Is diclofenac the only cause of the decline in vultures across Asia?
A large number of peer-reviewed scientific publications have established that diclofenac is the main and probably the only cause of the crash in vulture numbers across South Asia. These include studies on the causes of death of vultures in the wild, experiments demonstrating that diclofenac in livestock carcasses is toxic to vultures, measurements of the prevalence of diclofenac in livestock carcasses available to vultures, and population modelling that has determined that the measured prevalence and concentrations of diclofenac in carcasses are sufficient to account on their own for the observed decline in vulture numbers. However, this does not mean that other causes of mortality should be ignored. With the now tiny population of vultures present in Asia any additional causes of mortality are making the situation worse. In addition, the scarcity of vultures has forced people to find alternative methods for disposing of dead animals and this may reduce the vulture’s food supply in the longer term. However, the principal cause of the declines remains the presence of diclofenac (and similarly acting drugs) and only the complete removal of diclofenac from the environment will allow vulture numbers to have any chance of recovery.
Why are some scavenging species apparently not affected by diclofenac?
Some scavenging birds in Asia, including the black kite (Milvus migrans), have so far shown no signs of decline. Some birds tested with diclofenac, such as the domestic chicken (Gallus gallus domesticus) and the North American turkey vulture (Cathartes aura), have been found to have low susceptibility to diclofenac poisoning. There is considerable evidence for large differences between species in the toxicity of non-steroidal anti-inflammatory drugs (NSAIDs). The most striking example is the differing toxicity of aspirin within mammals. For humans and many other mammals aspirin is a safe and reliable pain killer, yet doses that we would take for a mild headache are highly toxic to domestic cats and related species. Similarly, while diclofenac is highly toxic to vultures from the genus Gyps, New World vultures (that are more closely related to storks) and crows can tolerate very high doses of diclofenac without dying. It is likely that physiological differences between species in the metabolism of the drug and in kidney function are responsible for the observed variations in toxicity.
Can breeding centres really work to bring about the recovery of vultures in Asia?
Vulture conservation breeding centres were an essential tool for the survival of the Californian condor (Gymnogyps californianus) and for the successful reintroduction of bearded vultures (Gypaetus barbatus) and Eurasian griffon vultures (Gyps fulvus) across Europe. The world population of California condor was down to just 24 birds, before the remaining wild individuals were captured. Over the following decade, conservation breeding and release increased this population, and in 2009 over 200 birds were present in three wild populations. The capture and breeding of a species is usually only implemented when wild populations are declining rapidly and other conservation measures will not have effect quickly enough. Numbers of white-backed vultures in Asia in 2007 had declined by more than 99.9% during the previous 14 years, with no indication that decline rates (44% a year) were slowing. Because of the scale and speed of these declines, capturing and breeding vultures in Asia is essential to ensure survival of the species. Without breeding centres, it is very likely that by the time diclofenac is completely removed from the environment that there will be no vultures left to make a recovery.
Is it realistic and desirable for vultures to return to their former abundance, as this was artificially high?
It has been suggested that the decline of vultures in the Indian subcontinent is acceptable because it represents a fall from abnormally high numbers supported by the huge and unnatural food supply provided by hundreds of millions of domesticated ungulates. It is certainly likely that the amount of food for vultures in the late 20th century was at an all-time high, and higher than it was when wild ungulates abounded before the advent of pastoralism and agriculture. However, the vulture population in the subcontinent has now declined so drastically that it is probably much lower than it has been at any time since the end of the last ice age. Even in protected areas where wild ungulates and their natural predators are present in large numbers, vultures are virtually absent. Although the high population of vultures before the decline was supported by human activity, the vulture population is now much lower than could be considered a “natural” level in the absence of humans. If toxic drugs are removed from domesticated ungulates, vulture populations are expected to recover very slowly because they only rear a maximum of one chick per year. In addition, there may be less food for them now because carcass disposal practices have changed while they were rare. However, the upper limit on their future population density should be much higher than at present, especially in and near protected areas where carcasses of wild ungulates are available.