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

Abstract

“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 and abiotic limiting factors. Then, through comparisons and applications 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 population-, community-, and ecosystem-level processes while keeping the research focused on ecological factors that directly or indirectly affect the size of a focal population. In the case of the endangered Red-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 shortage of suitable cavity trees, (3) loss and fragmentation of habitat, and (4) demographic isolation. As part of the research to identify other potentially limiting environmental factors in the Apalachicola National Forest of northern Florida, we studied a sample of 87 social units (each unit usually a mated pair of birds with or without helpers, but sometimes a single bird). Each unit was defending a cluster of cavity trees and a foraging territory of open longleaf pine (Pinus palustris) 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 significantly related to the ground cover composition and the extent of natural pine regeneration, both of which are indirect indicators of local fire history. This suggests that, in addition to the four main causes, environmental processes driven by the history of fire are also limiting the Red-cockaded Woodpecker population. Additional support for this idea comes from the fact that female Red-cockaded Woodpeckers on the Apalachicola Ranger District tend to lay larger clutches of eggs in the first breeding season after their territories have been burned. Because fire history affects soil nutrient dynamics, which in turn affect ground cover composition, our present hypothesis is that nutrient dynamics are affecting the health of animal populations in the system, including that of the Red-cockaded Woodpecker. The path by which this process operates, the particular nutrients involved, and its importance relative to other factors that limit the population need to be addressed experimentally. If nutrient dynamics are a previously unrecognized limiting factor for animal populations in this ecosystem, then the role of fire is not restricted to its ability to reduce vegetation in the midstory, and managers should acknowledge that different regimes of prescribed fire are likely to have different effects on animal, as well as plant, populations.