A decline in vulture populations has thrown off the balance of nature as well as underlining the effects of poaching, poisoning and other human interventions.

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MASAI MARA NATIONAL RESERVE, Kenya — Death feeds life on the Mara.

Each summer, 500,000 wildebeests die along the treacherous migration from the Serengeti National Park in Tanzania to the Masai Mara National Reserve in Kenya. And with death come the scavengers, none more important than the vulture.

But the birds that once feasted on that misfortune, the janitors that clean the grassy plains, are collapsing — part of a broader decline in vulture populations that throws off ecosystems and illustrates how far-reaching the effects of poaching, poisoning and other human interventions can be.

“The overall global picture for vultures is abysmal,” said Darcy Ogada, the assistant director of Africa programs at the Peregrine Fund. “Does this story echo that of the canary in the coal mine? Sure does.”

In the first major study of the 30-year decline of Pan-African vultures, Ogada and other scientists found that populations of eight species of vultures had declined an average of 62 percent.

Seven of those species had declined at a rate of 80 percent or more over three generations, according to the study, published this summer in the journal Conservation Letters.

In some parts of Africa, vultures are targeted by poachers who poison carcasses hoping to kill the birds so they will not circle overhead and signal park rangers. A vulture can spot a dead elephant in less than 30 minutes, but it can take a poacher more than an hour to hack off ivory tusks. No vulture, no warning.

Here on the Mara, one of the greatest natural strongholds left on the planet, the vultures are not directly targeted but are the unintended victims of poisoning of carcasses that is meant to kill large carnivores, like hyenas, in an effort to protect livestock.

Across Africa, the threats to wildlife are myriad, but much of the attention is focused on the stately animals of the savanna, like lions and elephants.

Vultures do not make for pretty postcards, and the local authorities are already stretched thin trying to protect the animals that tourists come to see.

“Everyone forgets about the Ugly Bettys of this world,” said Munir Virani, who directs the Africa and South Asia programs for the Peregrine Fund. “We are told all the time by the authorities that they are so busy working to protect elephants and rhinos and other animals that when it comes to the vultures, they are exhausted.”

Anthony Ole Tira, who is Masai and was raised on these lands and is now the co-owner of the Matira Bush Camp in the heart of the reserve, stood by a river crossing and pointed to scores of rotting carcasses.

One week earlier, 900,000 wildebeests, long in the face and often short on luck, had plunged headlong into the river in a panic. Thousands were trampled to death.

That was normal. The rotting remains were not.

“Ten years ago, this would have been cleaned by now,” he said. “There are a lot of places along the Mara River that are not as clean as they once were because there are not enough vultures.”

Researchers say they have seen what happens to an ecosystem when the vultures disappear.

In 2000, Virani was dispatched to India, where vultures were dying in great numbers but no one knew why.

“Everywhere I went, there were dead vultures,” he said. “But everywhere, their remains were in good condition.”

The initial hypothesis was that some type of infectious disease was behind the deaths. Soon it became clear that the killer was man-made.

A painkiller widely used to treat livestock was poisoning the birds that fed on their carcasses.

One carcass with the painkiller in its system could poison hundreds of birds, Virani said, and by 2006, when the painkiller was officially banned, the vulture population had already declined by 97 percent.

Over the same period, there was a drastic rise in cases of rabies in India, with feral dogs taking advantage of the decline in vultures and often spreading the disease to humans.

Virani described what he called apocalyptic scenes, with hordes of wild dogs numbering in the thousands, scavenging the remains of livestock. Estimates vary, but some put the feral-dog population in India now as high as 25 million.

Roughly 36 percent of the world’s rabies deaths — the majority of them children — occur in India, according to the World Health Organization. The battle against the virus is costing the government billions of dollars.

Over tens of millions of years, vultures have evolved into the most efficient cleaners in the natural world. Because of their highly acidic gastric juices, they can eat flesh infected with a variety of diseases without getting ill.

When the vultures feast on diseased meat, picking the carcass clean, the threat of wider infection ends.

But once the vultures are cleared from the skies, they are very hard to bring back.

Virani explained that vultures, despite their powerful digestive systems, are fragile.

Within the first four weeks of their lives, 50 percent of newborn vultures that leave the nest will die.

“They are naive,” Virani said. Many fall from their nests, while others succumb to natural causes.

In their first year of life, vultures have an extraordinary 90 percent mortality rate.

If they survive, they do not become sexually mature until their fifth year. Even then, their reproductive rate is low.

In Arizona, California and Utah, the Peregrine Fund and its partners have been working for years to bring back the critically endangered California condor, which by 1987 was almost completely wiped out by lead poisoning, with fewer than two dozen birds left. Nearly three decades later, there are around 400, fewer than half of them in captivity.

In Africa, Virani hopes that the population decline can be halted and reversed before it reaches the kind of critical situation found in India and other parts of the world.

“It is not too late,” he said.

The Peregrine Fund has started a program with the Masai people of East Africa to change attitudes about using poisons. It supported a young Masai man, Eric Ole-Reson, to study at Clemson University in South Carolina, and he has since returned to work with other Masai.

There are few things more important to the Masai than their cows. “The movable bank,” Tira calls them.

When a cow is killed by a lion or other predator, it is a threat to a family’s livelihood. So the Masai will poison carcasses in the hope of killing the killers. Inevitably, vultures come to feed and die.

Ole Sairowa, 67, a village elder, said that the use of poisons started two decades ago when the government provided “a dangerous white powder” to kill feral dogs. A decade later, he started to notice fewer vultures.

“Now we are worried they are not coming back,” he said.

Conservationists are working to find other ways to help the Masai protect their cows, including the testing of a system of solar-powered flashing lights to ward off predators at night.

But other threats to birds of prey remain. Virani cited the intensive effort to bring electricity to power-starved communities across Africa through the construction of wind farms and power plants as one that, if not carried out carefully, could endanger vultures and other birds.

For now, the vultures — which in flight look every bit as majestic as the beloved eagle — continue to play their role in the natural drama that unfolds during the migration.

On a recent morning, as first light broke over fields of red oat grass on the savanna, a female lion let roar. A male lion returned the call, and the hunt was on.

By the time it was over, five wildebeests had met their end, their carcasses obscured in the tall grass.

It took only a few minutes for vultures to begin circling overhead. First two, then a dozen, then scores. They waited until the lions began walking away before swooping in near the carcass.

Once the vultures pounced, it took them 20 minutes to pick the bones clean.

It seemed efficient. But Tira said the job used to be accomplished much faster, by many more vultures.

“In five minutes, they would be done,” he said. “If the vultures continue to disappear, can you imagine? This whole beautiful place will become one stink pit.”