Group size mediates effects of intraspecific competition and forest structure on productivity in a recovering social woodpecker population
| Authors: | J. E. Garabedian, C. E. Moorman, M. N. Peterson, J. C. Kilgo |
| Year: | 2021 |
| Type: | Scientific Journal |
| Station: | Southern Research Station |
| DOI: | https://doi.org/10.1111/acv.12757 |
| Source: | Animal Conservation |
Abstract
Conservation of endangered social wildlife in disturbance-prone forests is challengingbecause direct and indirect effects of management strategies developed at thetime of species’ listing when population density is low may change under highdensityconditions in locally recovered populations. Here, we used piecewise structuralequation modeling to evaluate direct and indirect drivers of productivity inthe federally endangered cooperatively breeding red-cockaded woodpecker Dryobates borealis (RCW) on Savannah River Site, South Carolina, USA. We estimateddirect and indirect relationships among group size, neighboring group sizes,fledgling production, density of cavity tree clusters occupied by RCWs, area satisfying threshold criteria of ≥22 stems ha1 of pines ≥35.6 cm diameter at breastheight (dbh), <1.4 m2 ha1 basal area (BA) of hardwoods 7.6–22.9 cm dbh, and<6% hardwood canopy cover, and area treated with prescribed fire, and testedwhether group size mediated indirect effects of area satisfying threshold criteria onfledgling production. Increases in area with ≥22 stems ha1 of pines ≥35.6 cm dbhand <1.4 m2 ha1 BA of hardwoods 7.6–22.9 cm dbh, and area treated withprescribed fire, but not area with <6% hardwood canopy cover, had direct positiveeffects on group size. Group size and area treated with prescribed fire, but not areasatisfying threshold criteria, had direct positive effects on fledgling production. Thedirect effect of neighboring group sizes on fledgling production was negative andsmaller relative to the direct positive effect of group size on fledgling production.Overall, our results indicate positivedirect effects of group size on fledgling productionoutweighed negative direct effects of neighboring group sizes, and that group size mediated positive indirect effects of area satisfying structural thresholdcriteria on fledgling production. These findings indicate that ongoing forest managementaimed to increase area with ≥22 pines ha1 ≥35.6 cm dbh and <1.4m2 ha1 BA of hardwoods 7.6–22.9 cm dbh will promote large group sizes, whichin turn improve fledgling production and offset costs of heightened competitionwith neighboring groups under high-density conditions. Additionally, positiveeffects of area treated with prescribed fire on RCW group size and fledgling productionindicate prescribed fire has unique contributions to woodpecker productivity,likely via direct effects on forest structure and potentially indirect effects onarthropod prey available to foraging RCWs. By simultaneously accounting for multipledrivers of productivity in social wildlife, our study contributes to the understandingof how increases in social wildlife population sizes can alter previously documented habitat-fitness relationships.