Problems with sampling desert tortoises: A simulation analysis based on field data

Journal of Wildlife Management
By: , and 



The desert tortoise (Gopherus agassizii) was listed as a U.S. threatened species in 1990 based largely on population declines inferred from mark-recapture surveys of 2.59-km2 (1-mi2) plots. Since then, several census methods have been proposed and tested, but all methods still pose logistical or statistical difficulties. We conducted computer simulations using actual tortoise location data from 2 1-mi2 plot surveys in southern California, USA, to identify strengths and weaknesses of current sampling strategies. We considered tortoise population estimates based on these plots as "truth" and then tested various sampling methods based on sampling smaller plots or transect lines passing through the mile squares. Data were analyzed using Schnabel's mark-recapture estimate and program CAPTURE. Experimental subsampling with replacement of the 1-mi2 data using 1-km2 and 0.25-km2 plot boundaries produced data sets of smaller plot sizes, which we compared to estimates from the 1-mi 2 plots. We also tested distance sampling by saturating a 1-mi 2 site with computer simulated transect lines, once again evaluating bias in density estimates. Subsampling estimates from 1-km2 plots did not differ significantly from the estimates derived at 1-mi2. The 0.25-km2 subsamples significantly overestimated population sizes, chiefly because too few recaptures were made. Distance sampling simulations were biased 80% of the time and had high coefficient of variation to density ratios. Furthermore, a prospective power analysis suggested limited ability to detect population declines as high as 50%. We concluded that poor performance and bias of both sampling procedures was driven by insufficient sample size, suggesting that all efforts must be directed to increasing numbers found in order to produce reliable results. Our results suggest that present methods may not be capable of accurately estimating desert tortoise populations.

Study Area

Publication type Article
Publication Subtype Journal Article
Title Problems with sampling desert tortoises: A simulation analysis based on field data
Series title Journal of Wildlife Management
DOI 10.2193/0022-541X(2005)069<0045:PWSDTA>2.0.CO;2
Volume 69
Issue 1
Year Published 2005
Language English
Publisher Bioone
Contributing office(s) Western Fisheries Research Center
Description 12 p.
First page 45
Last page 56
Country United States
State California
Other Geospatial Joshua Tree National Park, Ward Valley
Online Only (Y/N) N
Additional Online Files (Y/N) N
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