Long-term population monitoring of northern Idaho ground squirrel: 2016 implementation and population estimates

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We implemented the third full year of the new northern Idaho ground squirrel (NIDGS) long-term population monitoring strategy in 2016. This was the first completion of the 3-year rotating panel design for sampling 1,757 100-m x 100-m grid cells across occupied habitat. We conducted line-transect distance surveys across 915 cells and recorded 1,847 NIDGS at 485 cells (53%). From these data, program DISTANCE estimated a density of 1.51 squirrels/ha and a total population size of 2,659 squirrels (95% CI: 2,337–3,016). We post-stratified data based on relative density (higher, lower), with resulting densities of 1.71 squirrels/ha in stratum 1 compared with 0.77 squirrels/ha in stratum 2, and unadjusted population sizes of 2,340 and 301, respectively. We conducted presence–absence surveys at 14 sites with no cells selected for 2016 surveys, sites too small to have grid cells, or sites with an unknown status. To validate estimates of population size from distance-based line-transect surveys and recalculate the index to abundance, we conducted mark-recapture at 10 sites and captured 312 individuals. The probability of capture pooled across all sites was 0.43 with a resulting population estimate of 496. The average mark-recapture-to-survey ratio for these 10 sites was 1.35 for an adjusted index to abundance of 3,590 NIDGS. We compared the 1-year change in population estimates between 2016 and 2015 from the DISTANCE analyses of all data each year, from DISTANCE analysis of the 500 core grid cells intended to be surveyed every year, and from a pair-wise comparison of the 500 core cells. On average, 49% more NIDGS groups were detected on surveys in 2016 compared with 2015, and modeled densities and population estimates continued to increase from the apparent decline measured in 2014. Increases from 2015 to 2016 occurred in both strata, with 37% more detections in stratum 1 and 47% more in stratum 2. Using program PRESENCE we estimated probability of occupancy at 0.72 for grid cells in stratum 1 and 0.47 for stratum 2. These numbers were higher than in 2015, suggesting a broader distribution of NIDGS across our sampling frame in 2016. From 2014 to 2016 across all 1,757 grid cells in the sampling frame, 53% of cells had at least 1 NIDGS detected, and 65% of the core 500 cells were occupied.

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ELECTRONIC FILE - Zoology, Mammals

Endangered Species Section 6 Grant F15AF00965
and Cooperative Agreement No. F11AC00175