Cooperative Fish and Wildlife Research Units Program: Montana Cooperative Wildlife Research Unit
Education, Research and Technical Assistance for Managing Our Natural Resources

Summary. 1. Resource selection specialization may increase vulnerability of populations to environmental change. One environmental change that may negatively impact some populations is the broad decline of quaking aspen (Populus tremuloides), a preferred nest tree of cavity-nesting organisms who are commonly limited by nest-site availability. However, the long-term consequences of this habitat change for cavity-nesting bird populations are poorly studied. 2. I counted densities of woody plants and eight cavity-nesting bird species (six woodpeckers, Mountain Chickadee Poecile gambeli, House Wren Troglodytes aedon) over 29 years in 15 high elevation riparian drainages in Arizona, USA. I also studied nest tree use and specialization over time based on 4946 nests across species. 3. Aspen suffered a severe decline in availability over time, while understory woody plants and canopy canyon maple (Acer grandidentatum) also declined. The decline of understory plants resulted from increased elk (Cervus canadensis) browsing linked to declining snowfall, and the lack of tree recruitment caused the decline in canopy aspen and maple. 4. All six woodpecker species exhibited very high specialization (>95% of nests) on aspen for nesting, and densities of all species declined with aspen over time. Mountain Chickadees and House Wrens exhibited increasingly less specialization on aspen. Chickadees strongly increased in density over time, despite a relatively high specialization on aspen. Nest sites clearly were not limiting their population. House Wren densities declined modestly over time, but nest box addition experiments demonstrated that nest-site availability was not limiting their population. House Wren densities increased with understory vegetation recovery in elk exclosures compared with controls, demonstrating that the decline in understory vegetation on the broader landscape was the cause of their population decline. The increased densities in exclosures were facilitated by increased generalization in nest site use. 5. Synthesis and applications: Management should target species that specialize in resource selection on a declining resource, because these species are vulnerable to population problems. Species with greater resource selection generalization can reduce population impacts of environmental change. Resource generalization can allow a species, like the wren, to take advantage of a habitat refuge, such as provided by the elk exclosures. Yet, when habitat on the broader landscape is declining in quality, or higher quality refuges are lacking, then resource generalization cannot offset the broader negative changes, as demonstrated by the decline in wrens on the broader landscape. Ultimately, aspen is an important habitat for biodiversity, and land management programs need to protect and aid recovery of aspen habitats.