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Greater sage-grouse population trends: An analysis of lek count databases 1965-2015 Public Deposited

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  • Counts of males attending leks in the spring have been the primary means employed by states to monitor status of greater sage-grouse (Centrocercus urophansianus) populations for over 75 years. Despite limitations and potential biases, lek count data remain the only long-term, range-wide dataset available for evaluating trends in sage-grouse populations. Using lek data provided by each state, we calculated the number of active leks, the average number of males per active lek each year, and modeled trends in male counts using a set of mixed-effects, Bayesian hierarchical models at range-wide, management zone, and state spatial scales for the 1965-2015 period. Trends within high population density core areas were compared to peripheral areas at range-wide and management zone scales. Trend estimates were also modeled at range-wide scales for the most recent 10-year period (2005-2015). Summary statistics on average males per lek indicate large variability in sage-grouse population size over time at all spatial and temporal scales. Our results support previous findings that have documented a long-term (1965-2015) decline of greater sage-grouse range-wide. The long-term (1965-2015) decline in average males per lek was estimated at 0.83% per year range-wide, and 2.7% and 0.5% for Management Zones I and II, and 0.70, 1.38, and 0.06% per year for Management Zones IV, V, and VI, respectively. Management Zone III showed an increasing trend in average males per lek of 0.19% per year. In 5 of 6 management zones, annual decreases were greatest in the periphery, and lower in core areas. This suggests that denser sage- grouse populations located within the core appear to be insulated more readily from population decline than those on the periphery. Modeling indicated positive trends in average males per lek since 1965 in Wyoming, Utah and Idaho, with negative trends in the 8 other sage-grouse states and declines greater than 1% per year in North Dakota, South Dakota, and Montana. The number of males counted on leks range-wide in 2015 (80,284) increased by 63% compared to the number counted in 2013 (43,397), the most recent trough. A minimum breeding population of 424,645 was estimated for 2015, which does not include grouse on unknown leks.
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