Long-term population monitoring of northern Idaho ground squirrel: 2017 implementation and population estimates

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We implemented the fourth full year of the new northern Idaho ground squirrel (NIDGS) long-term population monitoring strategy in 2017, which was the first replicate of the 3-year rotating panel. The sampling frame consisted of 1,757 100-m x 100-m grid cells across occupied habitat. We conducted line-transect distance surveys across 885 cells and recorded 1,595 NIDGS at 481 cells (54%). From these data, program DISTANCE estimated a density of 1.21 squirrels/ha and a total population size of 2,120 squirrels (95% CI: 1,879–2,444). We post-stratified data based on relative density (higher, lower), with resulting densities of 1.36 squirrels/ha in stratum 1 compared with 0.68 squirrels/ha in stratum 2, and unadjusted population sizes of 1,857 and 267, respectively. Our adjusted index to abundance was 2,862 NIDGS. We compared the 1-year change in population estimates between 2017 and 2016 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. Based on analyses of all data, 20% fewer NIDGS groups were detected in 2017 compared with 2016. This decrease in detections was significant when comparing just the core cells surveyed every year. At the broader scale (i.e., the population estimate from all surveys), NIDGS abundance did not differ significantly this year from last year. We explored several environmental variables, including tree canopy cover, variation in elevation, topographic radiation aspect index, proximity to nearest occupied cell, and strata, as site covariates in occupancy modeling with program PRESENCE. The most parsimonious model included distance to nearest occupied cell and variable detection across visits. We applied this model to the full 1,757-cell sampling frame to generate a probability of occupancy map. Most cells (72%) had >50% probability of being occupied, only 3% had <5% probability, and 19 cells had 0. Probability of detection, given a cell was occupied, was estimated at 0.62 for visit 1, 0.55 for visit 2, and 0.73 for visit 3. We conducted presence–absence surveys at 7 sites where no grid cells were selected for surveys, visited 19 additional locations not included in the original grid selection process, and conducted exploratory surveys across 800 ha outside of known sites to determine if sites were still extant and document dispersal into new or treated areas.

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ELECTRONIC FILE - Zoology, Mammals

Endangered Species Section 6 Grant F15AF00965
and Cooperative Agreement No. F16AC00381