Long-term population monitoring of northern Idaho ground squirrel: 2015 implementation and population estimates

Publication Type:

Report

Authors:

Diane Evans Mack; Baker, Carly

Source:

Endangered Species Section 6 Grants F12AP00815 and F13AF01075, Idaho Department of Fish and Game, Boise, p.16 (2015)

Call Number:

U15MAC01IDUS

Keywords:

northern Idaho ground squirrel, Spermophilus brunneus brunneus, Urocitellus brunneus

Abstract:

We implemented the second full year of the new northern Idaho ground squirrel (NIDGS) long-term population monitoring strategy in 2015. We developed a revised sampling frame of 1,757 100-m x 100-m grid cells across occupied habitat, sampled from those cells with an equal-probability, spatially balanced sampling procedure called Balanced Acceptance Sampling (BAS), and used the resulting ranks to develop a 3-year rotating panel design of 919 cells surveyed each year, including 500 core cells sampled every year and all cells sampled within 3 years. We conducted line-transect distance surveys across 918 cells and recorded 1,389 NIDGS at 421 cells (46%). From these data, program DISTANCE estimated a density of 0.98 squirrels/ha and a total population size of 1,723 squirrels (95% CI: 1,461–2,007). We post-stratified data based on relative density (higher, lower), with resulting densities of 1.13 squirrels/ha in stratum 1 compared with 0.47 squirrels/ha in stratum 2, and unadjusted population sizes of 1,546 and 185, respectively. Our adjusted index to abundance was 2,757 NIDGS. We compared the 1-year change in population estimates between 2015 and 2014 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, 60% greater NIDGS groups were detected on surveys in 2015 compared with 2014, and modeled densities and population estimates from various subsets of data corroborate a rebound from the apparent decline measured in 2014. Our data suggest that the difference between 2015 and 2014 occurred across both high- and low- density sites. Using program PRESENCE we estimated probability of occupancy at 0.63 for grid cells in stratum 1 and 0.36 for stratum 2. We conducted presence–absence surveys at 5 sites where no grid cells were selected for surveys this year. To continue to validate estimates of population size from distance-based line-transect surveys, we recommend repeating a comparison of survey to mark-recapture estimates every 3–5 years.