25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Theory, Applications, and Controversies of Distance Sampling

Anthony J. Krzysik
School of Arts and Sciences, Embry-Riddle Aeronautical University, 3200 Willow Creek Road, Prescott, AZ 86301-3720

Line and strip transects have been used for almost a century to estimate abundances of both animal and plant populations. In actuality, square, rectangular, and circular quadrats are merely specialized geometries of transects. From the earliest use of transects for biological surveys, surveyors appreciated that corrections had to be applied in order to "adjust" for the logical consequence of visually missing objects that were located farther out from the surveyor's line of travel. A large number of simple to complex functions have been implemented over the years.

Distance sampling (DS) was developed and refined over several decades by Dave Anderson and Ken Burnham to estimate densities of biological organisms in the field, and software was available in 1993. DS software fits user-selected analytical functions and series expansion terms to actual field-survey data to optimally model and quantify the decreasing visibility of objects at increasing distances from the surveyor's transect line. We applied DS to surveying desert tortoises and their sign at Edwards Air Force Base in 1994 (DTC 1995) and at Marine Corps Air Ground Combat Center and Joshua Tree National Park in 1995 (DTC 1997).

A number of problems and controversies have arisen in the application of DS for wildlife surveys, particularly when estimating desert tortoise distribution and density patterns. Important issues include: assumptions inherent in the application of DS methodology, the relevance of tortoise fossorial behavior, the relevance of tortoise sign, the relevance of g₀, and the nature of the experimental design to incorporate distance sampling transects. These issues are discussed along with the theory of DS and its relationship to strip transects, advantages of DS, and application of DS for surveying desert tortoises.