174 



Invcnchnilcs — Our Lninii Rfsouncs 



For further information: 



Piiul A, Opler 



National Biological Service 



Intorination Transfer Center 



1201 Oak Ridge Dr. 



Suite 2110 



Fort Collins, CO 80525 



References 



Brown, J,W.. and PA. Opler. 1990. Patterns of butterfly 

 species density in peninsular Florida. Journal ol 

 Biogeography 17:615-622. 



Peigler. R.S.. and RA. Opler. 1993. Moths of western North 

 America. I . Distribution of Saturniidae of western North 

 America. Contributions of the C.P. Gillette Insect 

 Biodiversity Museum, Colorado State University, Fort 

 Collins. 24 pp. 



Smith. M.J. 1993. Moths of western North America. 2. 

 Distribution of Sphingidae of western North America. 

 Contributions of the C.P. Gillette Insect Biodiversity 

 Museum. Colorado State University, Fort Collins. 34 pp. 



Stanford. R.E.. and PA. Opler. 1993. Atlas of western USA 

 buttertlies, including adjacent parts of Canada and 

 Mexico. Published by authors. Denver. CO. 275 pp. 



Sphmx moth [Pro.sctpiuiis jiionita). 



The Tall-grass 

 Prairie 

 Butterfly 

 Community 



by 



Ann B. Swengel 



Scott R. Swengel 

 Baraboo, Wisconsin 



■ Study sites □ Tall-grass prairie biome 



Fig. 1. Original boundaries of the 

 tall-grass prairie biome in the 

 United States (Ri.sser et al, 1981 ) 

 and locations of study sites ( A.B. 

 Swengel. unpublished data). 



The piairie biome is a plant community dom- 

 inated by grasses and nongrassy herbs 

 (wildflowers or "forbs"). with some woody 

 shrubs and occasional trees. Prairie is classified 

 into three major types by rainfall and conse- 

 quent grass composition. The easternmost and 

 moistest division is the tall-grass prairie (Risser 

 et al. 1981). Althotigh tall-grass prairie once 

 broadly covered the middle of the United States 

 (Fig. 1). this biome is now estimated to be at 

 least 99% destroyed from presettlement by pio- 

 neers, who converted it for agricultural uses. 

 Praiiie loss continues through plowing, extreme 

 overgrazing, and development, but at varying 

 degrees. Prairie is also lost passively because 

 the near-total disruption of previous ecological 

 processes causes shifts in tloristic composition 

 and structure. 



As a result of this habitat destruction, butter- 

 flies and other plants and animals that are oblig- 

 ate to the prairie ecosystem are rare and primar- 

 ily restricted to prairie preserves. The Dakota 

 skipper (Hesperia dacotae) and the regal fritil- 

 lary {Spcyeria idalia) are federal candidates for 

 listing under the Endangered Species Act, and 

 additional prairie butterfly species are on state 

 lists as officially threatened or endangered. 

 Patches of original prairie vegetation remain in 

 preserves, parks, unintensively used farmlands 

 such as hayfields and pastures, and in unused 

 land. These remnants of prairie, however, are 

 isolated and often in some state of ecological 

 degradation. 



The existence of prairie depends on the 

 occunence of certain climatic conditions and 

 disturbance processes such as animal herbivory 

 and fire. These natural processes, however, are 

 severely disrupted today because of the destruc- 

 tion and fragmentation of the prairie biome. 

 Without management intervention, the vegeta- 

 tional composition and structure of prairie sites 

 are altered through invasion of woody species 

 and smothering under dead plant matter. Prairie 

 usually requires active management to main- 

 tain the ecosystem and its biodiversity, but it is 



difficult to know exactly which processes once 

 naturally maintained the prairie ecosystem. 

 Frequent fire, whether caused by lightning or 

 set by native peoples, is usually considered the 

 dominant prehistoric process that maintained 

 prairie; thus management for tall-grass prairie 

 in most states relies primarily or solely on fre- 

 quent fire (e.g., Sauer 1950: Hulbert 1973: Vogl 

 1974). Other researchers (e.g., England and 

 DeVos 1969), however, assert that prairie was 

 the result of grazing by large herds of ungulates 

 as in the Serengeti in Africa. 



Despite this scientific conflict, it appears 

 certain that successful management for main- 

 taining the prairie landscape and its native 

 species should be based on these natural 

 processes, whatever they were. The vast diversi- 

 ty and specificity of insects to certain plants and 

 habitat features make them fine-tuned ecologi- 

 cal indicators. Thus, butterfly conservation is 

 useful not only for maintaining these unique 

 species, but also for helping us monitor and 

 learn about the soundness of our general 

 ecosystem management. 



Survey and Classification 



We counted 90 butterfly species and 80,906 

 individuals in surveys from 1988 to 1993 at 93 

 prairies varying from 1 to 445 ha (3 to 1,100 

 acres) in the Upper Midwest (Illinois, Iowa, 

 Minnesota, Wisconsin) and southwestern 

 Missouri (Fig. I ). Most sites are managed prin- 

 cipally with fire, with bums averaging about 

 25% (range 0-99% or more) of the prairie patch 

 per year. Many Missouri sites are managed pri- 

 marily with summer haying along with a little 

 burning and cattle grazing. The vegetation in 

 each survey unit was relatively uniform. 



Any species observed 100 or more times was 

 designated a study species. Before analyzing 

 the results, we classified the study species by 

 habitat niche breadth: prairie specialist, grass- 

 land, generalist, and invader. We used popula- 

 tion indices (individuals observed per hr in each 



