Distance Sampling for Sonoran Desert Tortoises

Abstract

We used line transects and distance sampling in combination with radiotelemetry to estimate density of a desert tortoise (Gopherus agassizii) population in the Rincon Mountains near Tucson, Arizona, USA, as part of a longterm study evaluating the impact of urban development on tortoises. During 2000, 34 1-km transects were each sampled twice in the 368.5-ha study area. We observed 46 tortoises with midline carapace lengths >150 mm (subadults and adults) plus 7 juveniles on transects. For subadults and adults, the encounter rate was 0.63 tortoises/km, and the mean proportion of tortoises observable during radiotelemetry, conducted concurrently with transect sampling, was 82%. Corrected mean density based on line transects and radiotelemetry was 0.523 tortoises/ha (CV = 22.99, 95% CI = 0.29-0.79), and absolute abundance in the study area was estimated to be 193 (CV = 23.0%, CI = 107-291). Using the 2 independent coverages of transects as separate samples, the Lincoln-Petersen mark-recapture estimator produced an abundance estimate of 224 subadult and adult tortoises (CV = 53.9%, CI = 72-440). Transects measured on the ground over uneven topography resulted in 3% smaller estimates of density when compared to analysis with transect lengths determined from coordinates plotted on a map. Distance sampling appears to be a feasible method of estimating density of Sonoran Desert populations of the desert tortoise, but transect lengths should be based on mapped rather than measured distances to prevent biases caused by uneven topography.

Response to human presence during nocturnal line transect surveys in fallow deer (Dama dama) and wild boar (Sus scrofa)

“…This can be done (1) by comparing distance sampling estimates with independent estimates (e.g. Hounsome et al 2005;Focardi et al 2002;Swann et al 2002;Franzetti and Focardi 2006) or (2) by validating the key assumptions of the method (e.g. Focardi et al 2005;Butler et al 2005;Marques 2007).…”

An evaluation of distance sampling to estimate badger (Meles meles) abundance

“…Therefore, if it is known that a proportion of the population is not available for sampling on the line transect (g(0) < 1), then an independent method for estimating g(0) must be incorporated into the survey design (Buckland, Anderson et al, 2001). For example, Swann, Averill-Murray & Schwalbe (2002) included an estimate of the detectability of the Sonoran desert tortoise Gopherus agassizii, into their DISTANCE analysis. From radio-tracking they were able to estimate the proportion of the population that was undetectable during sampling, because the tortoises were buried underground.…”

Filter Media Pore Space Analysis Based on Geometrical Characteristics

“…Reptile species are declining on a global scale (Gibbons et al 2000). But despite the importance of providing unbiased abundance estimations to enable efficient conservation planning, few reptile studies have compared different methods for obtaining these estimates (see however Cassey and Ussher 1999, Rodda et al 2001, Swann et al 2002, Freilich et al 2005, Grant and Doherty 2007, Kacoliris et al 2009, Smolensky and Fitzgerald 2010). All of these previous abundance comparison studies rely on a different sampling design for each method tested.…”

“…This variation in sampling design may be partially responsible for the differences observed in the estimations, in spite of the care taken by the researchers to avoid potential sources of heterogeneity between the designs. This risk could be avoided in studies based on data collected using the same sampling scheme (Zylstra et al 2010); however, to our knowledge, such direct comparison has not yet been proposed, apart from a study that applied Lincoln–Peterson and distance sampling methods on twice‐repeated transects to estimate the population of the desert tortoise ( Gopherus agassizii ; Swann et al 2002).…”