SPECIES-CENTERED ENVIRONMENTAL-ANALYSIS - INDIRECT EFFECTS OF FIRE HISTORY ON RED-COCKADED WOODPECKERS

''Species-centered Environmental analysis'' (SCEA) is a procedure for diagnosing species-specific environmental factors that limit the size of a population. It attempts to identify presently recognized biotic a nd abiotic limiting factors. Then, through comparisons and application s of the principles of experimental design, it evaluates the relative importance of the factors and searches for new ones. The advantage of SCEA is that it frames ecological hypotheses in a context that spans p opulation-, community-, and ecosystem-level processes while keeping th e research focused on ecological factors that directly or indirectly a ffect the size of a focal population. In the case of the endangered Re d-cockaded Woodpecker (Picoides borealis), which lives in mature pine forests of the southeastern United States, four types of environmental factors have been shown to limit its numbers, even on public land: (1 ) insufficient habitat due to hardwood midstory encroachment, (2) a sh ortage of suitable cavity trees, (3) loss and fragmentation of habitat , and (4) demographic isolation. As part of the research to identify o ther potentially limiting environmental factors in the Apalachicola Na tional Forest of northern Florida, we studied a sample of 87 social un its (each unit usually a mated pair of birds with or without helpers, but sometimes a single bird). Each unit was defending a cluster of cav ity trees and a foraging territory of open longleaf pine (Pinus palust ris) forest. We then developed regression models for predicting within -population variation in the size, density, and productivity of social units from data on habitat variation. We found that variation in the bird variables was not significantly related to the sizes or densities of pine trees in these territories. It was, however, highly significa ntly related to the ground cover composition and the extent of natural pine regeneration, both of which are indirect indicators of local fir e history. This suggests that, in addition to the four main causes, en vironmental processes driven by the history of fire are also limiting the Red-cockaded Woodpecker population. Additional support for this id ea comes from the fact that female Red-cockaded Woodpeckers an the Apa lachicola Ranger District tend to lay larger clutches of eggs in the f irst breeding season after their territories have been burned. Because fire history affects soil nutrient dynamics, which in turn affect gro und cover composition, our present hypothesis is that nutrient dynamic s are affecting the health of animal populations in the system, includ ing that of the Red-cockaded Woodpecker. The path by which this proces s operates, the particular nutrients involved, and its importance rela tive to other factors that limit the population need to be addressed e xperimentally. If nutrient dynamics are a previously unrecognized limi ting factor for animal populations in this ecosystem, then the role of fire is not restricted to its ability to reduce vegetation in the mid story, and managers should acknowledge that different regimes of presc ribed fire are likely to have different effects on animal, as well as plant, populations.