Date of this Version
Swenson, E. & Powell, L. (2021). Clumped at the Waterhole: Density Estimate Biases and Methods of Correction. Undergraduate Honors Thesis, University of Nebraska-Lincoln.
Transect surveys and distance sampling are often used in southern Africa to obtain density and population estimates for species in large reserves or management zones. However, these estimates may be biased by unnaturally large concentrations of animals at waterholes that are near the path of the survey route. We used empirical survey data from the NamibRand Nature Reserve in Namibia to parameterize spatial simulations in which we distributed oryx (Oryx gazella) on a grid along a 50-kilometer transect. We created multiple simulations to determine how the proportion of animals in a region clumped at a waterhole might bias estimates of density and population sizes. Our results suggest that clumping near the transect may cause population size to be overestimated by 67% (5% of animals at the waterhole) to 967% (20%). We also assessed two possible methods to correct the biased population estimates. Censoring the observations and transect length immediately around the waterhole led to more precise estimates than statistically redistributing the animals at the waterhole to the surrounding landscape. Our methodology can be applied to scenarios in which a feature may cause the distribution of wildlife to violate assumptions for distance sampling during surveys in localized areas.