Rapid population declines and mortality clusters in three Oriental white-backed vulture Gyps bengalensis colonies in Pakistan due to diclofenac poisoning

Abstract

The population declines affecting Asian Gyps vultures are among the most rapid and geographically widespread recorded for any species. This paper describes the rates and patterns of mortality and population change over 4 years at three Oriental white-backed vulture Gyps bengalensis colonies in Pakistan: Dholewala (initially 421 pairs), Toawala (initially 445 pairs) and Changa Manga (initially 758 pairs). Vulture mortality led to the extirpation of two of these colonies (Changa Manga and Dholewala) in 3 years, and a decline of 54.3% in the third. Visceral gout, indicative of diclofenac poisoning, was the largest single cause of death in vultures examined. Annual adult mortality from diclofenac poisoning was significantly positively correlated with annual population declines at each colony indicating a direct causal relationship. Visceral gout occurred in temporal and spatial clusters suggesting multiple point sources of diclofenac exposure. The spatial and temporal distribution of dead vultures and approximate time since death were used to estimate minimum rates at which colonies encountered carcasses with sufficient diclofenac to cause mortality of 1.26–1.88 carcasses per colony per month. By estimating total carcass consumption at each colony, the percentage of carcasses contaminated with diclofenac was calculated as 1.41–3.02%, exceeding the minimum required to have caused the observed population decline. With populations declining by approximately 50% annually, the long term survival of Gyps vultures in South Asia will require the removal of diclofenac from vulture food and establishment of captive populations for future restoration once the environment is free from contamination.