By Carl Zimmer, The New York Times, November 2, 2015
|About 100,000 saigas died in Central Asia in May|
A mysterious die-off of endangered antelopes last spring in Central Asia was even more extensive than originally thought, killing more than half of the entire species in less than a month, scientists have found.
“I’ve worked in wildlife disease all my life, and I thought I’d seen some pretty grim things,” Richard A. Kock, of the Royal Veterinary College in London, said in a telephone interview. “But this takes the biscuit.”
At a scientific meeting last week in Tashkent, Uzbekistan, Dr. Kock and his colleagues reported that they had narrowed down the possible culprits. Climate change and stormy spring weather, they said, may have transformed harmless bacteria carried by the antelopes, called saigas, into lethal pathogens.
It is a scenario that deeply worries scientists. “It’s not going to be something the species can survive,” Dr. Kock said. “If there are weather triggers that are broad enough, you could actually have extinction in one year.”
Saigas are unusual creatures, noted by naturalists for their fleshy, cartoonish noses resembling an elephant’s trunk in miniature. Until the 20th century, great herds swarmed across the Central Asian steppes, migrating hundreds of miles each year.
Overhunting reduced their numbers to just 50,000 by the 1990s. Conservation measures had helped saigas rebound to a few hundred thousand individuals scattered across five distinct ranges from Russia to Mongolia.
But in May, Steffen Zuther of the Frankfurt Zoological Society and his colleagues visited the calving grounds of the largest population, called Betpak-dala, in Kazakhstan. They discovered huge numbers of corpses scattered across the steppes.
The deaths tapered off by the end of May, and initial estimates put the death toll at 120,000 saigas.
In June, Mr. Zuther and his colleagues began making aerial observations and counting survivors. They spotted far fewer animals than they had expected. The scientists now estimate that at least 211,000 saigas — 88 percent of the Betpak-dala population and over half of the species — died in May.
“It’s really horrific,” said Aline Kühl-Stenzel, the terrestrial species coordinator of the Convention on the Conservation of Migratory Species of Wild Animals.
Once individual saigas became ill, they typically died within hours. Entire herds were quickly wiped out. “This is really not biologically normal,” Dr. Kock said.
Necropsies revealed internal bleeding, and blood testing showed that the saigas suffered massive infections of bacteria called Pasteurella multocida and Clostridium perfringens.
These species are normally harmless constituents of the microbiomes of many animals. From time to time, however, they can explode into deadly infections.
In recent months, researchers have scoured the region for factors that could have weakened the animals and contributed to the infections. Mr. Zuther and his colleagues gathered plants along migration routes, but did not find unusual amounts of poisonous species. Signs of chemical toxins or radiation were not found in soil and water samples.
A number of researchers now suspect that rough weather prompted the die-off. The region saw unusually stormy conditions in May, compared to previous years. The saigas experienced sudden drops in temperature in the spring, along with bitter wind chills.
“In this part of the world, the temperature and wind can be really hammering,” said Navinder J. Singh of the Swedish University of Agricultural Sciences.
Other studies of grazing animals suggest that the stress caused by sudden climate changes can weaken animals, enabling Pasteurella and other bacteria to overcome the defenses of their hosts.
The timing may have played a deadly role. In May, the saigas had just dropped their winter coats to prepare for the hot summer. They were eating newly sprouted grass and needed energy to break it down as quickly as possible. Females were giving birth to calves and nursing newborns.
Dr. Singh and his colleagues are now looking back at earlier die-offs to see if they were preceded by the same weather conditions. The worst occurred in 1988, when an estimated 73 percent of the Betpak-dala population died.
There is an important difference between then and today, Dr. Kock and other experts noted: Climate change has raised the average May temperature several degrees across the saigas’ range.
Studies on bacteria have shown that high temperatures can incite some harmless species of microbes to start making toxins and other molecules required to sicken a host.
While other populations of saigas did not suffer in the die-off this year, they have faced their own challenges. One population, called the Ustiurt, numbered 200,000 in 1999.
But poaching has gradually reduced its numbers, and a border fence between Uzbekistan and Kazakhstan has made things worse by blocking migration. At the Tashkent meeting, researchers reported that the Ustiurt population was down to 1,000 animals.
At the meeting last week, conservation groups and government representatives from Kazakhstan, Mongolia, Russia, Turkmenistan and Uzbekistan agreed on a five-year action plan to protect the saigas. It includes measures to find ways to open up migration routes and set up captive breeding populations.
Dr. Milner-Gulland said the best defense against future die-offs may simply be to boost saiga populations with crackdowns on poaching and other measures.
“The main thing is to make sure all the populations are large and resilient enough that this kind of thing doesn’t send them to extinction,” she said.