Fire Effects on Fauna in Dry Prairies
In contrast to the effects of fire on vegetation, the effects of fire on the faunal component of dry prairie is largely unknown. However, most of the faunal taxa should be well adapted to the natural fire regime. In particular, those which are endemic to the dry prairie region of south-central Florida have evolved within a high fire frequency ecosystem. In fact, some if not most prairie faunal taxa that are dependent on dry prairie also appear to be dependent on recently burned sites (i.e., Florida grasshopper sparrow, see Walsh et al. 1995), or prefer recently burned sites over fire-suppressed dry prairies.
It had been thought that ground-nesting birds might be susceptible to mortality or lower reproductive success due to growing season prescribed burning (T. Dean, Department of Forestry and Wildlife, University of Massachusetts, personal communication 1998). The Florida grasshopper sparrow, Bachman’s sparrow and the eastern meadowlark are all ground-nesting birds of dry prairie. All initiate breeding in mid-March through April (Perkins et al. 1998), and have relatively short reproductive cycles (30 to 40 days from egg-laying through fledging). It had been hypothesized that natural frequent late spring and early summer lightning fires, now simulated with prescribed burns during the growing season from late March through early June, would destroy some nests and young birds, and that this would result in significant reduction in reproductive success and population levels. However, all three of these birds regularly re-nest if nests are destroyed (T. Dean, Department of Forestry and Wildlife, University of Massachusetts, personal communication 1998). In addition, multiple successful clutches are possible for all three taxa (T. Dean, Department of Forestry and Wildlife, University of Massachusetts, personal communication 1998). Recent studies have shown that adult Florida grasshopper sparrows and other ground-nesting avian taxa appear to be able to successfully avoid fires, and can occupy recently burned areas within 1 week after fire (Shriver et al. 1994, Shriver 1996). Although some nests and young birds could be lost to fire, growing season burning does not preclude successful reproduction in that season for any of these birds. Recent studies have shown that Florida grasshopper sparrows successfully fledge young after June fires (Vickery and Perkins 1997), and increase the length of the breeding season by as much as 8 weeks, extending as late as September, after these fires (Shriver 1996, Vickery et al. 1997). However it is still unknown whether reproductive success differs between summer-burned and winter-burned areas (FWS 1997).
Several factors suggest that growing season burns may benefit the Florida grasshopper sparrow. These sparrows preferentially nest in areas less than 1.5 years post-burn (Walsh et al. 1995, Shriver 1996). If such habitat patches were available within a mosaic of areas with differing post-burn times, they would most likely be the areas utilized for nesting. Prescribed burning during the nesting season would most likely be conducted on areas of 2 years or more post-burn, therefore the areas least likely to be currently utilized for nesting. Therefore, as long as a sufficient area less than 1.5 years post-burn is present within the population area, nesting season prescribed burning of adjacent areas should destroy relatively few nests. Those disrupted should renest and the extension of the breeding season by prescribed burning during the nesting season may have important consequences for a species that appears to have low reproductive success (Shriver 1996). Lengthening the breeding season by provision of newly burned areas effectively doubles the number of nesting opportunities, may increase the number of pairs that attempt additional clutches, and may provide sub-dominant males the opportunity to establish territories and breed for the first time (Shriver 1996). Further research is needed to test these new hypotheses.
The long-term effects of growing season fires on the overall productivity of these ground-nesting sparrows are largely unknown (T. Dean, Department of Forestry and Wildlife, University of Massachusetts, personal communication 1998). However, neither is it known how the traditional winter burning of dry prairie by the cattle ranchers and some current land managers may have affected the long-term population levels of such ground-nesting birds. Larger prairie birds, including sandhill cranes and caracaras, would possibly have greater potential to be affected by growing-season burns, since they require longer periods to produce young, 30 days incubation in cranes (Walkinshaw 1981) and 30 to 33 days for caracaras (Layne 1996, Morrison 1996). However re-nesting is still common in both species (Nesbitt 1988, Morrison 1996), and loss of nests would not preclude nesting in either bird. In addition, the breeding cycles of both species are in the dry season, before the peak time for natural growing season fires. Since caracaras nest in trees and cranes nest in wetlands, nest losses due to fire may be minimal. However, the indirect effects of fire on overall productivity are unknown.
The effects of fire on other prairie occupants are also largely unknown. Armadillo and gopher tortoise burrows and the numerous depressional ponds characteristic of the dry prairie landscape likely provide refuge from fires for herpetofauna and small mammals (T. Dean, Department of Forestry and Wildlife, University of Massachusetts, personal communication 1998).
The uncertainty of the effects of growing season burning on the Florida grasshopper sparrow has caused land managers to shift prescribed burns from optimal late spring and early summer burns (especially April and May). The acceptable burning seasons therefore are pre-avian breeding, dormant season (i.e. winter) burns (January to early March), or postponement of prescribed burning until post-avian breeding (July through September). Winter or dormant season burns generally result in less overall reduction of aboveground woody biomass, and reduction in flowering of grasses and other native forbs. Prescribed burning conducted during the post-breeding season (July through September), after the onset of summer convective thunderstorms, has the potential to cause significant shifts in species composition in dry prairie. Heavy rainfall events, which are typically more frequent from mid-June through the summer months, can produce two dramatic effects on vegetation in areas of post-breeding season burns: (1) more patchy burns (resulting in less overall reduction of woody vegetation) and (2) flooding of postburn regrowth, resulting in submersion of regrowth and stress to perennial groundcover grasses. The result is that annual cyperoids, such as Scleria reticularis (nut-rush), present in the seed bank are favored over perennial grasses and can locally co-dominate summer-burned areas (Orzell and Bridges, personal field observations). Whether this vegetative response is short-term or long-term is unknown, but it needs to be studied.
Encyclopedia ID: p188
