Population Indices Versus Correlated Density Estimates of Black‐Footed Ferret Abundance

Abstract

ABSTRACT Estimating abundance of carnivore populations is problematic because individuals typically are elusive, nocturnal, and dispersed across the landscape. Rare or endangered carnivore populations are even more difficult to estimate because of small sample sizes. Considering behavioral ecology of the target species can drastically improve survey efficiency and effectiveness. Previously, abundance of the black‐footed ferret (Mustela nigripes) was monitored by spotlighting and generating indices of relative abundance because reintroduced populations were slow to establish. Indices, however, lack variance estimates and are costly to generate for the black‐footed ferret. We therefore used spotlight surveys and live‐trapping in conjunction with a robust mark‐recapture estimator to improve abundance monitoring for the black‐footed ferret, one of North America's most endangered carnivores. We estimated abundance of the black‐footed ferret at Shirley Basin, Wyoming, USA, using correlated density estimates and Program MARK. We compared our results to 2 indices of relative abundance, minimum number alive and predicted number of ferrets from litter counts. The correlated density estimate for the black‐footed ferret (N̂R = 229; 95% CI = 161–298) was similar to minimum number alive (N̂R = 192) and predicted number of ferrets from litter counts (N̂R= 235). The efficiency and effectiveness of survey methods we used for the black‐footed ferret were high by carnivoran standards. Our results suggest that the sampling approach we utilized can be implemented for a fraction of the cost and effort required to generate 2 indices of relative abundance for the black‐footed ferret. Although we recommend managers implement a similar survey approach to monitor abundance of reintroduced populations of the black‐footed ferret, analysis with sparse data sets will be problematic. Until the black‐footed ferret becomes widespread and abundant at a reintroduction site, spotlighting will remain preferable as a means to generate indices of distribution and relative abundance for the black‐footed ferret.