Occupancy modeling of Black-backed Woodpeckers on burned Sierra Nevada forests

Abstract

Abstract. The Black-backed Woodpecker (Picoides arcticus) has been designated by the USDA Forest Service as a management indicator species for snags in burned conifer forests of the Sierra Nevada of California, USA. However, little is known about the characteristics that affect between-fire and within-fire habitat selection by the species in the region. Here we report on the first broad-scale survey of Black-backed Woodpeckers on wildfire-affected forests of the Sierra Nevada. We implemented a Bayesian hierarchical model to: 1) estimate Black-backed Woodpecker occupancy probability in fire areas burned within a time window of 1-10 years; 2) identify relationships between occupancy probability and habitat covariates (fire age, change in canopy cover pre-to-post fire, and snag basal area), elevation, and latitude; and 3) estimate detection probability and relate it to survey interval length and survey type (passive v. broadcast). We included random fire-area effects in our model of occupancy probability to accommodate clusters of nonindependent points surveyed within the larger set of fire areas. Mean occupancy probability was estimated to be 0.097. Elevation (after controlling for latitude) had the strongest effect on occupancy probability (higher occupancy at higher elevation) followed by latitude (higher occupancy at northerly sites). Fire age was also important; occupancy probability was about 43 higher on the youngest compared to oldest fires. Although the direction of regression coefficients were in the expected direction (positive), snag basal area and canopy cover change were of minor importance in affecting occupancy probability. There was some indication, however, that the importance of snag basal area increased with fire age. Weak links between occupancy and canopy cover change suggested the species uses a range of burn severities, and such heterogeneity may promote habitat longevity. Our estimate of overall detection probability (across all survey intervals) was 0.772. We found strong effects of survey interval length (higher for longer interval) and, especially survey type (higher for broadcast survey) on detection probability. Our modeling framework and implementation illustrates the flexibility of the Bayesian hierarchical approach for handling complexities such as estimating derived parameters (and variances) and modeling random effects, and should prove generally useful for occupancy studies.

Occurrence patterns of Black-backed Woodpeckers in green forest of the Sierra Nevada Mountains, California, USA.

“…Playbacks were broadcast at a standardized volume (90 db) using FOXPRO ® ZR2 digital game callers (FOXPRO Inc., Lewistown, Pennsylvania, USA). Playback surveys have been shown to significantly increase detection probability for this species compared to individual passive point count surveys (Saracco et al 2011). Playback surveys were only conducted once per transect visit after all passive point count surveys were completed to avoid influencing detection probability on passive surveys via individuals drawn toward the broadcast from large distances away.…”

“…Throughout its range, it occurs at highest densities in conifer forests that have recently burned (within 1-5 years) at high severity (Murphy and Lehnhausen 1998, Hutto 2008, Saracco et al 2011. In western North America, especially in the Sierra Nevada and Rocky Mountain ranges, research and management has primarily focused on burned forest habitat (Dixon and Saab 2000, U.S. Forest Service 2007, Hutto 2008, Saracco et al 2011, Bond et al 2012. Their status and occurrence patterns in Sierra Nevada green forest are unknown, although small numbers of nesting pairs have been documented outside of areas that recently burned at moderate or high severity (Raphael and White 1984).…”

Pyrodiversity promotes avian diversity over the decade following forest fire

“…Variable selection was conducted a priori based on prior experience modelling species occurrence over a wide variety of post-fire forest conditions in the Sierra Nevada [41] as well as focused studies of habitat usage by birds in other western post-fire ecosystems (e.g. [42]).…”

“…Using previous work in this system as a baseline [41], we modelled detectability as a function of three covariates:…”

Seeking parsimony in landscape metrics

“…Specifically, we included a random effect of the patch in which the site was located, because some patches contained .1 sampling location. This allowed us to separate variability in the response due to a site being in a different patch from the variability in the characteristics of that site (Saracco et al 2011). To determine the relative strength of covariate relationships, we defined a significance threshold using the 95% credible interval (CRI) of posterior distributions.…”