Non‐native grazers affect physiological and demographic responses of greater sage‐grouse

Abstract

Non‐native ungulate grazing has negatively impacted native species across the globe, leading to massive loss of biodiversity and ecosystem services. Despite their pervasiveness, interactions between non‐native grazers and native species are not fully understood. We often observe declines in demography or survival of these native species, but lack understanding about the mechanisms underlying these declines. Physiological stress represents one mechanism of (mal)adaptation, but data are sparse. We investigated glucocorticoid levels in a native avian herbivore exposed to different intensities of non‐native grazing in the cold desert Great Basin ecosystem, USA. We measured corticosterone, a glucocorticoid in feathers for a large sample (n = 280) of female greater sage‐grouse (Centrocercus urophasianus) from three study areas in Northern Nevada and Southern Oregon with different grazing regimes of livestock and feral horses. We found that greater feral horse density was associated with higher corticosterone levels, and this effect was exacerbated by drought conditions. Livestock grazing produced similar results; however, there was more model uncertainty about the livestock effect. Subsequent nesting success was lower with increased feather corticosterone, but corticosterone levels were not predictive of other vital rates. Our results indicate a physiological response by sage‐grouse to grazing pressure from non‐native grazers. We found substantial among‐individual variation in the strength of the response. These adverse effects were intensified during unfavorable weather events, highlighting the need to reevaluate management strategies in the face of climate change.

Climate and non‐native herbivores influence reproductive investment by Greater Sage‐grouse

“…Female sage-grouse initiated nests at lower rates when livestock were more abundant and precipitation was low during the previous year. We report elsewhere that a proportion of the study population expressed elevated levels of corticosterone in their feathers when livestock use was high and precipitation was low (Behnke et al, 2022). While there is a several-month lag between the growth of new feathers and the next breeding season, it is possible that carryover effects from the physiological state of females during feather growth influenced their propensity to breed the next spring Fairhurst et al, 2017;Hansen et al, 2016).…”

Greater sage‐grouse chick mortality risk relative to livestock grazing management, environmental factors, and morphometric measurements

“…Several studies across the sage‐grouse range have been initiated to evaluate grazing effects on sage‐grouse vital rates (Cutting et al 2019, Conway et al 2021, Behnke et al 2022). No studies to date have investigated the relationships between grazing and chick survival or mortality risk.…”