Mark‐Recapture Distance Sampling for Aerial Surveys of Ungulates on Rangelands

Abstract

Aerial surveys are an efficient technique for counting animals over large geographic areas such as rangelands. In southwestern rangelands, aerial surveys are routinely conducted for ungulates, with the implicit understanding that abundance estimates represent an undercount. Distance sampling can correct for visibility bias, but assumes perfect detection on the survey line, a condition often violated in aerial surveys of ungulates. The incorporation of mark‐resight methods into the distance‐sampling framework, termed mark‐recapture distance sampling (MRDS), corrects for both visibility bias and imperfect detection on the survey line. However, the use of MRDS introduces logistical and technical constraints that may not be practical for aerial surveys. We conducted aerial surveys of ungulates on rangelands in south Texas, USA, to evaluate the feasibility and performance of MRDS relative to conventional distance sampling (CDS) and multiple covariate distance sampling (MCDS). We conducted surveys prior to and after leaf‐fall in 2013–2015 to test the hypothesis that distance sampling corrected for changes in visibility bias. We surveyed white‐tailed deer (Odocoileus virginianus), nilgai (Boselaphus tragocamelus), collared peccary (Pecari tajacu), and feral swine (Sus scrofa) on 4 sites. Each site was surveyed seasonally for 2 years; twice both prior to (Nov) and after leaf‐fall (Feb). Probability of detection on survey lines for each species was high (range = 0.82–0.97) and the average for each species was similar between seasons (0.89 and 0.92 during pre and post leaf‐fall, respectively). The MRDS density estimates often were only ~10% greater than CDS and MCDS estimates; all population estimates had overlapping 95% confidence intervals. Further, CDS and MCDS estimates were similar indicating that measured covariates (seat position, vegetation type, and cluster size) contributed little towards detection probabilities. Despite similar average probability of detection for all species before and after leaf‐fall (0.47 and 0.51, respectively), deer and nilgai population estimates were 22–59% lower during fall surveys than winter surveys. The discrepancy between consistent probability of detections with different population estimates suggests that availability bias, which cannot be addressed with distance sampling, was an issue. Overall, the MRDS technique addressed imperfect detection on the survey line and generated probabilities of detection in the survey area consistent with previous studies done in Texas. However, the extra costs (~US$13,000) and logistical hurdles to preserve observer independence for a small increase in precision of population estimates may not be justifiable. © 2020 The Wildlife Society.

Effectiveness and costs of helicopter-based shooting of deer

“…Aerial survey data from NSW and ACT sites were analysed using mark-recapture distance sampling (MRDS; Amos et al 2014;Burt et al 2014;Peterson et al 2020). Models included a mark-recapture submodel for estimating g(0), the probability of detecting deer on the transect line, and a multi-covariate distance sampling (MCDS) submodel for estimating the distance sampling detection function using observation-specific covariates, including habitat (open or wooded), distance class, observer position (front or rear) and deer group size.…”

The broad scale impact of climate change on planning aerial wildlife surveys with drone-based thermal cameras

“…These accurate estimates open new opportunities to estimate populations at different times of the year. Helicopter-based deer surveys in Texas are usually conducted during autumn or winter 2,45,46 before or after the bulk of recreational harvest. Our findings indicate that thermal contrast would be too poor for drone surveys during autumns which suggest that crewed helicopters with would be the preferred platform.…”

Error in demographic ratios from aerial surveys of white‐tailed deer in semi‐arid rangelands

“…Aerial platforms are an efficient way to survey wildlife populations over spatially extensive areas (Samuel and Pollock 1981, Bleich et al 1994, Peterson et al 2020). Data collected during aerial surveys are used to generate estimates of abundance or population size, adult sex ratios, male age structure, and offspring recruitment rates; all of these are used to direct management or conservation decisions (Gasaway et al 1986, Zabransky et al 2016).…”

“…White‐tailed deer are managed as a publicly owned, renewable natural resource over a range of management intensities and harvest regimes (Adams and Hamilton 2011). Landowners and state agencies routinely use aerial surveys for the management of white‐tailed deer populations (Beringer et al 1998, Potvin et al 2004), particularly on rangelands in the southwestern United States (DeYoung et al 1989, Peterson et al 2020).…”