Application of distance sampling for assessing abundance and habitat relationships of a rare Sonoran Desert cactus
Accurate abundance estimates of plant populations are fundamental to numerous ecological questions and for conservation. Estimating population parameters for rare or cryptic plant species, however, can be challenging and thus developing and testing new methods is useful. We assessed the efficacy of distance sampling for estimating abundance and habitat associations of the endangered Pima pineapple cactus (Coryphantha scheeri var. robustispina), a rare plant in the Sonoran Desert of southwestern North America that has traditionally been surveyed with census-based methods. Distance sampling (DS) involves measuring distances between focal objects and samples of lines or points, and modeling detection functions that adjust estimates for variation in detection probability (P). Although often used in animal systems, DS remains largely untested for plants. We encountered 105 live individuals along 36.9 km of transects in 11 study plots placed across much of the geographic range of the species, and estimated an average density of 1.47 individuals/ha (CV = 0.139). Compared to values from intensive censuses, density estimates from DS were underestimated by only 2.3% on average and highly correlated on the untransformed (r = 0.84) and logarithmic (r = 0.93) scales. Estimates of P averaged 0.49 and declined as soils became increasingly dominated by larger soil substrates, and somewhat with increasing vegetation volume and decreasing cactus height. Local densities increased with increasing slope and soil substrate size and decreased with increasing vegetation volume (P ≤ 0.024). Combined with careful survey design, DS offers an efficient method for estimating population parameters for uncommon and cryptic plants.
KeywordsAbundance estimation Detection probability Distance sampling Habitat Pima pineapple cactus Population size Coryphantha scheeri var. robustispina
We thank S. Mann, M. Garcia, and R. Villa for field assistance, M. Baker and S. Hart for long-term monitoring data, and P. and J. King for access. J. Crawford of USFWS, D. Atkinson of Arizona Department of Agriculture (ADA), and S. McMahon of University of Arizona provided helpful comments. This study was funded by ADA through USFWS Cooperative Endangered Species Conservation Fund Grant Program (Project No. Segment 19, 2015-2017-04).
BFP largely conceived of the study that was designed by ADF, IWM, and BFP. Data were gathered by ADF, IWM, S. Mann, M. Garcia, and R. Villa. ADF completed the analyses with assistance from IWM and JMG. The first draft of the manuscript was written by ADF with all authors commenting on and contributing to subsequent drafts. All authors read and approved the final manuscript.
Compliance with ethical standards
Conflict of interest
No potential conflict of interest was reported by the authors.
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