Estimating Species Interactions in a Woodpecker Tree-Hole Community at the Individual, Population, and Community Levels

The endangered red-cockaded woodpecker (Picoides borealis) is a keystone species in the southeastern United States where it excavates tree holes in living pines trees. An understanding of the interactions among the species using the tree holes may indicate whether they affect red-cockaded woodpeckers negatively. In the Apalachicola National Forest of northern Florida, I conducted a series of experiments at the individual, population, and community level to determine the nature of these interactions. Short-term interactions at the individual level were quantified by removing red-cockaded woodpeckers and red-bellied woodpeckers (Melanerpes carolinus), the two numerically dominant species in the assemblage and by temporarily occluding tree holes. Reciprocal effects of each species on the other were demonstrated over periods as short as 48 hours. Surprisingly, at the population level, red-cockaded woodpecker group size and female persistence were reduced in areas where red-bellied woodpeckers were removed. The immigration rate of floater red-bellied woodpeckers was inadvertently increased and thereby increased the disruption within red-cockaded woodpecker family groups. At the community level, Markov models were used to measure the interaction among members of the species assemblage. Time and location did not affect the matrix transition probabilities. In a district of the forest where the red-cockaded woodpecker population is declining, the removal of red-bellied woodpeckers allowed the population of flying squirrels (Glaucomys volans) to increase. In a district where the red-cockaded woodpecker is stable, the removal of red-bellied woodpeckers did not have any effect. When matrices were projected at equilibrium, 50% more red-cockaded woodpeckers and 50% less red-bellied woodpeckers were predicted in the stable population than in the declining population. The interaction of both red-bellied woodpeckers and flying squirrels with red-cockaded woodpeckers is probably contributing to the decline of red-cockaded woodpeckers in parts of the Apalachicola National Forest and possibly elsewhere. This study underscores the need for a combination of empirical approaches that allow investigators to identify mechanisms of species interactions at the individual level, the fitness effects at the population level, and the overall effects on the assemblage at the community level.