Illinois Natural History Survey - University of Illinois

Insects and Fire: Too Much of a Good Thing?

Insects comprise the largest component of biodiversity in native prairies, where they play a variety of important roles, including being the major pollinators of native plants. Thus, management that maintains a diverse insect fauna is crucial to the conservation of these ecosystems. Traditionally, managers have relied heavily on prescribed fire to encourage production and flowering of native grasses and forbs, and to discourage growth of woody and invasive species. Although this method has had a dramatic, positive impact on native plant diversity and structure in prairies, many entomologists question the assumption that prairie insects are adapted to fire and, thus, are not harmed by prescribed burns. Anecdotal reports suggest that fire reduces insect abundance, species richness, and diversity, but more detailed studies have revealed that some species recover quickly from fire. Nevertheless, the subject of the effects of fire on prairie insects remains controversial, in part because the factors that determine the rate of recovery of insect populations after fire are not well understood.

Post-burn recovery results from a combination of recolonization and in situ survival, the relative importance of which remains unknown, and apparently varies depending on burn characteristics and the proximity of unburned refugia. Insect diversity in prairie remnants could be managed more effectively if we knew whether and under what conditions insects survive fire in situ. If in situ survival is rare, then maintaining unburned refugia nearby is necessary to enable insects to recolonize a burned site.

In a study currently under way, we are attempting to assess the relative contributions of recolonization and in situ survival to overall post-fire recovery of insect populations by monitoring the insect faunas of two Illinois remnant prairies--Richardson Wildlife Foundation in Lee County and Windfall Prairie in Vermilion County--following prescribed burns. Following a spring burn, each prairie was divided into burned and unburned units and equal numbers of enclosed and unenclosed plots were monitored in each unit throughout the growing season. Insects found within the enclosures, constructed of "no-see-um" netting, presumably survived the fire in situ. Those found in the unenclosed plots either survived in situ or recolonized the site from elsewhere. By comparing the faunas of burned and unburned and enclosed and unenclosed plots, we were able to assess the contributions of in situ survival versus recolonization to overall recovery of insect populations.

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Results of this first year of monitoring suggest that although fire is not necessarily detrimental to insect species richness, prescribed burns can have a significant impact on species composition, INHS researchers Mary Harper and Richard Larimore record the vegetation cover in an insect enclosure at Richardson Wildlife Foundation.

primarily because certain species do not normally survive fire in situ. At both sites, although overall species diversity in both enclosed and unenclosed plots was initially reduced as a result of the fire, by the end of July no significant differences in diversity were found between the burned and unburned treatments. Some species extirpated by fire quickly recolonized the burned areas, seemingly from adjacent unburned refugia. At Windfall Prairie, Laevicephalus minimus, a leafhopper that specializes on side-oats grama, was initially absent from the burned unit, but adult individuals began showing up in the uncaged plots by mid-June, apparently having migrated from the unburned part of the prairie. Other less mobile species failed to appear despite an abundance of suitable host plants. The grass-feeding aphid, Izyphia flabella, was abundant in the unburned plots but remained absent from the burned plots, and probably will require more than a single season to recolonize the burned area. This suggests that differences in species composition between burned and unburned treatments resulted because some species not only failed to survive in situ but also were unable to recolonize the burned area.

This study repesents a preliminary step towards a greater understanding of the effects of fire on prairie insects. Analyses of additional data on vegetation, burn temperature, and burn intensity will address additional unanswered questions crucial to our understanding of the effects of fire on prairie insects. Hopefully, as more data become available in subsequent years, consistent patterns will begin to emerge that will enable us to make specific recommendations for managing insect diversity in native prairies.

C.H. Dietrich, Center for Biodiversity; M.G. Harper, R.L. Larimore, and P.A. Tessene, Center for Wildlife Ecology



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