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Wildlife managers often need to estimate population abundance to make well-informed decisions. However, obtaining such estimates can be difficult and costly, particularly for species with small populations, wide distributions, and spatial clustering of individuals. For this reason, DNA surveys and capture-recapture modeling has become increasingly common where direct observation is consistently difficult or counts are small or variable. We compared the precision, as indicated by the coefficient of variation (CV), and cost-effectiveness of 2 methods to estimate abundance of desert bighorn sheep (Ovis canadensis nelsoni) populations: traditional ground-based mark-resight and fecal DNA capture-recapture. In the Marble Mountains in the Mojave Desert of southeastern California, USA, we conducted annual ground-based mark-resight surveys and collected fecal samples at water sources concurrently during the dry seasons (Jun-Jul) of 2016 and 2017. Fecal DNA samples were genotyped to identify unique individuals. The Lincoln-Peterson bias-corrected estimator and Huggins closed-capture recapture models were used to estimate abundance for the ground-based mark resight and fecal DNA capture-recapture, respectively. We compared costs between the 2 methods for our study and used simulations to estimate costs for a variety of possible sampling scenarios for our study system based on field-based estimates. Population abundance estimates from fecal DNA capture-recapture achieved much greater precision (CV = 5-7%) than estimates derived from ground-based mark-resight (CV = 21-56%). Our simulations indicated that for a population of 100, 2 sampling occasions, and resight probability of 0.20, the lowest CV obtained by mark-resight was approximately 12%. We predict the cost of abundance estimates for this level of precision (CV = 12%) from fecal DNA capture-recapture would be 28% of the cost of ground-based mark-resight (i.e., a 72% cost reduction). We conclude that fecal DNA capture-recapture is a highly cost-effective alternative for estimating abundance of relatively small populations (≤300) of desert bighorn sheep. More broadly, integrating simulated study designs with cost analyses provides a tool to identify the most effective method for estimating abundance over a wide variety of sampling scenarios.

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Monitoring sex and age ratios (i.e., population composition) is common practice in deer (Odocoileus spp.) management because these estimates are used to determine population vital rates, as well as assess effects of hunting regulations and other management actions. However, there is longstanding recognition of potential biases in sex and age ratios that can arise from differences in deer behavior and sightability, or from methodological issues, such as using roads for sampling. Biologists often use roads for composition surveys because of convenience and roads are often considered to provide coverage of the sampling area and, consequently, unbiased estimates of sex and age ratios. We tested the long‐standing belief that low‐use roads through mule deer (O. hemionus) winter range provided an unbiased sampling scheme by designing and conducting a random‐route survey in conjunction with the road survey through the same winter range in Round Valley, California, USA, during 2015 and 2016. In addition, we conducted a helicopter survey of the same area in 2016. We found adult male:adult female (buck:doe) ratios were 31–45% greater from the random‐route survey compared with the road survey. There was no difference in buck:doe ratios between road and helicopter surveys, but the random‐route survey had 55% greater buck:doe ratios than the helicopter survey. The helicopter survey also had 2.2–4.7 times greater proportions of unclassified bucks than the road or random‐route surveys. There were no differences in fawn:doe ratios among the 3 survey methods. We conducted additional spatial analyses to determine if the buck:doe ratio was greater along random‐routes because bucks were avoiding roads or because the random‐route survey covered different areas. We found that bucks were not avoiding roads; rather, the roads did not adequately cover the winter range area. There were differences between all 3 methods, but our main message is that the assumption that roads provide adequate coverage of winter range and unbiased estimates of population composition should be evaluated. To evaluate that assumption, we recommend conducting ≥1 survey using random routes and comparing composition estimates with road surveys. Further, if information on buck antler‐class is required for management decisions, a ground survey is preferable to a helicopter survey. © 2020 The Wildlife Society.

Monitoring big game populations is necessary for making well‐informed management decisions. In the eastern Sierra Nevada in California, USA, mule deer (Odocoileus hemionus) herds have been monitored using traditional mark‐resight methods on winter ranges since the 1990s. Although mark‐resight methods work well for estimating abundance, animal capture for marking is expensive, invasive, and risky. We were motivated to evaluate fecal DNA‐based capture‐recapture for estimating abundance on winter ranges based on the effectiveness of this new technique in other populations in California. This method has been successful for estimating abundance when animals are concentrated around a required resource or pellets are collected from game trails, but it has not been evaluated using randomly selected transects on winter ranges where traditional methods work well. We compared precision (CV) and cost of fecal DNA capture‐recapture surveys to aerial mark‐resight surveys for estimating population abundance. In the Round Valley and Goodale study areas in the eastern Sierra Nevada, we conducted annual aerial mark‐resight surveys and collected fecal pellet samples in Round Valley during winter 2020. We genotyped fecal DNA from pellets to identify unique individuals and sex. Population abundance estimates from aerial mark‐resight surveys were more precise in Round Valley (CV = 6.6%) compared to abundance estimates from fecal DNA capture‐recapture (CV = 20.4%). But based on simulations from Round Valley data, to obtain the same CV (CV = 10%), fecal DNA capture‐recapture was about 63% less expensive than aerial mark‐resight using very high frequency collars and 70% less expensive than aerial mark‐resight using global positioning system collars. In contrast, fecal DNA capture‐recapture did not succeed in the Goodale study area because of weather and logistical constraints. Determining which method is best depends on the situation, but fecal DNA capture‐recapture provides another tool for estimating population size on winter range. We conclude that fecal DNA capture‐recapture is a cost‐effective alternative for estimating abundance of ungulates on winter ranges when weather is not limiting and when researchers can survey an adequate proportion of the study area and collect an adequate number of samples.