Vultures are often cartoon-ish characters in parched deserts. No one wants them around because in western films it means they are just waiting for you to die.
Cultural jokes aside, vultures in some parts of the world are in danger of disappearing. And according to a new report from University of Utah biologists, such a loss would have serious consequences for ecosystems and human populations alike.
The primary threat to vultures, according to the report published today in Biological Conservation, is the presence of toxins in the carrion they consume. On many continents, vultures are the unfortunate victims of poisoned carcasses -- especially impactful because dozens -- or even hundreds -- of vultures can feast on a single carcass. Populations of most vulture species around the world are now either declining or on the brink of extinction.
Losses of vultures can allow other scavengers to flourish, and proliferation of such scavengers could bring bacteria and viruses from carcasses into human cities. So the scavenger you know is better than the scavenger you don't.
Risk factors for decline
The new paper examined factors affecting the extinction risk of more than 100 bird species, including 22 species of vultures, which eat carrion exclusively, and other scavenging birds that have broader diets.
Their results suggest several inherent ecological traits that likely contribute to vultures' extinction risk, including their large body masses, slow reproductive rates and highly specialized diets. The greatest external threat to vultures, however, is poisoning.
Poisoning on three continents
Poisoning is the greatest extinction risk facing vultures, and impacts 88 percent of threatened vulture species. The poisons come in many forms.
In North America, the California condor, a vulture, experienced sharp declines until only 22 individuals remained by 1982. The leading cause of decline? Toxic lead bullet fragments in the gut piles left behind by hunters after animals had been field-dressed. Intensive conservation efforts helped the species to rebound. The condors now number well over 400, and range over large areas of California, Arizona, Utah and Baja California, Mexico.
In the mid-1990s India experienced a precipitous vulture decline, with more than 95 percent of vultures disappearing by the early 2000s. The cause was eventually traced to diclofenac, a veterinary anti-inflammatory drug that relieved pain in cattle, but proved highly toxic to vultures. Hundreds of vultures would flock to each cattle carcass. And if the cow had recently been treated with diclofenac, hundreds of vultures would die. Because of this highly gregarious feeding behavior, less than one percent of cattle carcasses contaminated with diclofenac could account for the steep vulture decline. Fortunately, international cooperation led to a total ban on veterinary diclofenac use. The numbers of vultures have stabilized, and are now showing signs of slowly increasing.
Now, the center of the vulture crisis is in sub-Saharan Africa. Potent newly affordable poisons are used to control predatory pests, such as lions or jackals. The poisons are so toxic that they can cascade through ecosystems: birds, mammals and insects are often found littering the area around these poisoned carcasses. But, as the predominant scavenger, vultures take the brunt of the poisoning and face the largest number of casualties. For example, an elephant carcass poisoned in Namibia in 2007 killed as many as 600 vultures. In other cases, vultures are the victims of poachers who poison carcasses so that vultures do not give away the location of illegally taken animals.
Rise of the facultative scavengers
In vultures' absence, other scavenger populations increase to take advantage of all of the uneaten carrion. By some estimates, in Central America, South America and Africa, vultures eat more meat than all predators combined. Without vultures, animals that eat carrion as a part of their diet (called facultative scavengers, as opposed to vultures, which eat only carrion) proliferate to take advantage of the available nutrients in a dead carcass.
Crows, rats, dogs -- any of these species can suddenly become abundant and dominant, to the point of crowding out the remaining vultures. Hundreds of vultures on a carcass can easily frighten away packs of dogs, but when only a few vultures are left, the dogs can rule.
Such changes in populations of certain animal groups can upset the balance of food webs, though any ecological balance of nature has turned out to be more mythological than evidence-based. Ecology is always shifting.
The poisoning that is killing vultures is also affecting many other organisms throughout ecosystems. But vultures are the most sensitive canaries in ecological coal mines. The story of the California condor shows that recovery is possible, but at a high cost that countries in the developing world may not be able to pay.