What Causes Mass Mortality Events?

One of the most extreme incidents that can occur in nature, mass mortality events (MMEs) are a single, disastrous event which obliterates large swathes of a species within a short space of time. Scientists have known about MMEs since the 1940s, though reported incidences of the phenomenon have been on the increase over recent years.

So what are the root causes behind these extraordinary and catastrophic happenings? Is there a pattern to be observed? And is there a way to prevent them occurring in the future?

Recent MMEs

Perhaps the most newsworthy example of an MME to occur within recent memory is the incident which devastated the saiga population in Kazakhstan in 2015, killing well over 150,000 antelopes in just the space of a few weeks. The species was already on the critically endangered list before the incident, but saw their population decimated and their precarious hold on existence loosened even further. Experts believe the event was caused by blood poisoning due to a latent bacterium inside the antelopes’ bodies.

Meanwhile over in Australia, the indigenous fruit bats (otherwise known as flying foxes) have also been suffering as a result of extreme temperatures in recent times. Although their constitution is well-adjusted to deal with the heat of an Australian summer, temperatures in excess of 40°C can prove too much for them. On a single day in 2014, over 45,000 flying foxes succumbed to the intense heat in southern Queensland.

Probable cause

These are just two instances among hundreds of MMEs that have taken place in the last 60 years. The most comprehensive study on the subject, published in the journal Proceedings of the National Academy of Sciences in 2015, analysed 727 recorded MMEs going back as far as 1940. In total, 2,407 different animal populations had their numbers severely impacted by these events.

The biggest cause of an MME was found to be disease, affecting 26.3% of all documented examples. Pollution caused by human activity was the second biggest factor, responsible for 19.3% of events, while biotoxicity (including phenomena such as blue green algae) accounted for 15.6%. Other climatic factors, such as oxygen stress, starvation, thermal stress and extreme weather events, were found to have contributed to 24.7% of events. Though it’s difficult to say conclusively that climate change is directly responsible for any such event, the evidence linking them is certainly growing.

What can be done?

Given the fact that MMEs have been occurring with increasing frequency over recent years (with incidences jumping up by around one more every year), and that manmade pollution and climate change are believed to be key contributors to the problem, it’s clear that humans have a role to play in reducing the likelihood of an MME.

Boosting conservation efforts for endangered species, reducing our carbon footprint and curbing pollution are all effective ways in which we can rein in this volatile phenomenon. Given that many marine populations (such as starfish and coral reefs) are also at high risk from MMEs, precise water temperature monitoring in known habitat regions could also be an effective strategy. But with certain species dwindling ever smaller in number, it’s a race against extinction to prevent MMEs from becoming the regular fixture they threaten to be.