Update, Nov. 4 | Carl Zimmer has filed a fresh update from the scientists investigating the mass die-off of saiga populations. They see signs of weather conditions triggering a toxic change in normally harmless bacteria.

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The numbers and images that describe a mass dying of the critically endangered saiga, the world’s northernmost antelope species, on the grassy steppe in the Betpak-dala region of Kazakhstan are stunning.

Hastily bulldozed pits brim with corpses. The count of dead animals, according to the United Nations Environment Program, is more than 120,000 out of the 250,000 alive in the most recent survey.

Below, you can read why I see good odds that this eco-cataclysm was caused by a communal binge on noxious weeds that thrive in the region in warm, wet springs. (It has been wet there. The closest city to the area where the die-off occurred, Astana, reported that a month’s worth of rain fell on a single day, May 16.) [An update from the field, appended below, indicates odds of a dietary cause are dropping.]

The enormous new saiga die-off is particularly devastating to conservation biologists because efforts to cut poaching (for meat and “medicinal” horns) were gaining steam in recent years. (Visit the websites of the Saiga Conservation Alliance and Wildlife Conservation Society for more.)

The ancient species had numbered more than a million a century ago and — in a crash similar to that of the American bison in the 19th century — was reduced to a few tens of thousands of animals at its nadir. That crash has been blamed on booming meat demand after the fall of the Soviet Union, but some research points to subtler issues. In 2009, the saiga conservationist Elena Bykova described the 1990s crash as “the fastest decline ever recorded for a mammal species.”

Over the past week, speculation on the cause has focused on various diseases, including ailments related to bacteria found in some specimens by an international team that has raced to the region.

This excerpt from the U.N. news release, though, stresses that those pathogens are not triggers:

[I]t is becoming clear that two secondary opportunistic pathogens, specifically Pasteurella and Clostridia, are contributing to the rapid and wide-spread die-off. However, the hunt for the fundamental drivers of the mass mortality continues since these bacteria are only lethal to an animal if its immune system is already weakened.

Andy Coghlan has written an excellent overview for New Scientist, including this reaction from Richard Kock, a scientist from the Royal Veterinary College in Hatfield, England, who has done extensive research on the species:

It’s very dramatic and traumatic, with 100-percent mortality…. I know of no example in history with this level of mortality, killing all the animals and all the calves. [postscript has more updates from Kock]

Carl Zimmer has filed a news story for The Times with additional context.

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Friday morning, I began sifting the literature on saiga mortality and found strong hints of a possible cause in a study of smaller saiga die-offs in 2010 (12,000 animals) and 2011 (just 450) in the animal’s westernmost population, in the Urals.

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The paper has a ponderous title — “Examination of the forage basis of saiga in the Ural population on the background of the mass death in May 2010 and 2011.” (If you read Russian, please provide the journal name!)

But it’s full of fascinating relevant information.

The first thing that struck me was the similarity in timing. Those events and this year’s mass dying were in mid to late May.

The reported symptoms in the dead and dying animals are the same, as well: foaming at the mouth, diarrhea and bloating.

The paper on the Urals deaths also notes that well before the 2010 and 2011 events, there had been previous die-offs including in 1955, 1956, 1958, 1967, 1969, 1974, 1981 and 1988.

By email, I reached Til Dieterich, one of the authors of the paper on the Ural-region dyings. He said:

As with an airplane crash, there is not only one cause behind this and it is not unprecedented…. In 1988 there was a mass death with 434,000 animals dead (68 percent of the population) more or less in the same region.

For more detail, read the abstract from his paper, co-written with Bibigul Sarsenova:

Mass death of Saiga antelopes took place from 18 to 21 May 2010 in the north west of West Kazakhstan province northeast and southeast of Borsy (about 12.000 dead animals found). In August and September the forage basis of Saiga antelope in the mass death area was investigated. Mass growth of potentially poisonous Brassicacea species for ruminants could be found on abandoned fields in the area (Lepidium perfoliatum, Lepidium ruderale, Descurainia sophia and Thlaspi arvense). Due to favorable warm and wet weather conditions in spring 2010 the mass growth of these annual Brassicacea species occurred on a big scale. Even though Saiga is capable to eat large amount of this plants, they are poisonous to ruminants when consumed in large amounts. In addition lush growth of Brassicacea and Poacea species (Poa bulbosa, Eremophyrum triticeum, Leymus ramosus, Elytrigia repens) providing high protein forage, can cause the observed symptoms of foamy fermentation, diarrhea and bloating. The animals thus could have been killed by extreme bloating and/or acute pulmonary edema (“fog fever”) after foraging on wet and highly nutritious “fog pastures.” Qualitative investigations in the field confirmed that the animals ate most above-mentioned species…. In addition the animals have been congregating for calving, which does contribute to a higher background stress. The results of the investigation suggest that a combination of at least some of the above listed factors is responsible for the tragic events.

Here’s the paper’s concluding section:

In both years the Saiga death events started just after the females and their 1–2 week old young started to move again. During the first 10 days of the calving time the females did not leave their young and not even move to the nearby water places for drinking. In both cases the calving sites where some meters higher and covered either by mainly steppe vegetation (2010, Stipa-Festuca Steppe) or Leymus ramosus grassland on fallow fields. Thus the moist pastures where presumably more intensively used during the death event. Nevertheless the heavy rain events just before or during the death event, did certainly lead to very moist fodder especially in the morning hours. In 2010 even fog was reported by the locals just before the dying started. The local people also reported, that Lepidium species do cause diarrhea in cattle and after heavy rain events herders do not let their livestock out to the pastures before noon. Wet and warm weather conditions have also been reported for the Betbak Dala Population during the spring death events in 1981 and 1988. The animals have also been calving for the first time in the Borsy area usually using pastures further south in the semi desert region. Part of the Saiga population did actually calve further south in the semi-desert area 2011 and no deaths were reported here. Wet weather conditions in spring combined with lush pastures are thus obviously problematic to Saiga.

The paper includes recommendations to limit risks of such events going forward:

With this evidence on hand we recommend in similar wet years to keep Saiga off such dangerous pastures and train the responsible rangers in identifying the described dangerous conditions. If it turns out difficult or dangerous for the Saiga population to keep them off dangerous pastures, the relevant areas should just be cut during the time when the animals are immobile during the first 10 days of calving. Cutting the dangerous pastures will prevent excessive development of toxins and protein in the plants. Even if the plants are eaten dry the risk of negative effects is minimized.

The authors warn against expanding agriculture in saiga territory, noting that plowing or herbicide use would simply lead to mass growth of the weedy toxic species in the cleared area. “This will enlarge the risk of pasture problems even more,” they wrote.

There’ll be much more to report in the coming days.

Richard Kock sent this cautionary note by email, stressing that weeks of analysis will be required to nail down what happened:

We just need to try to gather all the evidence we can from the animals and the environment. In the end this will guide us to the pathogenesis of this problem. Until then much of what you will read is speculation.. We have a pretty good idea of the proximate cause of death but less understanding of possible triggers but noxious weeks I doubt, as the steppe was looking pretty good over this die off and the animals are highly selective so not likely to make mistakes on diet. Rich grass might upset the stomach and rain and warmth can promote this so perhaps a factor but we will need a lot of work to prove this. Hopefully it will be done.

A new update from Kock cuts against a dietary cause:

It is dose that matters and no evidence any single plant was in abundance in the rumen other than grasses. The ingestion of nutrient dense grasses might have led to some ruminal disorder and subsequent tympany and toxicosis. The animals in these outbreaks were eating mainly wheat grass from my limited examination of grazed plants and a few more of their normal food plants – no evidence for pure plant toxicity remember they are highly selective feeders! But pasture analysis would help including toxicology to rule this out and also looking at other potential toxic factors – perhaps in water although plenty of this around this year – I doubt algal toxicosis as a trigger but all this should be ruled out.

Kock, in an interview with the BBC, has pointed more toward environmental conditions as a trigger, and stressed that die-offs are not unprecedented:

“What we’re seeing is sort of a perfect storm of different factors,” Prof Kock explained. Two different bacteria, pasteurelosis and clostridia, have been found in every dead animal studied. These bacteria are naturally found in the animals’ respiratory and gut systems, so something must have reduced the immunity of the animals. One possible trigger is climatic. This year a very cold winter was followed by a wet spring, and this may have affected the immune competence of the animals, making them more vulnerable to the bacteria. This, or some other trigger, pushed the animals past a threshold at which the bacteria overcame Saiga immune defenses and became deadly enough to transmit to their calves. “There’s no infectious disease that can work like this,” said Prof Kock. He added that the wave of Saiga deaths was not unprecedented. “[This] die-off syndrome has occurred on a number of occasions.” In 1984, 2010 and 2012 there were massive die-offs, but none of these claimed such a massive proportion of the population. Despite these huge losses, Saiga antelope are surprisingly well adapted to recover quickly from population crashes. “Its strategy for survival is based on a high reproductive rate, so [the Saiga] produce triplets and have the highest fetal biomass of any mammal. It’s built, in a sense, to recover from collapse,” Prof Kock said. [Read the rest.]

The Saiga Conservation Alliance has set up an emergency fund to help support analysis of the die-off and steps to protect the species.