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

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.

Importance of Lunar and Temporal Conditions for Spotlight Surveys of Adult Black-Footed Ferrets

“…Spotlight surveys are the most commonly used technique to monitor ferrets (Biggins et al 2006a). Surveyors locate ferrets via eyeshine and identify individuals using PIT tags Johns 1987, Biggins et al 2006a) and dye marks (e.g., Grenier et al 2009, Jachowski et al 2010, Eads et al 2011a, 2011b, thereby tallying numbers of located ferrets. These data are used to estimate population size (Forrest et al 1988, Grenier et al 2009, information used to assess recovery success (sensu U.S. Fish and Wildlife Service 2006).…”

“…Surveyors locate ferrets via eyeshine and identify individuals using PIT tags Johns 1987, Biggins et al 2006a) and dye marks (e.g., Grenier et al 2009, Jachowski et al 2010, Eads et al 2011a, 2011b, thereby tallying numbers of located ferrets. These data are used to estimate population size (Forrest et al 1988, Grenier et al 2009, information used to assess recovery success (sensu U.S. Fish and Wildlife Service 2006). However, the timing and effort spent accomplishing spotlight surveys vary among sites, which increases the difficulty of comparing population sizes among sites and, thus, assessing success of the ferret reintroduction program.…”

“…A PITreader loop antenna circumscribed an occupied burrow opening and acquired animal-specific identification numbers as the tagged ferret emerged from the burrow through the antenna Johns 1987, Biggins et al 2006a). We identified nontagged ferrets via dye marks (Grenier et al 2009, Jachowski et al 2010, Eads et al 2011a, 2011b. Thus, ferrets were identified to individual and sex.…”

“…Although across-site standardization is complicated by intersite variation in climate, logistical constraints (e.g., money and numbers of surveyors), and physiographic features (e.g., rough vs. flat terrain), a generalized approach seems plausible. A recent spotlight-based method of estimating ferret population size (Shirley Basin, WY; Grenier et al 2009), if useful at other sites, could provide a methodological template. As demonstrated herein, managers should continue to concentrate postbreeding surveys in August-September (Biggins et al 1986, Fagerstone and Biggins 1986, Grenier 2008.…”

Black-Footed Ferret Digging Activity in Summer

“…To identify a ferret, we placed an automatic PIT-reader loop antenna on the occupied burrow opening; identification numbers were recorded whenever a tagged ferret passed near the antenna (Biggins et al 2006a). We identified ferrets without PIT tags via unique dye markings applied in early to mid-June of each field season by TML (Grenier et al 2009, Jachowski et al 2010, Eads et al 2011a, 2011b.…”