Quantifying Purported Competition with Individual- and Population-Level Metrics

Abstract

Competitive species interactions may contribute to population declines. Purportedly, Red-bellied Woodpeckers (Melanerpes carolinus), a common species, and Red-cockaded Woodpeckers (Picoides borealis), an endangered species, compete for roosting and nesting cavities in living pine trees. To determine whether behavioral interactions measured at the individual level manifest themselves at the population level, we conducted field experiments designed to test whether the presence of Red-bellied Woodpeckers resulted in a decrease in fitness to Red-cockaded Woodpeckers. As part of a 4-year study examining the nature of interspecific interactions in two populations of Red-cockaded Woodpeckers (one stable, the Apalachicola Ranger District; one declining, the Wakulla Ranger District) in the Apalachicola National Forest, Florida, we conducted a set of Red-bellied Woodpecker removal experiments. Paradoxically, following the removal of Red-bellied Woodpeckers, we observed decreases in Red-cockaded Woodpecker group size, proportion of nests that were successful, and proportion of individuals remaining on territories. Removal of Red-bellied Woodpeckers may have exaggerated the immigration rate of Red-bellied Woodpeckers to Red-cockaded Woodpecker territories. The Red-cockaded Woodpeckers in the Apalachicola Ranger District likely can withstand pressure from immigrating Red-bellied Woodpeckers given that their population has remained relatively stable despite the presence of Red-bellied Woodpeckers. A major factor of population persistence in the Wakulla Ranger District was the high turnover rate of adult female Red-cockaded Woodpeckers, a phenomenon that was exacerbated by removal of Red-bellied Woodpeckers. Relying solely on observations of apparently competitive interactions between individuals may not necessarily provide information about population-level outcomes. Paradoxically, removing species that appear to be competitors may harm species of concern.