Correcting Waterhole-Driven Population Biases in Arid Ecosystems: A Case Study of Oryx (Oryx gazella)

Transect surveys and distance sampling are widely used to estimate wildlife population densities, but these methods can be biased when animals aggregate near features such as waterholes or other resources that occur along survey routes. Using empirical data from the NamibRand Nature Reserve in Namibia, we developed spatial simulations to examine how clumping of oryx (Oryx gazella) near water sources affects density and population estimates. We simulated surveys along a 50 km transect and varied the proportion of the population concentrated at waterholes (5–20%). Our analyses from the simulated surveys show that such aggregation can cause substantial positive bias, as population estimates were inflated by 67% to 967% relative to the known population size. We evaluated two correction approaches: censoring observations and transect segments near waterholes and redistributing animals from waterholes across the landscape. Both methods reduced bias when applied to our simulated survey data, but censoring was simpler and consistently produced more accurate estimates. These findings demonstrate that nonrandom animal distributions near linear survey features can severely compromise distance sampling assumptions. Accounting for such biases is essential for producing reliable population estimates, particularly in arid and semi-arid systems where wildlife strongly congregates around limited water sources.