Benefit of permanent non-fire refugia for Lepidoptera conservation in fire-managed sites in Wisconsin, USA
Published source details
Swengel A.B. & Swengel S.R. (2007) Benefit of permanent non-fire refugia for Lepidoptera conservation in fire-managed sites. Journal of Insect Conservation, 11, 263-279
Published source details Swengel A.B. & Swengel S.R. (2007) Benefit of permanent non-fire refugia for Lepidoptera conservation in fire-managed sites. Journal of Insect Conservation, 11, 263-279
The effects of ecosystem management with fire on insects are subjects both of research and controversy. Butterflies specialized to live in native prairie and savanna/barrens vegetation are often assumed to be fire adapted, given their restriction to ecosystems considered fire-dependent. Other studies have contested the assumption that frequent extensive burning benefits (or does not irretrievably harm) butterflies requiring native herbaceous vegetation, and recommend greater reliance on alternative managements like mowing, haying, light grazing, and/or brush-cutting. Most above-ground insects in the herbaceous layer appear to die in fires and specialists have the most negative short-term response, needing to rebuild their populations from unburned refugia. For those butterfly species listed as threatened or endangered at the state (Wisconsin) or federal levels, legal regulation of site management occurs, encouraging the creation of permanent non-fire refugia and greater use of mechanical cutting than burning.
Study sites: From the early 1990s through 2005, butterfly transect surveys were conducted annually at the same sites (prairies and savannas/barrens) in three regions of Wisconsin, USA. Specialist butterfly population indices were analyzed at three sites: Crex Meadows (12180 ha); Bauer-Brockway (125 ha); and a smaller site, Muralt Bluff (25 ha), where a permanent 'non-fire refugium' was established during this study.
This term signifies that a management unit (although possibly quite small) is kept unburned through and after the cycle of rotational fire management elsewhere in the site, not just temporarily unburned for that year or that rotation.
Butterfly abundance indices: Population indices in the refugium were compared to indices at comparison sites (which had consistent management throughout this study) in the same region. When the individual site was managed differently than the comparison sites, a difference in butterfly population trend between them can be attributed to that difference in management. When a site was managed with fire earlier in the study but developed a refugium later (or vice versa), we also compare results at that site between the earlier and later periods.
Two large sites: At Crex Meadows, all significant changes in numbers of four specialist species (including one legally listed species: the Karner blue Lycaeides melissa samuelis, federally listed as endangered), skewed toward relatively higher abundance in the refugium (14 ha, last burned in 1988) during the later period (1998-2005) vs. 10 comparison subsites (fire-managed in both periods) than expected from observations in the earlier period (1991-97). At Bauer-Brockway, the same pattern occurred in the refugium (4 ha) for frosted elfin Callophrys irus (state-listed as threatened), after the rest of that unit (9 ha) had its first management fire (in spring 2002). Specialist butterfly population trends were positive in the refugia at both Crex Meadows and Bauer-Brockway, while the comparisons usually had less favorable trends, or otherwise had similar trends.
Small site: At Muralt Bluff, regal fritillaries Speyeria idalia (state-listed as endangered), were more concentrated in the refugium (3 ha, last burned in 1991) during the earlier period (1991-97) but were more abundant in both periods in the refugium than the other units there (fire-managed in both periods). In the earlier period at Muralt Bluff, this species significantly declined, the opposite trend of comparison sites (which always had never-burned refugia), but significantly increased in the later period (1998-2005), similar to comparison sites. The refugium did not benefit the prairie-specialist Ottoe skipper Hesperia ottoe (not legally listed), which declined significantly in the earlier period and was not recorded in the later period. However, the refugium at Muralt Bluff was not a core area for Ottoe skipper. Although the species occurred in all parts of the transect survey route at this site, this skipper's core area was in an area that was fire-managed in both the earlier and later period.
Conclusions: Formerly burned units began functioning as refugia only >6-8 years after the last fire and continued to increase in benefit for years after that. In fire-managed and fire-prone sites, it is beneficial to establish permanent non-fire refugia (units kept unburned through cycles of rotational fire elsewhere in the site) for Lepidoptera conservation, placed where the most specialists will benefit and managed by more sympathetic techniques (e.g. mowing) if needed. Some sites (as opposed to units within sites) with unusually high richness and abundance of fire-sensitive specialists are particularly good candidates for large non-fire refugia. This study indicates that legal regulation for legally listed butterfly species that encourages permanent non-fire refugia and more emphasis on mechanical than fire management have benefited listed species as well as co-occurring unlisted specialist Lepidoptera. These regulations were developed and implemented based on the biology of individual species, but had multiple-species benefits.
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