Problems with Sampling Desert Tortoises: A Simulation Analysis Based on Field Data
Freilich JE, Camp RJ, Duda JJ, Karl AE. 2005. Problems with Sampling Desert Tortoises: A Simulation Analysis Based on Field Data. The Journal of Wildlife Management. 69(1):45-56
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 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 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 data using 1-km2 and plot boundaries produced data sets of smaller plot sizes, which we compared to estimates from the plots. We also tested distance sampling by saturating a 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 . The 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. Further-more, 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.
- Type
- Journal Article
- Authors
- Freilich, Jerome; Camp, Richard; Duda, Jeffrey; Karl, Alice
- Units
- JOTR
- Keywords
- Desert Tortoise, Distance Sampling, estimator performance, field survey methods, Gopherus agassizii, Joshua Tree National Park (JOTR), Mark-Recapture, plot-size choice, Population Estimates, Simulation, threatened endangered species, Tortoises