Landscape and local effects on occupancy and densities of an endangered wood-warbler in an urbanizing landscape

Abstract

Context Golden-cheeked warblers (Setophaga chrysoparia), an endangered wood-warbler, breed exclusively in woodlands co-dominated by Ashe juniper (Juniperus ashei) in central Texas. Their breeding range is becoming increasingly urbanized and habitat loss and fragmentation are a main threat to the species' viability.Objectives We investigated the effects of remotely sensed local habitat and landscape attributes on point occupancy and density of warblers in an urban preserve and produced a spatially explicit density map for the preserve using model-supported relationships. Methods We conducted 1507 point-count surveys during spring 2011-2014 across Balcones Canyonlands Preserve (BCP) to evaluate warbler habitat associations and predict density of males. We used hierarchical Bayesian models to estimate multiple components of detection probability and evaluate covariate effects on detection probability, point occupancy, and density. Results Point occupancy was positively related to landscape forest cover and local canopy cover; mean occupancy was 0.83. Density was influenced more by local than landscape factors. Density increased with greater amounts of juniper and mixed forest and decreased with more open edge. There was a weak negative relationship between density and landscape urban land cover. Conclusions Landscape composition and habitat structure were important determinants of warbler occupancy and density, and the large intact patches of juniper and mixed forest on BCP ([2100 ha) supported a high density of warblers. Increasing urbanization and fragmentation in the surrounding landscape will likely result in lower breeding density due to loss of juniper and mixed forest and increasing urban land cover and edge.

Density and nest survival of golden‐cheeked warblers: Spatial scale matters

“…Arguably we did not monitor nests across a wide enough range of landscapes to observe the strong fragmentation or edge effects observed for songbirds elsewhere in the Midwest (Robinson et al , Thompson et al ). Our intensive monitoring plots were generally representative of the BCP, with the only noticeable differences being that plots had slightly more canopy cover and taller canopy height at the local forest scale, and more urban land cover in the surrounding landscape (Reidy et al ; Table ). Interestingly, within these monitoring plots, warbler nests tended to be in taller trees, in areas of more mixed and juniper woodland at the 100‐m and 1‐km scale, and in areas of less open edge and urban land cover compared to plot means (Reidy et al ).…”

“…We derived measures of composition and structure at a forest (100‐m radius) and landscape scale (1‐km radius) around each survey point and nest using ArcMap 10.0 (ESRI). We created a finer resolution and more up‐to‐date map of vegetation cover by intersecting the Texas Ecological Systems phase 1 (TESP1) vegetation classification with a light detection and ranging (LiDAR)‐based map of vegetation height (Reidy et al ). The TESP1 vegetation classification was a 10‐m resolution vegetation classification developed for central Texas derived from satellite imagery and slope, aspect, landscape position, hydrology, and soil type data (Elliott et al ).…”

“…Because the hazard‐rate and half‐normal key functions received almost identical support (ΔAIC c = 0.016), we then compared support for 16 models comprised of all combinations of the variables observer, slope, and total tree basal area, as well as an intercept‐only (null) model, using the hazard‐rate and half‐normal key functions (8 models each). We predicted the observer would affect detection probability because observer is a well‐documented source of detection variation (Peak and Thompson , Reidy et al ). We predicted that detection may be inversely related to tree volume and slope steepness because bird song likely carries farther in areas of lower tree volume and across open space.…”

“…Fish and Wildlife Service [USFWS] 1992), a region that historically contained some of the largest and most contiguous patches of juniper‐oak woodland (Wahl et al , Groce et al , Duarte et al ). Although the majority of the BCP was juniper or mixed juniper‐oak woodland (City of Austin et al ), perceived woodland quality varied across the preserve, as measured by warbler density; density was highest in the northeastern portion of the BCP and lowest in the southwest (Reidy et al ). Dominant tree species included Ashe juniper, Texas red oak ( Q. buckleyi ), plateau live oak ( Q. fusiformis ), shin oak ( Q. sinuata var.…”

“…Warbler habitat is generically described as juniper‐oak woodland, but there is substantial variation within woodland vegetation structure and composition that may affect perceived or realized habitat quality as measured by density or nest (or territory) success (Peak and Thompson , ; Stewart et al ; Reidy et al ; Sesnie et al ). Previous studies focused separately on relationships between vegetation structure or woodland characteristics on warbler occupancy (Magness et al , Farrell et al ), density (Peak and Thompson , Reidy et al , Sesnie et al ), and productivity (Peak and Thompson ), and only one investigated multiple spatial scales with fine‐scale and coarse‐scale vegetation, and terrain data (DeBoer and Diamond ). Collectively these studies highlight the conservation value of contiguous mature woodland to occupancy, density, and nest success, but they do not provide the level of inference that can be gained by simultaneously considering both density and nest survival at multiple spatial scales.…”