Wildfire, climate, and invasive grass interactions negatively impact an indicator species by reshaping sagebrush ecosystems

Abstract

Iconic sagebrush ecosystems of the American West are threatened by larger and more frequent wildfires that can kill sagebrush and facilitate invasion by annual grasses, creating a cycle that alters sagebrush ecosystem recovery post disturbance. Thwarting this accelerated grass–fire cycle is at the forefront of current national conservation efforts, yet its impacts on wildlife populations inhabiting these ecosystems have not been quantified rigorously. Within a Bayesian framework, we modeled 30 y of wildfire and climatic effects on population rates of change of a sagebrush-obligate species, the greater sage-grouse, across the Great Basin of western North America. Importantly, our modeling also accounted for variation in sagebrush recovery time post fire as determined by underlying soil properties that influence ecosystem resilience to disturbance and resistance to invasion. Our results demonstrate that the cumulative loss of sagebrush to direct and indirect effects of wildfire has contributed strongly to declining sage-grouse populations over the past 30 y at large spatial scales. Moreover, long-lasting effects from wildfire nullified pulses of sage-grouse population growth that typically follow years of higher precipitation. If wildfire trends continue unabated, model projections indicate sage-grouse populations will be reduced to 43% of their current numbers over the next three decades. Our results provide a timely example of how altered fire regimes are disrupting recovery of sagebrush ecosystems and leading to substantial declines of a widespread indicator species. Accordingly, we present scenario-based stochastic projections to inform conservation actions that may help offset the adverse effects of wildfire on sage-grouse and other wildlife populations.

Greater sage‐grouse vital rates after wildfire

“…This continued use of the landscape affected by the Holloway fire appeared to have an acute fitness cost in terms of daily nest survival and adult survival during the first 2 years post‐fire. Thus, reduced vital rates likely contribute to observed declines in sage‐grouse population trend following wildfire (Coates et al ).…”

“…This dependence on sagebrush is acute during the nesting period (Connelly et al ), with most nests located under sagebrush shrubs (Patterson , Popham and Gutiérrez , Connelly et al ). Thus, removal of sagebrush at broad scales can affect vital rates and population persistence (Swenson et al , Coates et al ).…”

“…However, wildfires differ from prescribed fires in timing, intensity, and size and may affect sage‐grouse populations more severely (Slater , Baker ). Recently, researchers have examined the demographic response of sage‐grouse to fire disturbance, and observed generally negative effects of large wildfire or prescribed fire on spring lek counts, recruitment rates, rate of population change (Blomberg et al , Connelly et al , Coates et al ), and sage‐grouse survival (Lockyer et al ).…”

Broad‐scale occurrence of a subsidized avian predator: Reducing impacts of ravens on sage‐grouse and other sensitive prey

“…The anthropogenic influence index can also be used to identify areas where targeted management actions, particularly those that reduce access to resource subsidies, could be most effective. To explore this concept, we provide an applied example using existing information about spatial variation in sage‐grouse habitat suitability and breeding populations within our study area (sage‐grouse concentration areas; Coates, Ricca, et al., ; Doherty et al., ). First, we reclassified predicted raven occurrence into four classes (low to high probability).…”

“…First, we reclassified predicted raven occurrence into four classes (low to high probability). We then intersected these classes with sage‐grouse concentration areas (available in Coates, Ricca, et al., ) to obtain a relative index of the potential for ravens to impact breeding sage‐grouse populations. We sought to identify regions within sage‐grouse concentration areas where (A) raven mitigation actions would be best suited to sage‐grouse habitat improvements (e.g., raven occurrence driven primarily by natural effects), as opposed to (B), areas where management actions could effectively target anthropogenic subsidies (e.g., raven occurrence strongly influenced by anthropogenic effects).…”