Date of this Version
Swenson, E. & Powell, L. (2020). Transect survey biases and correction methods in Southern Africa. Poster presentation, UCARE Research Fair, Spring 2020, University of Nebraska-Lincoln.
In southern Africa, transect surveys and distance-based analyses are often used 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 on or near the path of the transect. We used empirical survey data from the Namibrand Nature Reserve in southwest Namibia to parameterize spatial simulations in which we distributed gemsbok (Oryx gazella) on a grid along a 50-kilometer transect. We created multiple simulations with and without waterholes to determine how the proportion of animals clumped at the waterhole might affect biases in the estimates of density and population sizes. Our results suggest that clumping near the transect may cause population size to be overestimated by at least 50%. We also assessed two possible methods to correct the biased density and population estimates. Censoring the observations and transect length immediately around the waterhole led to more precise estimates than statistically redistributing the observed animals at the waterhole to the surrounding landscape. Our methodology can be applied to any scenario in which a landscape feature may cause the distribution of wildlife to violate the assumptions for distance-based survey methods in localized areas.