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The Tall-grass Prairie Butterfly Community


by
Ann B. Swengel
Scott R. Swengel
Baraboo, Wisconsin
The prairie biome is a plant community dominated 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 consequent grass composition. The easternmost and moistest division is the tall-grass prairie (Risser et al. 1981). Although 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 pioneers, who converted it for agricultural uses. Prairie 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 floristic composition and structure.

Fig. 1. Original boundaries of the tall-grass prairie biome in the United States (Risser et al. 1981) and locations of study sites (A.B. Swengel, unpublished data).
As a result of this habitat destruction, butterflies and other plants and animals that are obligate to the prairie ecosystem are rare and primarily restricted to prairie preserves. The Dakota skipper (Hesperia dacotae) and the regal fritillary (Speyeria 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 occurrence of certain climatic conditions and disturbance processes such as animal herbivory and fire. These natural processes, however, are severely disrupted today because of the destruction and fragmentation of the prairie biome. Without management intervention, the vegetational 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 maintain 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 frequent 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 maintaining the prairie landscape and its native species should be based on these natural processes, whatever they were. The vast diversity and specificity of insects to certain plants and habitat features make them fine-tuned ecological 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. 1). Most sites are managed principally with fire, with burns averaging about 25% (range 0-99% or more) of the prairie patch per year. Many Missouri sites are managed primarily 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, grassland, generalist, and invader. We used population indices (individuals observed per hr in each unit) to identify which units had relatively greater densities of particular species and which factors might account for this variation. Details regarding the survey and statistical methodologies are provided elsewhere (A.B. Swengel, unpublished data).

Management and Distribution
Regal fritillaries (Speyeria idalia) mating on pale purple coneflower (Echinacea pallida). Courtesy A.B. Swengel
The overwhelming destruction of prairie habitat has had disastrous consequences for prairie-specialist butterflies, not just because of the outright loss of appropriate living space but also because of habitat fragmentation. Because prairie-specialist butterflies are rarely encountered outside of these fragmented prairie patches, populations at different sites may have minimal gene flow and are rarely able to recolonize sites of local extinctions. For example, the regal fritillary is the most widespread prairie butterfly species, but it requires larger habitat patches or connected networks of habitat patches to maintain populations. The arogos skipper (Atrytone arogos iowa) and ottoe skipper (Hesperia ottoe) also occur widely in the prairie biome but are more restricted in their habitat requirements, resulting in more localized and spotty distributions. The Dakota skipper and poweshiek skipper (Oarisma poweshiek) are most restricted in range, occurring only in northern prairie, and have further habitat restrictions within that range. As a result, the northern Midwest (northwestern Iowa, western Minnesota, and the eastern Dakotas) is the region where tall-grass prairie conservation has the most potential for maintaining the greatest diversity of prairie-specialist butterflies.
Our surveys show that the management occurring at a prairie critically affects whether prairie-specialist butterflies exist at the site and at what abundance. Although each butterfly species has its own response to fire, the prairie specialists show a pronounced and statistically significant decline after fire; this decline persists 4 or more years (A.B. Swengel, unpublished data). Species with the broadest habitat adaptation (invaders) are most abundant in recently burned units and least abundant in units left unburned the longest. Species of intermediate adaptations (grasslands, generalists) showed milder, intermediate trends.
Unintensive haying management (a single annual or biennial cutting with removal of the clipped vegetation) is more favorable for butterfly diversity. Such haying is more beneficial for butterflies sooner after treatment and causes a less pronounced variation in butterfly abundance between different treatment years. In general, butterflies are more abundant in the first years after haying than after burning; specialists account for much of this difference (Fig. 2). Our limited opportunities to test light grazing show that it may also serve specialist butterflies better than fire.

Fig. 2. Abundance of all, nonspecialist (grassland, generalists, and invaders), and prairie-specialist study species in the first years of fire and hay management, Missouri study sites.
Prairie-specialist butterflies apparently respond to different management types because of varying degrees of mortality (e.g., fire causes more direct mortality than haying or grazing) and because of differences in continuity of required habitat resources (e.g., fire removes all cover but is followed by regrowth of thick cover, while unintensive haying and grazing can more consistently maintain moderate cover). Management also indirectly affects butterfly populations by altering the abundance and occurrence of plants they depend on as well as the vegetational structure and physical features they require.
These results are consistent with butterfly conservation experience around the world, particularly in Europe and Australia (Butterflies Under Threat Team 1986; Kirby 1992; New 1993). Thus, simply preserving habitat is not sufficient to conserve insect biodiversity; suitable management approaches and land uses compatible with the habitat's native biodiversity must be preserved. It is possible to maintain plants successfully without protecting the associated animals, but it is impossible to maintain the associated animals successfully without protecting the plants.
It appears desirable for managers to aim for diversity and patchiness in prairie-management approaches within and among sites rather than broadly applying a single management formula for prairie everywhere. Whether or not a site is managed specifically to conserve insects, declines and extirpations of insects specialized to the habitat indicate that ecological degradation has already occurred there, while maintenance of these species indicates success in ecosystem conservation. Because we found that management with mechanical cutting or light grazing appears most effective for maintaining both the prairie habitat and its associated specialist insects (seeming to indicate an ecosystem adaptation to herbivory), we recommend that these methods should have a primary role in modern prairie management for the conservation of biodiversity. There is cause for optimism, however, because no known prairie butterfly species have gone extinct, despite their rarity. Instead, these species have persisted on habitat remnants, showing that appropriate habitat preservation and management should translate into readily measurable conservation successes.
For further information:
Ann B. Swengel
909 Birch St.
Baraboo, WI 53913
References
Butterflies Under Threat Team. 1986. The management of chalk grassland for butterflies. Joint Nature Conservation Committee, Peterborough, U.K. 80 pp.

England, R.E., and A. DeVos. 1969. Influence of animals on pristine conditions on the Canadian grasslands. Journal of Range Management 22:87-94.

Hulbert, L.C. 1973. Management of Konza Prairie to approximate pre-whiteman influences. Pages 14-19 in L.C. Hulbert, ed. Third Midwest prairie conference proceedings. Kansas State University, Manhattan.

Kirby, P. 1992. Habitat management for invertebrates: a practical handbook. Royal Society for the Protection of Birds, Bedfordshire, U.K. 149 pp.

 New, T.R. 1993. Conservation biology of Lycaenidae (butterflies). IUCN, Gland, Switzerland. 173 pp.

Risser, P.G., E.C. Birney, H.D. Blocker, S.W. May, W.J. Parton, and J.A. Wiens. 1981. The true prairie ecosystem. Hutchinson Ross Publishing Co., Stroudsburg, PA. 557 pp.

Sauer, C. 1950. Grassland climax, fire and management. Journal of Range Management 3:16-20.

Vogl, R.J. 1974. Effect of fire on grasslands. Pages 139-194 in T.T. Kozlowski and C.E. Ahlgren, eds. Fire and ecosystems. Academic Press, New York.


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