Population Ecology of Sage-grouse in the Great Basin: Predictable Patterns in a Variable Environment
AuthorBlomberg, Erik John
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The greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse) is an iconic species endemic to the sagebrush (Artemesia spp.) ecosystems of western North America. Sage-grouse have experienced dramatic contemporary declines in distribution and abundance, coincident with human disturbance of sagebrush habitats. These declines have led to sage-grouse being listed as a candidate species for protection under the United States Endangered Species Act, and listing as an endangered species by the Committee on the Status of Endangered Wildlife in Canada. Increased legal protection, coupled with long-term interest in sage-grouse conservation, has prompted considerable interest in sage-grouse ecology and the response of populations to environmental variation across multiple spatial and temporal scales. My dissertation research focused on the population ecology of sage-grouse in Eureka County, NV, and is comprised of 4 main chapters. In chapter 2, I used robust design and Pradel capture-mark-recapture models to evaluate 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. These 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 low in areas with a substantial exotic grassland footprint even following years of favorable rainfall. I 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. In my 3rd chapter, I investigated tradeoffs associated with reproductive costs to survival for female greater sage-grouse in our study system, while also considering reproductive heterogeneity by examining covariance among current and future reproductive success. I analyzed survival and reproductive histories from 328 unique female sage-grouse captured between 2003 and 2011, and examined the effect of reproductive success on survival and future reproductive success. Female survival was variable within years, and this within-year variation was associated with distinct biologic seasons. Monthly survival was greatest during the winter (November - March; ΦW = 0.99 ± 0.001 SE), and summer (June - July; ΦS = 0.98 ± 0.01 SE), and lower during nesting (April - May; ΦN = 0.93 ± 0.02 SE) and fall (August - October; ΦF = 0.92 ± 0.02 SE). Successful reproduction was associated with reduced monthly survival during summer and fall. This effect was greatest during the fall, and females that successfully fledged chicks had lower annual survival (0.47 ± 0.05 SE) than females who were not successful (0.64 ± 0.04 SE). Annual survival did not vary across years, consistent with a slow-paced life history strategy in sage-grouse. In contrast, reproductive success varied widely, and was positively correlated with annual rainfall. I found evidence for heterogeneity among females with respect to reproductive success; compared with unsuccessful females, females that raised a brood successfully in year t were more than twice as likely to be successful in year t+1. In chapter 4, I used 8 years of banding data from male sage-grouse in eastern Nevada, and capture-mark-recapture analyses, to evaluate the effect of breeding propensity on annual and long-term trends derived from lek counts. I estimated the proportion of variance in annual lek count trends that corresponded with an independent estimate of λ, versus variance associated with breeding propensity. Annual male breeding propensity (the probability a male attends a lek at least once) during the study ranged from a low of 0.56 (± 0.22 SE) to a high of 0.87 (± 0.11 SE). Variance in annual lek count trends was associated with both realized λ (semipartial R2 = 0.57), and sampling error associated with breeding propensity (semipartial R2 = 0.40). I found substantial discrepancies between lek count and realized λ in 3 out of 7 intervals, whereas estimates of long-term λ were extremely similar between count-based and capture-mark-recapture methods (λ = 0.90 ± 0.05 SE and λ = 0.91 ± 0.05 SE, respectively). Male density during the previous year appeared to have the most substantial influence on breeding propensity, perhaps driven by density-dependent competition and availability of food resources. Lek counts are well-suited for deriving long-term estimates of sage-grouse population growth, whereas short-term estimates of λ should be viewed cautiously if breeding propensity is not directly incorporated. In my fifth chapter, I developed an alternative approach for classifying diet of pre-fledging sage-grouse using carbon (δ13C) and nitrogen (δ15N) stable isotopes in feather tissue. Sequential sampling of δ13C and δ15N from feather tissue that was synthesized throughout growth allowed me to distinguish between plant and invertebrate contributions to chick diet during the first 28 days post-hatch. Feathers became progressively depleted in δ15N throughout growth, and Bayesian mixing models confirmed that the proportional contribution of invertebrate nitrogen declined with chick age. I estimate that invertebrate contributions to the protein in chick diets decreased from 33% at 1 week of age, to 14% at 4 weeks of age, consistent with previous research that used traditional diet sampling methods. I found a quadratic relationship between diet composition and chick size at 28 days; chicks that consumed a mixed-diet during growth had larger tarsi and body mass than chicks that were more strictly herbivorous or insectivorous. These growth patterns were consistent with an optimal diet strategy, where supplemental nutrients provided by invertebrates decreased in importance as the digestive capacity of chicks increased and facilitated greater herbivory. In contrast to δ15N, δ13C produced anomalous results that we believe were the product of digestive development as chicks aged. My research has significant implications for sage-grouse persistence in a changing climate, and demonstrates that multiple aspects of sage-grouse ecology are tied to water balance in the sagebrush ecosystem. In climate change results in more frequent drought and/or increased spread of exotic grasslands, negative impacts to sage-grouse populations may be expected.