Butterfly use of agricultural fields under different management techniques in Dartmouth, Massachusetts

Miller, Lauren
Jon Atwood, PhD
Department of Environmental Studies
2007
In Massachusetts agricultural land offers valuable habitat to many grassland species, a guild that has declined within the last 100 years, mostly as a result of land-use change. Although Dartmouth Massachusetts lost the highest amount of agricultural land in the entire state from 1985 to 1999, it continues to support small farms and open space. Grassland birds have been used to measure diversity in grassland ecosystems, however, local mowing practices especially in hayfields is detrimental to breeding bird success. But just because fields do not support birds, it doesn’t mean they are not diverse. As an alternative to birds, butterflies are an ideal object of study because they are sensitive to habitat conditions, and follow similar patterns of abundance and diversity as birds in grassland ecosystems. Because there are more species of butterflies in grassland ecosystems, butterfly diversity may be a better representation of overall diversity than birds. To look at the butterfly communities in this agricultural region of South Dartmouth, Massachusetts, I chose seven fields ranging from 9 acres to 20 acres. I looked at fields of two different management types; three fields managed strictly for hay production, and 4 fields managed for breeding grassland birds. Three mowing patterns were used on these fields; mow for hay production in mid June and a subsequent mow later that summer, delayed mow after July 1st, and no mow (fall mowing after butterfly activity is over). Fields were surveyed every two weeks in the summer of 2006 for a total of 8 survey periods. Butterfly behavior and nectaring observations were also made. A total of 27 species and 1517 individual butterflies were observed during the one-year study. Using Shannon and Simpson’s diversity indices, grassland bird managed fields (delayed mow and unmown) supported a more diverse butterfly assemblage than the hay production fields. Grassland bird manage fields also supported the highest abundance of habitat-sensitive butterflies, which cannot tolerate high levels of disturbance. In this study the delayed mow fields had mid-season larval and nectar resources for butterflies, and reduced mowing frequency lowered disturbance to habitat-sensitive butterfly species. Multiple mown fields supported a generalist butterfly community dominated by orange sulfur, mainly due abundance of alfalfa with a frequent mowing schedule. Even multiple mown fields supported habitat sensitive butterflies, and offered a variety of nectar resources. Red clover was the most common nectar resources used by adult butterflies. Goldenrod, asters, alfalfa and dandelion were also utilized often. In conclusion, butterfly habitats should contain both larval and nectar resources. Habitats that offer a mosaic of patches mown at different times will benefit local butterfly populations.

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