Using Distance Sampling to Estimate Density and Abundance of Saussurea Weberi Hultén (Weber's Saw-Wort) Author(s): Robert A. Schorr Source: The Southwestern Naturalist, 58(3):378-383. 2013. Published By: Southwestern Association of Naturalists DOI: http://dx.doi.org/10.1894/0038-4909-58.3.378 URL: http://www.bioone.org/doi/full/10.1894/0038-4909-58.3.378 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. 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MAEDA, K., AND K. TACHIHARA. 2010. Diel and seasonal occurrence Associate Editor was Mark Pyron. THE SOUTHWESTERN NATURALIST 58(3): 378–383 USING DISTANCE SAMPLING TO ESTIMATE DENSITY AND ABUNDANCE OF SAUSSUREA WEBERI HULTEN´ (WEBER’S SAW-WORT) ROBERT A. SCHORR Colorado Natural Heritage Program, Colorado State University, Fort Collins, CO 80523-1475 Correspondent: [email protected] ABSTRACT—Saussurea weberi Hult´en (Weber’s saw-wort) is a rare alpine calciphilic plant of the Rocky Mountains with disjunct populations in Colorado, Wyoming, and Montana. In Colorado, populations of S. weberi are scattered among <20 locations within the Mosquito Range and Hoosier Ridge regions. Saussurea weberi has specific habitat requirements, and its populations or populations of its pollinators may be susceptible to changing climatic conditions and use of off-road vehicles. It is scarce and discontinuous throughout its range, and there is no reliable estimate of density or abundance. Most estimates of abundance are based on cursory visits and unreplicated counts. To quantify size of the population at one of its centers, I used distance-sampling techniques to sample the population at Horseshoe Cirque, Park County, Colorado. I estimated density and abundance and examined variability of estimates to provide information for understanding the size of populations and for guidance in future protocols for sampling. RESUMEN—Saussurea weberi Hult´en es una planta alpina poco comun´ de suelos de sales de calcio en las Rocky Mountains con poblaciones disyuntas en Colorado, Wyoming y Montana. En Colorado se dispersan poblaciones de S. weberi en <20 lugares dentro de las regiones del Mosquito Range y Hoosier Ridge. Saussurea weberi tiene requisitos particulares de h´abitat, y sus poblaciones o las poblaciones de sus polinizadores pueden resultar susceptibles a condiciones cambiantes de clima y el uso de veh´ıculos de todo terreno. Es una planta escasa y discontinua por toda su distribuci´on geogr´afica y no existen estimaciones confiables de su densidad ni September 2013 Notes 379 abundancia. La mayor´ıa de las estimaciones de abundancia se basa en visitas superficiales y conteos no replicados. Para cuantificar el tamano˜ de la poblaci´on en uno de sus centros, se usaron las t´ecnicas de muestreo de distancia para muestrear la poblaci´on en Horseshoe Cirque en el condado de Park del estado de Colorado. Hice estimaciones de la densidad y la abundancia y evalu´e la variabilidad de las estimaciones para proporcionar informaci´on que sirva para entender el tamano˜ de las poblaciones y para proveer orientaci´on para los protocolos de muestreos futuros. Saussurea weberi Hult´en (Weber’s saw-wort) is a rare Schemske et al., 1994). For populations of rare plants, alpine plant of the Rocky Mountains of Colorado, the consequences of not knowing size and structure of Wyoming, and Montana. It is a member of a monophy- populations can have dire consequences for management letic tribe of some of the earliest branches of the (Morgan, 1999). Professional botanists have made pre- Asteraceae family (Bremer, 1994) and is a short (<25 liminary estimates of populations of S. weberi, but no cm), perennial herb with lanceolate leaves and tightly- formal estimate has been made (B. Glisson, in litt.). The clustered purple flowers (Keil, 2006). Populations of S. largest populations in Colorado may have as many as weberi are found at elevations >2,900 m on exposed 4,500 individuals near Mount Bross, and >1,000 individ- subalpine and alpine slopes of gravelly scree, usually uals near Mount Sheridan in the Mosquito Range derived from limestone or dolomite (O’Kane, 1988; B. (Colorado Natural Heritage Program Biodiversity Track- Glisson, in litt.). In Colorado, there are 17 known ing and Conservation Systems database, BIOTICS, www. locations for S. weberi, and nearly all are on federal lands cnhp.colostate.edu). These estimates were made using in the Mosquito Range or Hoosier Ridge areas of Park, nonrandom, unreplicated counts of individuals per area Lake, and Summit counties. It has no special federal that were extrapolated to the presumed population conservation status under the Endangered Species Act, extent. Unfortunately, such estimates are suspect at best. but the United States Forest Service considers it a For the population at Mount Bross, estimates vary widely sensitive species in Wyoming and Montana, and the from 86–4,500 individuals (B. Glisson, in litt.). Improved Bureau of Land Management considers it a sensitive techniques for estimating populations are available that species in Colorado (B. Glisson, in litt.). NatureServe would improve the precision of estimates of abundance, (http://www.natureserve.org/explorer/) ranks S. weberi increase sampling efficiency, and provide an understand- as a globally-imperiled species throughout its range. ing of the uncertainty of estimates. Estimates of populations vary widely from a few individ- Distance sampling is a process of estimating abun- uals to >10,000 in one locally-abundant population in dance of organisms by modeling the detection function Wyoming, and all locations are lacking rigorous estimates for observed organisms. Instead of estimating the number of the size of populations (B. Glisson, in litt.). of individuals within a set geographic area, distance- The threats to S. weberi are driven by its specific sampling models the area from which organisms are requirements for habitat, the limited availability of detected (Buckland et al., 2001). This area is modeled appropriate habitat, and the potential impacts of envi- using the probability of observing organisms as a function ronmental stochasticity on populations of the plant and of the distance of those organisms from a point or the pollinators (B. Glisson, in litt.). The elevations at transect. As the observer travels