Northern Prairie Wildlife Research Center
Anderson Prairie was initially planted with a variety of prairie grasses and forbes in May 1988 and burned in its entirety on an annual basis before 1994. In 1994, the prairie was divided into 14 plots at least 30 by 115 m in size (Fig. 1). Plots are burned in the spring between early-April and early-May and managed as follows: two plots are burned annually (plots 5 and 6), with the remaining 12 plots divided into three year burn cycles. Plots used in our study included plots 2, 4, and 12, which were burned in 1994; plots 1, 7, and 9, which were burned in 1995; and plots 3, 11, and 14, which were burned in 1996.
|Figure 1. Burn unit layout within Anderson Prairie, a reconstructed prairie in Decorah, Winneshiek County, Iowa. Sampled in our study were plots 2, 4, and 12 burned in 1994; plots 1, 7, and 9 burned in 1995; and plots 3, 11, and 14 burned in 1996.|
We collected ground beetles by using pitfall traps for 15 one-week periods during the 1996 field season. Traps were first placed in the prairie 9 April 1996 (Julian date = 100) when day-time temperatures were consistently above freezing and sampling continued on a biweekly basis until 28 October 1996 (Julian date = 302) when the night-time temperatures dropped consistently below freezing. Each pitfall trap was constructed from one 473 ml plastic cup (9 cm dia) placed into the ground so the lip of the cup was at or slightly below the ground surface. In each cup fitted with a funnel constructed from a 207 ml casual cup insert (Sweetheart® Cup Company, Chicago, Illinois) we placed approximately 50 ml of propylene glycol preservative, diluted at a 1:1 ratio with water. During each trapping period, four traps were placed in each of the nine plots in a transect at 10 to 15 m intervals. After each biweekly trapping period, all traps were removed and the holes left by each trap were filled. Traps, which were damaged and apparently disturbed by large vertebrates, were noted and excluded from the study. We identified ground beetles to species by using keys presented in Lindroth (1961-1969) and Noonan (1991), and standardized names of Bousquet and Larochelle (1993). Voucher specimens are housed in the reference insect collection of the Hoslett Museum of Natural History, Luther College, Decorah, Iowa.
Ground Beetle Faunal Analysis
We determined the total number of ground beetles and species richness for each burn treatment for the entire trapping season. We calculated species diversity (Shannon's diversity index H') and evenness (J) of the ground beetle fauna for each burn treatment (Krebs 1989). Percent similarity among the three treatments, sometimes called the Renkonen index (Krebs 1989), was also determined.
Using only three one-week samples taken during the peak activity periods as determined by both species and numbers of ground beetles collected, we performed a second discrete sampling period analysis. This peak began at the end of June (Julian date = 191) and continued to the beginning of August (Julian date = 219).
We performed statistical comparisons among burn treatments on the total number of ground beetles, species richness, and average number of ground beetles caught per trap per day by using a two-way analysis of variance (Minitab 1994) for the continuous full season data, with each plot serving as a replicate. We used the non-parametric Friedman test (Minitab 1994) to analyze differences in species accumulation between treatments blocked by date over the sampling season.