Population dynamics and the effect of drought in the threatened Nubian ibex

Abstract

Estimating key state variables such as abundance, survival, and recruitment of wild populations, and their interaction with local conditions such as precipitation, is essential for state‐dependent decision making and management. Estimation of state variables remains challenging, especially in species such as mountain ungulates inhabiting rugged and remote terrains. Camera traps present a technological advance that can mitigate some of the challenges in state estimation and can be used to enhance existing methods. Here, we combined camera traps, individual markings, and GPS tracking to estimate abundance and survival for the threatened Nubian ibex (Capra nubiana) population of the Judean Desert, Israel. Since precipitation is a key determinant for recruitment in desert ungulates, we complemented our estimates with kid‐female ratios and regional precipitation records. Between 2016 and 2019, we individually marked 48 ibex of which 38 were also fitted with GPS collars within the Judean Desert. We performed 6 days of camera‐trap surveys each year to estimate abundance for the north Judean Desert population using mark‐resight in a robust‐design framework. Weight‐averaged abundance estimates for 2017–2019, respectively, were as follows: trueN ¯ ˆ $\hat{\bar{N}}$ = 267 (95% CI = 117–418, SE = 51.27), 201 (95% CI = 115–288, SE = 36.85), and 226 (95% CI = 154–299, SE = 36.94) for females, and trueN ¯ ˆ $\hat{\bar{N}}$ = 291 (95% CI = 154–299, SE = 36.94), 160 (95% CI = 108–212, SE = 25.19), and 283 (95% CI = 181–384, SE = 29.76) for males. By augmenting data from telemetry with camera traps, we estimated survival via known fates for the entire Judean Desert (north, mid, and south populations). Weight‐averaged survival estimates for 2016–2019, respectively, were: trueS ¯ ˆ $\hat{\bar{S}}$ = 0.99 (95% CI = 0.97–1.00, SE = 0.012), 0.90 (95% CI = 0.24–0.99, SE = 0.153), 0.87 (95% CI = 0.62–0.97, SE = 0.081), and 0.88 (95% CI = 0.69–0.96, SE = 0.062) for females, and trueS ¯ ˆ $\hat{\bar{S}}$ = 0.77 (95% CI = 0.26–0.97, SE = 0.203), 0.64 (95% CI = 0.33–0.86, SE = 0.151), 0.85 (95% CI = 0.60–0.96, SE = 0.087), and 0.88 (95% CI = 0.69–0.96, SE = 0.062) for males. The interannual trends of abundance and survival estimates corresponded with observed kid‐female ratios and the regional precipitation records, reflecting a population decline following 2017's drought and an increase following the increased precipitation in 2018. Our findings substantiate the efficacy of using camera traps to estimate Nubian ibex's key state variables, as well as detecting population trends driven by environmental conditions. Despite being costly, the capacity of statistically‐robust methods to estimate key state variables is indispensable, and camera traps make similar methods increasingly more feasible in remote and inaccessible populations.