Scientists have long been aware of mass mortality events (MMEs), but concern is growing around the increase in their frequency and magnitude. According to recent research, climate change and warming temperatures are increasingly a significant factor, both directly and indirectly.
MMEs result in tens or even hundreds of thousands of deaths in a short period of time, threatening extinction for some species and endangerment for others.
A recent example that took scientists by surprise was the 2015 MME affecting hundreds of thousands of saiga - antelopes that live in the wilderness of Kazakhstan.
Some were mothers that had travelled to this remote wilderness for the annual calving season, while others were their offspring, just a few days old. Each had died in just a few hours from blood poisoning. In the 30C heat of a May day, the air around each of the rotting hulks was thick with flies.
The same grisly story has been replayed throughout Kazakhstan. In this springtime massacre, an estimated 200,000 critically endangered saiga – around 60% of the world’s population – died.
Richard Kock witnessed the devastation firsthand:
“All the carcasses in this one of many killing zones were spread evenly over 20 sq km,” says Kock, professor of wildlife health and emerging diseases at the Royal Veterinary College in London. “The pattern was strange. They were either grazing normally with their newborn calves or dying where they stood, as if a switch had been turned on. I’ve never seen anything like that.”
But scientists know this was not the first time the antelopes experienced an MME: it happened in 1981 and again in 1988. What was the cause?
After 32 postmortems, they concluded the culprit was the bacterium Pasteurella multocida, which they believe normally lives harmlessly in the tonsils of some, if not all, of the antelopes. In a research paper published in January in Science Advances, Kock and colleagues contrasted the 2015 MME with the two from the 1980s. They concluded that a rise in temperature to 37C and an increase in humidity above 80% in the previous few days had stimulated the bacteria to pass into the bloodstream where it caused haemorrhagic septicaemia, or blood poisoning.
Though blaming global warming for specific die-offs is difficult, scientists are certain the events will become more numerous as warming persists.
As in the case of the saiga antelopes, other species also have been adversely impacted as they are unable to adapt to rapidly changing climates.
The most thorough study of its kind published in the Proceedings of the National Academy of Sciences in 2015 uncovered 727 accounts of MMEs involving 2,407 animal populations since 1940. It found that not only are reports of MMEs on the increase – by about one event a year – but the number of animals killed in each event is on the rise for birds, fish and marine invertebrates.
The study found that disease was the biggest factor in MMEs, playing a role in a quarter of them. Around 19% were directly linked to human behaviour such as pollution. Factors linked directly to climate – including extremes of hot and cold, oxygen stress and starvation – collectively contributed to about a quarter.
But as was seen with the saiga - as well as another significant MME involving starfish off America's west coast - rising temperatures can lead to increased risk of disease.
That kind of temperature-related outbreak is now thought to lie behind one of the biggest die-offs ever observed in the natural world, in which hundreds of millions of starfish off the west coast of America began to “melt” into white gloop. More than 20 species of starfish along the coast from Mexico to Alaska were hit by the sea star wasting disease, a condition caused by a parvovirus – the group of viruses that cause gastrointestinal problems in animals. The virus left the starfish vulnerable to bacterial infection. Within one or two weeks of infection, white cuts appeared on their bodies and the creatures became listless. Some ripped off their infected arms and tried to walk away. But for most the disease was deadly. Like the bacteria that triggered the MME in saiga, the virus appears to have been present in starfish for decades – if not longer. Samples stored in museums since the 1940s tested positive.
As MMEs increase in both frequency and intensity, ecosystems will be thrust into disarray with rapidly changing food chains. The implications are potentially dire.
Kock says conservationists need to pay close attention to ensure no opportunities to mitigate MMEs are missed.
“The tragedy is, we will probably see more events like the event that affected the saiga,” he says. “Evolution takes millions of years and if we have a shift in environmental conditions, everything that’s evolved in that particular environment is under different pressures. Microbes adapt and can respond to changes quickly, but mammals take hundreds of thousands of years or millions of years to adapt. That’s the real worry.”