Northern Prairie Wildlife Research Center
Previous studies have reported two periods of peak activity for ground beetles during the year, with the first peak occurring from late-May to June and the second peak from mid-August to late-September (Kirk 1971, Niemelä et al. 1989). We found only one adult activity peak, which began in late-June and lasted into early-August. This single peak may have been due to an unusually cool summer, but in northeastern Iowa apparently the climate allows only this single mid-summer ground beetle activity peak.
When examining short-term sampling periods as a valid representation of full season trends, Niemelä et al. (1990) showed that samples of ground beetle assemblages obtained by trapping periods of 20 days or more are similar enough to a continuous whole season sample to be used in several types of ecological studies. In comparing the full-season and short-term results of our study, many of the same trends relative to the burn treatments were apparent in both the short-term and full-season sampling regimes. Species richness did show dissimilarities between the discrete and full-season results, most likely due to the continued accumulation of species from recolonization detected by continuous sampling not detected during the discrete sample period. We suggest however, that the trends in abundance, diversity, and evenness were most important and agree with Niemelä et al. (1990) that short-term sampling for ground beetles is valid for detecting the effect of management techniques such as fire on ground beetle fauna. We also found that the most recently burned plots contained the richest carabid fauna when sampled throughout the beetle-active season. This species richness was not apparent until August, however, and may be the consequence of recolonization of burned areas the first year after a fire. Although species rich, the most diverse beetle assemblage did not occur until several years after a fire.
Previous research on the effects of burning prairies on ground beetles is mixed. Rickard (1974) found no significant difference in the abundance of the carabid Calosoma luxatum Say between burned and unburned stands of shortgrass prairie. Van Amburg et al. (1981) found a greater abundance of adult carabid beetles in burned tallgrass prairie plots than in unburned plots. In contrast, Seastedt et al. (1986) found carabid larvae were more abundant in unburned plots than burned tallgrass prairie plots.
Our study showed that ground beetle diversity did increase over several years immediately following fire in a tallgrass prairie environment. However, before the effects of burning on ground beetles can be determined, we must identify which species are prairie "specialists" and how these species respond to fire. Some species, such as C. platyderus, were "pyrophilic", i.e., are most abundant immediately following a burn, while other species, such as Chlaenius tricolor Dejean and Poecilus lucublandus (Say) were "pyrophobic", i.e., increasing in their abundance several years post-burn. Although fire may cause a reduction in the abundance of carabid beetles immediately after a burn (Holliday 1984), some species are apparently attracted to recently burned areas (Evans 1971, Holliday 1984). The greater number of species collected in plots burned in 1996 may have been due to increased activity by the ground beetles allowed by the removal of the dense leaf litter, which rapidly builds up in unburned tallgrass prairies. Greenslade (1964) recognized that vegetation impedes ground beetle activity. Possibly ground beetle activity may be affected more by alteration of the vegetation architecture by fire than by the fire directly (McCoy 1987). Continued research should further clarify how disturbance by fire and plant regeneration interact with individual species to influence the ground beetle assemblage found in tallgrass prairies.