Erik J. Blomberg

Characteristics of climate and landscape disturbance influence the dynamics of greater sage-grouse populations

Life histories determine how organisms interact with their environment and predict how populations respond to environmental change. Understanding relationships between life histories and environmental variability is therefore crucial for effective conservation under changing environmental conditions. Greater sage-grouse (Centrocercus urophasianus) is a species of major conservation concern in western North America following widespread disturbance of the sagebrush ecosystems to which they are endemic. Using robust design and Pradel capture-mark-recapture models, we evaluated the influence of climatic processes and disturbance associated with post-wildfire exotic grass invasion on annual survival, per-capita recruitment, and population growth of breeding male sage-grouse in eastern Nevada, USA. Climatic processes, indexed by annual rainfall and maximum summertime temperatures, had a strong relationship with recruitment and adult survival, respectively. The range of variation in recruitment during the study was greater than the range of variation in survival, consistent with a life-history strategy that features lengthened lifespan to capitalize on periodically favorable reproductive conditions. Annual variation in precipitation variables (e.g., rainfall or snow depth) explained as much as 75% of the annual variance in population size during the study. Our results are consistent with bottom-up regulation of sage-grouse populations, where abundance is determined in large part by climate-driven variation in resource availability. Exotic grasslands had a negative influence on recruitment that was interactive with annual rainfall; recruitment was consistently low in areas with a substantial exotic grassland footprint even following years of favorable rainfall. We found males breeding at leks with substantial exotic grassland impacts had lower annual survival compared to males at leks surrounded by native sagebrush habitats. However, models containing an interaction between exotic grasslands and maximum summer temperature were not clearly superior to models that considered only additive effects of the two variables. This research has significant implications for sage-grouse persistence in a changing climate, where more frequent drought and increased spread of exotic grasslands will have negative impacts on sage-grouse populations.

Carryover effects and climatic conditions influence the postfledging survival of greater sage‐grouse

Prebreeding survival is an important life history component that affects both parental fitness and population persistence. In birds, prebreeding can be separated into pre- and postfledging periods; carryover effects from the prefledging period may influence postfledging survival. We investigated effects of body condition at fledging, and climatic variation, on postfledging survival of radio-marked greater sage-grouse (Centrocercus urophasianus) in the Great Basin Desert of the western United States. We hypothesized that body condition would influence postfledging survival as a carryover effect from the prefledging period, and we predicted that climatic variation may mediate this carryover effect or, alternatively, would act directly on survival during the postfledging period. Individual body condition had a strong positive effect on postfledging survival of juvenile females, suggesting carryover effects from the prefledging period. Females in the upper 25th percentile of body condition scores had a postfledging survival probability more than twice that (Φ = 0.51 ± 0.06 SE) of females in the bottom 25th percentile (Φ = 0.21 ± 0.05 SE). A similar effect could not be detected for males. We also found evidence for temperature and precipitation effects on monthly survival rates of both sexes. After controlling for site-level variation, postfledging survival was nearly twice as great following the coolest and wettest growing season (Φ = 0.77 ± 0.05 SE) compared with the hottest and driest growing season (Φ = 0.39 ± 0.05 SE). We found no relationships between individual body condition and temperature or precipitation, suggesting that carryover effects operated independently of background climatic variation. The temperature and precipitation effects we observed likely produced a direct effect on mortality risk during the postfledging period. Conservation actions that focus on improving prefledging habitat for sage-grouse may have indirect benefits to survival during postfledging, due to carryover effects between the two life phases.

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

Significance The Great Basin of western North America is larger than 75% of countries worldwide and is comprised mostly of a “sagebrush sea” threatened by a novel disturbance cycle of wildfire and annual grass invasion. The greater sage-grouse is a sagebrush-obligate species whose populations generally track declines in sagebrush, and is highly influential in shaping state and national land-use policy. Using three decades of sage-grouse population count, wildfire, and climate data within a modeling framework that allowed for variable postfire recovery of sagebrush, we provide quantitative evidence that links long-term declines of sage-grouse to chronic effects of wildfire. Projected declines may be slowed or halted by targeting fire suppression in remaining areas of intact sagebrush with high densities of breeding sage-grouse. 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.

Intraseasonal variation in survival and probable causes of mortality in greater sage-grouse Centrocercus urophasianus

The mortality process is a key component of avian population dynamics, and understanding factors that affect mortality is central to grouse conservation. Populations of greater sage-grouse Centrocercus urophasianus have declined across their range in western North America. We studied cause-specific mortality of radio-marked sage-grouse in Eureka County, Nevada, USA, during two seasons, nesting (2008-2012) and fall (2008-2010), when survival was known to be lower compared to other times of the year. We used known-fate and cumulative incidence function models to estimate weekly survival rates and cumulative risk of cause-specific mortalities, respectively. These methods allowed us to account for temporal variation in sample size and staggered entry of marked individuals into the sample to obtain robust estimates of survival and cause-specific mortality. We monitored 376 individual sage-grouse during the course of our study, and investigated 87 deaths. Predation was the major source of mortality, and accounted for 90% of all mortalities during our study. During the nesting season (1 April - 31 May), the cumulative risk of predation by raptors (0.10; 95% CI: 0.05-0.16) and mammals (0.08; 95% CI: 0.03-013) was relatively equal. In the fall (15 August - 31 October), the cumulative risk of mammal predation was greater (M(mam) = 0.12; 95% CI: 0.04-0.19) than either predation by raptors (M(rap) = 0.05; 95% CI: 0.00-0.10) or hunting harvest (M(hunt) = 0.02; 95% CI: 0.0-0.06). During both seasons, we observed relatively few additional sources of mortality (e.g. collision) and observed no evidence of disease-related mortality (e.g. West Nile Virus). In general, we found little evidence for intraseasonal temporal variation in survival, suggesting that the nesting and fall seasons represent biologically meaningful time intervals with respect to sage-grouse survival.

Integrating spatially explicit indices of abundance and habitat quality: an applied example for greater sage-grouse management.

Summary Predictive species distributional models are a cornerstone of wildlife conservation planning. Constructing such models requires robust underpinning science that integrates formerly disparate data types to achieve effective species management. Greater sage‐grouse Centrocercus urophasianus, hereafter ‘sage‐grouse’ populations are declining throughout sagebrush‐steppe ecosystems in North America, particularly within the Great Basin, which heightens the need for novel management tools that maximize the use of available information. Herein, we improve upon existing species distribution models by combining information about sage‐grouse habitat quality, distribution and abundance from multiple data sources. To measure habitat, we created spatially explicit maps depicting habitat selection indices (HSI) informed by >35 500 independent telemetry locations from >1600 sage‐grouse collected over 15 years across much of the Great Basin. These indices were derived from models that accounted for selection at different spatial scales and seasons. A region‐wide HSI was calculated using the HSI surfaces modelled for 12 independent subregions and then demarcated into distinct habitat quality classes. We also employed a novel index to describe landscape patterns of sage‐grouse abundance and space use (AUI). The AUI is a probabilistic composite of the following: (i) breeding density patterns based on the spatial configuration of breeding leks and associated trends in male attendance; and (ii) year‐round patterns of space use indexed by the decreasing probability of use with increasing distance to leks. The continuous AUI surface was then reclassified into two classes representing high and low/no use and abundance. Synthesis and applications. Using the example of sage‐grouse, we demonstrate how the joint application of indices of habitat selection, abundance and space use derived from multiple data sources yields a composite map that can guide effective allocation of management intensity across multiple spatial scales. As applied to sage‐grouse, the composite map identifies spatially explicit management categories within sagebrush steppe that are most critical to sustaining sage‐grouse populations as well as those areas where changes in land use would likely have minimal impact. Importantly, collaborative efforts among stakeholders guide which intersections of habitat selection indices and abundance and space use classes are used to define management categories. Because sage‐grouse are an umbrella species, our joint‐index modelling approach can help target effective conservation for other sagebrush obligate species and can be readily applied to species in other ecosystems with similar life histories, such as central‐placed breeding.

Individual and environmental effects on egg allocations of female Greater Sage-Grouse

ABSTRACT The average number of eggs in a clutch and the size of those eggs play a role in individual fitness. We explored sources of variation in egg allocations of female Greater Sage-Grouse (Centrocercus urophasianus) in the American Great Basin over a 10-yr period, as well as range-wide variation in clutch size, using our data and other published values. We tested for environmental and individual effects on clutch size (n = 390) and egg volume (n = 2,486) in a mixed-modeling framework, with random-effect terms that described variation among individual females (i.e. heterogeneity) and allowed us to calculate repeatability for clutch and egg size. The strongest influence on clutch size was the timing of nest initiation, which varied by as much as 67 days within years and showed a negative linear relationship with clutch size. Once this pervasive effect was accounted for, we also found positive effects of annual precipitation and nest-site elevation. In wetter years and at more productive high-elevation sites, females laid larger clutches, which suggests that some degree of large-scale resource availability affects clutch size. The fixed effects in our models explained ∼34% of the total variance in clutch size, and individual random effects explained an additional 15% (repeatability = 0.15). In contrast to clutch size, little measurable variation in egg volume could be attributed to the fixed effects we considered, and ∼60% of the variance in egg volume was associated with random effects (repeatability = 0.59). Prenesting female body condition influenced clutch size, and this effect was most pronounced for replacement clutches. We found repeatability for clutch and egg size to be within the range of published estimates for other avian taxa. Across studies, mean clutch size increased with latitude, demonstrating that Greater Sage-Grouse follow geographic patterns in clutch size that are consistent with other avian taxa.

Fine-scale genetic structure among greater sage-grouse leks in central Nevada

BackgroundMating systems that reduce dispersal and lead to non-random mating might increase the potential for genetic structure to arise at fine geographic scales. Greater sage-grouse (Centrocercus urophasianus) have a lek-based mating system and exhibit high site fidelity and skewed mating ratios. We quantified population structure by analyzing variation at 27,866 single-nucleotide polymorphisms in 140 males from ten leks (within five lek complexes) occurring in a small geographic region in central Nevada.ResultsLek complexes, and to a lesser extent individual leks, formed statistically identifiable clusters in ordination analyses, providing evidence for fine-scale geographic genetic differentiation. Lek geography predicted genetic differentiation even at a small geographic scale, which could be sharpened by strong site fidelity. Relatedness was also higher among individuals within lek complexes (and leks), suggesting that reproductive skew, where few males participate in most of the successful matings, could also potentially contribute to genetic differentiation. Models incorporating a habitat resistance surface as a proxy for potentially reduced movement due to landscape features indicated that both geographic distance and habitat suitability (i.e. preferred habitat) predicted genetic structure, with no significant effect of man-made barriers to movement (i.e. power lines and roads). Finally, we illustrate how data sets containing fewer loci (<4000) had less statistical precision and failed to detect the full degree of genetic structure.ConclusionOur results suggest that habitat features and lek site geography of sage-grouse shape fine scale genetic structure, and highlight how larger data sets can have increased precision and accuracy for quantifying ecologically relevant genetic structure over small geographic scales.

Observer effects strongly influence estimates of daily nest survival probability but do not substantially increase rates of nest failure in Greater Sage-Grouse

ABSTRACT Researchers have long recognized that the process of observing nests may influence nest success by increasing depredation risk or causing females to abandon nests. The effects of observer-related nest abandonment may also reduce estimates of daily nest survival when time to nest fate is reduced and the average nest exposure period within the sample becomes shorter. We used an 8-yr record of observer visitations of nesting female Greater Sage-Grouse (Centrocercus urophasianus) in central Nevada, USA, to assess whether observers influenced either the probability of daily nest survival or the overall probability of nest success. Because nest success is influenced by factors other than visitation, we also accounted for the overall quality of a nest (i.e. probability of success in the absence of observer effects) based on potential confounding factors. Nest visitation (no flushing) had no effect on daily nest survival, regardless of nest quality or female age. Flushing a female from a nest substantially lowered the probability of the nest surviving the following day (maximum ~0.22). When considered in the context of the entire nest exposure period, however, flushing a female once produced only a marginal reduction in overall nest success (~0.015). Additionally, females that were younger, or that were associated with low-quality nests, were more likely to abandon the nest after flushing, compared with older females or those associated with higher-quality nests. Observer-related abandonment, however, introduced a negative bias (~0.07) into estimates of overall nest survival by reducing the average timing of nest fate and thereby lowering daily nest survival. Our results suggest that the act of flushing female Greater Sage-Grouse may bias estimates of nest survival low, but this is due to an effect on estimates of daily nest survival, rather than an actual influence on the probability of nest fate.

Prefledging Diet is Correlated with Individual Growth in Greater Sage-Grouse (Centrocercus urophasianus)

ABSTRACT. The period of growth prior to fledging is a critical life stage for most birds, and shifts in diet during ontogeny may affect growth and development. We used nitrogen (&dgr;15N) stable isotopes in feather tissue to quantify dietary composition and evaluate the relationship between diet and growth in prefledging Greater Sage-Grouse (Centrocercus urophasianus). Sequential sampling of &dgr;15N from feather tissue that was synthesized throughout growth allowed us to evaluate changes in plant versus invertebrate contributions to chick diet during the first 28 days posthatch. Feathers became depleted in &dgr;15N throughout growth, and Bayesian mixing models suggested that the proportional contribution of invertebrate nitrogen declined with chick age. We estimate that invertebrate contributions to the protein in chick diets decreased from 39% at 1 week to 23% at 4 weeks of age, which is consistent with previous research on sage-grouse that used traditional diet-sampling methods. Chicks with feather &dgr;15N values that spanned a larger range, but had an intermediate mean, had the longest tarsi and greatest mass at 28 days. These chicks also displayed a more rapid transition to herbivory during the prefledgling period. These patterns are consistent with greater importance of invertebrates during early growth but also suggest that a rapid transition to a more herbivorous diet ultimately results in the highest growth rates. Sequential sampling of feather &dgr;15N provided useful information on temporal patterns in chick diet that were directly relatable to growth during the prefledging stage, and we encourage replication of our approach in other systems.

Evaluating vegetation effects on animal demographics: the role of plant phenology and sampling bias

Abstract Plant phenological processes produce temporal variation in the height and cover of vegetation. Key aspects of animal life cycles, such as reproduction, often coincide with the growing season and therefore may inherently covary with plant growth. When evaluating the influence of vegetation variables on demographic rates, the decision about when to measure vegetation relative to the timing of demographic events is important to avoid confounding between the demographic rate of interest and vegetation covariates. Such confounding could bias estimated effect sizes or produce results that are entirely spurious. We investigated how the timing of vegetation sampling affected the modeled relationship between vegetation structure and nest survival of greater sage‐grouse (Centrocercus urophasianus), using both simulated and observational data. We used the height of live grasses surrounding nests as an explanatory covariate, and analyzed its effect on daily nest survival. We compared results between models that included grass height measured at the time of nest fate (hatch or failure) with models where grass height was measured on a standardized date – that of predicted hatch date. Parameters linking grass height to nest survival based on measurements at nest fate produced more competitive models, but slope coefficients of grass height effects were biased high relative to truth in simulated scenarios. In contrast, measurements taken at predicted hatch date accurately predicted the influence of grass height on nest survival. Observational data produced similar results. Our results demonstrate the importance of properly considering confounding between demographic traits and plant phenology. Not doing so can produce results that are plausible, but ultimately inaccurate.