The destruction and degradation of North American prairie habitats, especially in the eastern tall-grass region, have considerably reduced the abundance of associated animals. The species most affected are those restricted to remaining prairie fragments. Prairie birds and butterflies often have specific habitat requirements, and reduction of the tall-grass prairie has caused serious declines in their populations. Because some species of prairie sparrows and butterflies have similar habitat requirements, trends in their populations are sometimes correlated.

Prairie Sparrows

In eastern and central North America, birds nesting in grasslands and ground-nesting birds have declined more than birds of any other North American behavioral or ecological guild (Knopf 1994). Data from the U.S. Geological Survey Breeding Bird Survey illustrate that, between 1966 and 1993, Henslow's sparrows have declined 91% rangewide, grasshopper sparrows have been reduced by 66%, and dickcissels by 39% (Fig. 1). Henslow's sparrows and grasshopper sparrows are among the fastest declining North American songbirds (Peterjohn et al. 1994). The Henslow's sparrow has more specialized habitat and management needs (Kahl et al. 1985; Zimmerman 1988; Smith and Smith 1992) than the other two species. The grasshopper sparrow, which is rarer and declining more rapidly than the dickcissel, seems to be the more sensitive of the two species to habitat changes and management methods (Skinner et al. 1984; Smith and Smith 1992; Zimmerman 1992).

Records indicate that these sparrows had declined considerably even before the Breeding Bird Survey began. Nineteenth-century sources summarized by Herkert (1994) reveal that Henslow's sparrow was at that time one of the most abundant birds in Illinois. Much of the Henslow's sparrow population losses actually occurred decades before Breeding Bird Survey monitoring began (Graber and Graber 1963; Herkert 1994), with a decline of 90% occurring between 1958 and 1979 in Illinois (Illinois Natural History Survey 1983). These three sparrow species have all undergone long-term range reductions in the eastern parts of their breeding ranges (Fretwell 1973; Smith and Smith 1992). The range of Henslow's sparrow is also contracting from the north (McNicholl 1988), and numerous local populations have disappeared in recent decades (Hands et al. 1989; Illinois Natural History Survey 1983).

Prairie Butterflies

No long-term monitoring program comparable to the Breeding Bird Survey is available for butterflies, and population dynamics are not as well documented for prairie butterflies as for birds. However, butterflies requiring prairie habitat have clearly experienced long-term declines, both along the fringes of their core ranges in the central United States and within the prairie province. The extinction wave of the regal fritillary (Fig. 2) from east to west and the species' increasingly localized occurrence within the prairie region are well documented (Swengel 1993). The Dakota skipper and the Poweshiek skipperling (Fig. 3) have also become more localized and restricted to prairie fragments (Opler and Krizek 1984; Johnson 1986).

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.

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).

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.