Accuracy, precision, and cost-effectiveness of conventional dung density and fecal DNA based survey methods to estimate Asian elephant (Elephas maximus) population size and structure

Non-invasive DNA-based capture–mark–recapture (CMR) methods have been developed to estimate population size and other parameters and have the advantage that samples can be collected without the need to see or disturb the animals. There are, however, few comparisons of DNA-based CMR estimates of animal population size with estimates from non-genetic methods. We compared the results of a dung-density based survey of an Asian elephant ( Elephas maximus ) population with a simultaneous fecal DNA-based CMR survey of the same population. We found 545 dung-piles along 116 line transects, converted dung-pile density to elephant density using rates of defecation and dung disappearance, and derived a population estimate of 141 (95% CI = [95, 208]) elephants. We collected 267 fecal samples during three CMR sampling sessions: 215 (81%) yielded sufficient genotypic information for analysis and gave an estimated population size of 132 (95% CI = [120,149]) elephants, closely matching the estimate produced by the dung-density method but with greater precision. The DNA-based method also provided information on population structure. We conclude that DNA-based CMR methods provide more precise abundance estimates, and more data about population structure and dynamics, than dung density-based methods. Fecal DNA-based CMR methods also require less time in the field and can be used when dung density methods are impracticable. Finally, fecal DNA based CMR methods are now cheaper than dung density based methods when line transect survey costs are approximately equal to CMR survey costs and dung decay rate monitoring costs are greater than laboratory costs (which will usually be the case).