- The expansion of native, woody plants is a global phenomenon with characteristics and effects that are often indistinguishable from exotic invasions. These expansions have largely been driven by altered fire regimes and favorable climatic conditions. In the Great Basin of western North America, expansion of conifers such as western juniper (Juniperus occidentalis) is a considered a primary threat to sagebrush (Artemisia spp.) ecosystems. The greater sage-grouse (Centrocercus urophasianus; sage-grouse), a sagebrush obligate gallinaceous bird, utilizes sagebrush during every phase of its lifecycle. Sage-grouse have declined across their range and currently occupy approximately 56% of their pre-European settlement distribution. Management agencies are actively removing conifers that established after European settlement (~1870) in the Great Basin to restore sagebrush ecosystem function and benefit species such as sage-grouse that are inextricably tied these landscapes. The literature on the response of sage-grouse to these broad-scale management actions is limited. My study utilized a “before-after-control-impact” study design to investigate the response of sage-grouse populations, habitat selection, and habitat connectivity to conifer removal. I was able to build upon a radio telemetry data set that began in 2010 and with my work included a total sample of 417 females, their nests (n = 378), and broods (n = 233) for demographic and resource selection modeling.
In Chapter 2, I developed a hierarchical population model in a Bayesian framework that integrated telemetry data and lek (i.e., breeding arena) count data to estimate sage-grouse vital rates and population growth rates in a treatment area with conifer removal (Treatment) and a control area (Control). The model indicated that dynamics in population growth rates (λ) approximately tracked each other in the Treatment and Control. However, starting in 2013 (conifer removals initiated in 2012), λ in the Treatment steadily increased relative to the Control and was 11.2% higher by 2017. This trend was driven by increases in juvenile, adult, first nest, and yearling survival in the Treatment relative to the Control. These findings indicated that conifer removal is an effective technique for restoring sagebrush ecosystem function and increasing sage-grouse population growth rates.
In Chapter 3, I estimated resource selection functions for sage-grouse nest site and breeding season habitat selection. Conifer cover and conifer removal variables were among the most influential predictors of nest site and breeding season habitat selection. Sage-grouse selected nest sites in and near older conifer removal areas and were 16% less likely to nest in an area for every 1% increase in conifer cover within 400 m of the nest. During the breeding season, sage-grouse selected habitat closer to conifer removals and were 26% more likely to use a removal each year post-removal (1–5 years post-removal). By 2017, 75% of the treatment area was in the medium-high and high probability of use categories resulting from conifer removal, up from 49% in 2010. These findings demonstrated the efficacy of conifer removal for increasing usable space for sage-grouse in landscapes affected by conifer expansion.
In Chapter 4, I modeled the landscape connectivity in the Treatment with resistance surfaces generated with a resource selection function. The change in resistance resulting from conifer removal was quantified within the habitat utilized by sage-grouse, which was modeled with Brownian bridge movement models and straight-line, movement pathways. Regardless of the habitat use metric used, there was a reduction in landscape resistance in the areas used by sage-grouse in the post-conifer removal period. When comparing areas used by females with broods and those without, breeding season home ranges of females without broods experienced greater increases (≤81% increase) in connectivity after conifer removal. These differences in landscape resistance held when comparing females with broods and those without in a given post-removal year. These findings indicated that conifer removal is an effective method of restoring landscape connectivity in landscapes affected by post-European settlement expansion of conifers. The difference in benefits for females with and without broods may have demographic implications and needs further investigation.
I sought to evaluate the response of sage-grouse to landscape-scale conifer removal. Collectively, I demonstrated increased population growth rates, usable space, and landscape connectivity resulting from conifer removal. While the extent of sagebrush ecosystems affected by conifer expansion in the Great Basin and the larger sage-grouse range is immense, these findings indicated that targeted conifer removal is an effective management technique to improve the ecosystem function of these landscapes and benefit sage-grouse and likely other sagebrush obligates.