Population viability analysis of Critically Endangered white-rumped vultures Gyps bengalensis

Abstract

More than a decade has passed since the catastrophic population decline in Gyps species was reported from South Asia, but much uncertainty remains about quantifying their short-term extinction risk. To estimate the future extinction risk of the white-rumped vulture Gyps bengalensis in Nepal, we conducted counts at 7 nesting colonies between 2002 and 2012. We com- pared 3 methods of estimating abundance based on count data and calculated mean population growth rates and cumulative probabilities of extinction given the abundance estimates from each method. The first 2 methods of abundance estimation were traditional indices: mean and maxi- mum values of all counts. The third method was a mixture modeling approach that corrected raw counts by a detection parameter. The results of the traditional indices were characterized by high uncertainty levels as reflected in the wide confidence intervals, which limited their capacity to make predictions about the fate of the populations with any confidence. The mixture modeling method provided more reliable results; there was a 51% probability of populations facing quasi- extinction (i.e. ≤20 vultures) in 13 yr and a 99% probability of quasi-extinction in 18 yr. Because the mixture modeling method provided more precise predictions while requiring minimal addi- tional effort, population biologists using count data are encouraged to employ such model-based estimators. The white-rumped vulture populations in Rampur are in danger of disappearing within 2 decades, so conservation efforts should be expedited to prevent the loss of this species.