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Fig. 2. Regal fritillary on a purple coneflower. |
Fig. 3. Powershiek skipperling. |
Although declining grassland birds and prairie-specialist butterflies share the same habitats, their abundances are not necessarily correlated, in part due to differences in the geographic scales of their habitat use. For example, Henslow's sparrows and grasshopper sparrows are short-distance migrants, wintering primarily in the southern United States, while dickcissels winter mostly in northern South America (Fretwell 1973). Prairie-specialist butterflies are year-round residents on particular prairie patches, with relatively little dispersal among patches (Opler 1981; Opler and Krizek 1984; Moffat and McPhillips 1993). Although birds depend upon suitable habitat and resources being available at seasonally appropriate times in widely distributed regions, butterflies need resources and conditions to be consistently available within a particular habitat patch.
Key habitat features and required resources differ between birds and butterflies. Both butterflies and birds show preferences for either wet lowlands or dry upland habitats; however, vegetational structure is particularly important only to birds (Hopkins 1991). Henslow's sparrow prefers large grassland expanses with consistent patches of dense cover provided by dead vegetation, whereas grasshopper sparrows favor shorter, more open vegetation with sparse cover (Kahl et al. 1985). Butterflies often have specific associations with plant species, especially in the larval life stage (for example, larval food plants), and as adults they show some degree of preference for certain nectar flowers (Opler 1981; Opler and Krizek 1984).
It may be difficult to census butterfly and bird populations in the same survey because of their differing daily and seasonal activity patterns. Songbirds are active early and late in the day, a pattern that is weak or lacking in most grassland birds (Kantrud 1981). Butterflies tend to be more active in the warmer and sunnier parts of the day (Opler and Krizek 1984). Songbirds are more detectable during the breeding season in late spring to early summer, when they vocalize more. Butterflies are easier to see during their adult life stage, the timing of which varies considerably by species (Opler and Krizek 1984).
Population Trends
Despite the difficulties associated with simultaneous surveys of grassland birds and butterflies, some covariances, are apparent between populations of the two groups (Fig. 4). In our study, prairie-specialist butterfly species were more strongly correlated with grassland sparrows than with butterflies less restricted in habitat. The regal fritillary, the most widely occurring prairie-specialist butterfly, showed the most consistent co-occurrence with grassland sparrows.
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Fig. 4. Percent correlation in abundance between species pairs. A 100% correlation indicates a total correspondence in abundance patterns; 0% correlation indicates a complete noncorrespondence in abundance patterns. Percentages are based on Pearson's product-moment correlations of log-transformed observations of each species per hour per survey, in surveys from mid-June to 31 July 1988-1995 at 104 prairie sites in Illinois, Iowa, Minnesota, Missouri, and Wisconsin (methods described in Swengel 1996). Although Henslow's sparrows historically occurred at sites in the range of the Dakota skipper and the Poweshiek skipperling, they do not occur at these sites today. |
Prairie birds and butterflies present both conservation concerns and opportunities for preservation, Though their populations have been much reduced through habitat loss, no known species of prairie-specialist butterfly or North American grassland sparrow has yet become extinct. Conservation activities that are effective at maintaining one prairie species may confer benefits to others. For example, in southwestern Missouri, prairie conservation management rotates midsummer haying (annually to triennially, though usually biennially) to benefit the greater prairie-chicken (Solecki and Toney 1986). This rotated haying also supports large populations of prairie-specialist butterflies (Swengel 1996) and grassland sparrows (Skinner et al. 1984). Grassland birds and butterflies also benefit from nonintensive grazing and hay-cutting regimes (Skinner 1975; Kantrud 1981; Smith and Smith 1992; Swengel 1996), and birds particularly benefit from idling of croplands, as in the United States Conservation Reserve Program, which rewards farmers for tilling a smaller percentage of their land (Johnson and Schwartz 1993).
This resource is based on the following source:
Swengel, Ann B., and Scott R. Swengel. 1998. Highlight Box: Tall-
grass Prairie Butterflies and Birds. Pages 446-447 in M. J. Mac,
P. A. Opler, C. E. Puckett Haecker, and P. D. Doran, eds. Status
and Trends of the Nation's Biological Resources, Vol. 2. U.S.
Department of the Interior, U.S. Geological Survey.
This resource should be cited as:
Swengel, Ann B., and Scott R. Swengel. 1998. Highlight Box: Tall-
grass Prairie Butterflies and Birds. Pages 446-447 in M. J. Mac,
P. A. Opler, C. E. Puckett Haecker, and P. D. Doran, eds. Status
and Trends of the Nation's Biological Resources, Vol. 2.
Jamestown, ND: Northern Prairie Wildlife Research Center Online.
http://www.npwrc.usgs.gov/resource/2000/grlands/grlands.htm
(Version 21JAN2000).
Acknowledgments
We thank the funding providers of the surveys used in Fig. 4: Lois Almon Small Grants Research Program, The Nature Conservancy Minnesota Chapter, Wisconsin Department of Natural Resources, U.S. Fish and Wildlife Service, and W. and E. Boyce.
See Literature Cited for references
Authors
Ann B. Swengel
Scott R. Swengel
909 Birch Street
Baraboo, Wisconsin 53913
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