Estimating the Abundance of Christ's Indian Paintbrush ('Castilleja Christii') Using Distance Sampling Methodology

摘要

Christs Indian paintbrush (Castilleja christii) was designated a candidate for listing under the Endangered Species Act in 1999. The global distribution of Christs Indian paintbrush is restricted to 0.81 km2 at the top of Mount Harrison in south-central Idaho, entirely within the Sawtooth National Forest. Population estimates traditionally consisted of counting all detectable plants, but this method produced results with high rates of error. Distance sampling is an alternative method that produces a density estimate with confidence intervals that allows direct statistical analyses between years and individual transects. The objectives of this study were to: (1) evaluate the use of distance sampling for Christs Indian paintbrush population trend analysis and (2) use distance sampling to acquire an accurate estimate of the overall population size of Christs Indian paintbrush. Three transects were established in each of the 3 communities (graminoid, mountain big sagebrush (Artemisia tridentata ssp. vaseyana), and snowbed), for a total of 9 transects. We estimated the density of Christs Indian paintbrush using 0.50-m cluster distances at all 9 transects and with 0.25-m cluster distances at a subset of these transects. Distance sampling worked best in the graminoid and snowbed communities, probably because of the uniform distribution of associated vegetation. Distance sampling did not work well at some of the mountain big sagebrush distance sampling transects, likely caused by the obstruction of plants by mountain big sagebrush. Both the 0.25- and 0.50-m cluster distances worked well in most cases, although the 0.25-m cluster distance produced narrower confidence intervals and better probability detection models. We recommend continuing distance sampling at the 9 transects using the 0.25-m cluster distance. The predicted population size across all three communities was 819,126-1,716,033 plants, but there are severaled data for the spatial extent of the communities within the distribution of Christs Indian paintbrush. These results represent the baseline for distance sampling of Christs Indian paintbrush and should provide information from which to objectively measure population trends.