that bison were recently present, the team counted bison chips, which would normally deteriorate within a few weeks, making the presence of bison chips telltale signs of the recent presence of bison.
These observations found that areas where the bison normally assemble receive regular ground disruption from the bison hooves, grazing and horning (rubbing horns on small shrubs and trees). These activities open the canopy, and reduce and prohibit the growth of woody vegetation that may shade out grassland flowers that butterflies and moths require for food sources.
The more highly disturbed areas of the pasture, including along fence lines and bison trails, are more heavily populated with eastern tailed–blues and their endangered blue cousins, Karner blue butterflies.
The less frequented grassland areas, where bison graze, are frequented by American coppers, great spangled fritillaries, Acadian hairstreaks, clouded sulfurs and little wood satyrs.
The grasslands of the bison pasture are covered in sunflowers, flowering spurge (not to be confused with the invasive leafy spurge) and goldenrods. Small white moths of the Ennominae family can often be seen darting in the undergrowth. You can often see these moving through in low shrubs, looking like nondescript little white moths.
Among these butterflies, the fritillaries alone are most likely to be found in heavy shrubs and trees, feeding on the bergamot that can grow among the oak shrubs. With this exception, other butterflies are not to be found in the areas that are not kept open and grassy by the bison.
Bison wallowing is especially helpful to butterfly and moth populations, breaking up the tough grassland sod and allowing the hardened grassland seeds to sift down through the thick grasses into the prairie soil.
Wild blue lupine is among these plants which require rubbing and scraping to break up their tough, thick, drought–resistant seed pods. Lupine plants are often seen growing around the edges of new wallows, and are certain to be found growing around abandoned wallows. This produces a constant turnover of habitat for the Karner blue butterfly and frosted elfin, which require wild blue lupine as a food source.
In comparison, the data collected over recent years shows that Karner blue females in particular are found more often around wallows than any other areas of the bison pasture, most likely because these areas have a high abundance of lupine plants for laying eggs and caterpillar growth. The Karner blue males were most likely to be located in areas with high amounts of bison chips.
Butterflies in general are known to mineralize, or seek out dead or decaying organic material to feed on salts and other minerals. The Karner blue males may be using the abundance of bison chips to acquire much needed nutrients. Whatever the case, it is apparent that the bison, and the Karner blues in particular, are especially linked.
The summer of 2014 was an especially good year for the bison pasture. In the fall of 2013 the wildlife area staff conducted maintenance that opened up the oak shrubs in the thicker parts of the pasture. This effort, combined with the heavy rains in the spring, produced a massive amount of flowering grassland plants. Butterfly populations exploded from the previous year, adding several species to the list of frequently observed butterflies and moths, and quadrupling the Karner blue butterfly population. The butterflies and moths at Sandhill can be observed every year, and are most abundant around the observation tower in the bison pasture. You canít miss it — it's by the sign with the big bison.
Anna Hess is a natural resource manager for the Minnesota DNR, managing the Lake Superior North Shore area.
Bob Hess has more than 45 years of experience in natural resource management and is currently the coordinator of the Karner Blue Butterfly Recovery Program for the Department of Natural Resources.
Joy Hess is a lifelong naturalist, gardener, rug weaver and knitter, doing research studies in the sandy plains of Wisconsin.
Bev Paulan is a DNR aircraft pilot, botanist and assistant to the Karner Blue Butterfly Recovery Program.
Julie Hess is a senior paper process engineer, moonlighting as a naturalist during the warm seasons.