On Saigas and Sea Stars

Critically endangered and elusively shy, you’d be forgiven for not knowing about the saiga antelope. Native to Central Asia, these creatures had largely escaped the public eye until one very strange, devastating event. In May 2015, over 200,000 saiga antelope, representing nearly 60% of the species, dropped dead on the steppes of Kazakstan. The fatalities included expectant mothers and young calves, bringing this critically endangered species a step closer to the edge. 

A female saiga antelope

The saigas were spread over 65,000 square miles, but it only took only three weeks for them to die. Researchers initially believed an infectious disease to be the culprit. However, the animals died off too quickly across such a vast area for this to be possible. 

After performing autopsies on over 30 saigas, researchers eventually discovered that a usually harmless bacterium found in their respiratory system, Pasteurella multocida, was to blame. They suggested that the unusually high humidity and temperature experienced in May 2015 somehow caused the bacteria to turn deadly. The exact mechanism is not understood, nor is climate likely to be the only factor at play, but high humidity may cause the bacteria to grow to toxic levels, thus killing the saigas. 

Temperature-related die-offs aren't just happening on land, but in our oceans, too. In 2014, millions of sea stars died gruesome deaths off the coast of North America. Sea star wasting disease (SSWD) causes lesions to form on the bodies of the sea star. Soon, the surrounding tissue starts to decay. Sea stars pull off infected limbs, but this is not enough to save them. In only a matter of days, the body of the sea star decays completely. Although the research isn't conclusive (some of the die-offs happened in winter), evidence points to increased sea temperatures affecting the progression and spread of the disease agent.

An ochre starfish, one of the species worst affected by Sea Star Wasting Disease

Although climate change cannot be linked to individual weather events, such as unusually humid summers, it makes extreme events more likely. As a result, these catastrophic die-offs, called 'mass mortality events' (MMEs), may increase in frequency across several taxa.

MMEs are also happening close to home. The UK frog population is expected to be more at risk of intense Ranavirus outbreaks in the years to come as the temperature increases. These outbreaks are also expected to happen earlier in the year, putting tadpoles, and therefore the future of the species, at risk.

"Climate change isn't something that's just happening in faraway places -- it's something real and present that's already had hard-to-predict impacts on wildlife in our own back gardens here in the UK," said Stephen Price of the Zoological Society of London and lead author on the paper linking Ranavirus outbreaks and climate change.

MMEs can have knock-on effects if the species affected is a keystone species, such as the ochre starfish in the kelp forests off the west coast of the USA. The ochre starfish predates on sea urchins, marine invertebrates that graze on kelp. Usually, this keeps the sea urchin population in check, but declines in sea star populations cause urchin populations to swell. This leads to overgrazing on kelp forests, which has well-documented, catastrophic effects on marine biodiversity.

It is clear that mass mortality events pose a real threat to ecosystems, but more research needs to be done to fully understand how climate change can exacerbate the problem.