Climate change linked to mass poisoning of hundreds of elephants

Climate extremes may have been behind 350 elephants dropping dead in southern Africa.

During the middle of 2020, reports of elephants dying in their hundreds emerged from Botswana’s Okavango Delta. The ongoing pandemic at the time meant that research on the ground was limited and so the cause of the animals’ deaths was never fully explained.

A team of British and Botswanan scientists now believe they have the answer. Using information from aerial surveys and satellite observations, they have identified 20 waterholes which had very high levels of cyanobacteria. This is a group of algae known for producing toxins, some of which would be more than powerful enough to kill elephants.

The researchers believe the blooms were linked to a dramatic shift from dry to wet conditions between 2019 and 2020. The rapid influx of rain is thought to have resulted in large amount of nutrients being suspended in the water of the pools, creating the perfect conditions for the algae to bloom.

Such extreme shifts are expected to become more common due to climate change, with lead author Davide Lomeo saying that there is “no reason to think an event like this won’t happen again.”

“Southern Africa is projected to become drier and hotter under climate change, and as a result waterholes across this region will likely be drier for more months of the year,” Davide explains. “Our findings point to the potential negative effects on water quantity and quality, and the catastrophic repercussions on animals, this could have.”

“This work, conducted in collaboration with the local authorities, underscores the severe ecological consequences of toxic algal proliferation. It emphasises the critical need for comprehensive water quality surveillance across all waterbodies, including the smallest ones, and the effectiveness of satellites to identify diverse sources of contamination.”

The findings of the study were published in the journal Science of the Total Environment.

Algae blooms and their causes

Cyanobacteria are a widespread group of blue-green algae. There are thousands of different species which, for the most part, spend their lives carrying out photosynthesis and releasing oxygen.

In certain conditions, however, the algae can release toxic compounds known as cyanotoxins that are lethal to a wide range of animals. But, as our algae expert Dr Anne Jungblut explains, it’s not well-known why cyanobacteria produce them.

“Cyanotoxins are energetically costly, meaning there must be a good reason for them to be produced,” she explains. “Otherwise, these genes would have been lost during the process of evolution.”

“At the moment, however, we’re not sure what this reason is. Communication, defence, or the uptake of iron have all been suggested, but there is no clear consensus. The environmental triggers are equally elusive, but it could be linked to temperature or nutrients.”

While cyanobacteria were among the suggested causes of the mass elephant deaths in 2020, evidence tying the microbes to the event was limited. Poaching, drought and infection by other bacteria were also suggested causes, but restrictions due to the COVID-19 pandemic meant that investigation on the ground was limited.

This is where Davide came in. He was investigating how historic animal deaths can be linked to algal blooms as part of his PhD, which was supervised by the Natural History Museum, Kings College London and Plymouth Marine Laboratory. He believed that satellite observation could help to shed light on what killed the elephants.

“Algal blooms are routinely monitored by satellite, but this data isn’t often used to investigate mass mortality events,” he explains. “As the scale of elephant deaths from unknown causes was much higher than anything that’s been seen before, it’s important to use all sources of evidence to investigate what happened to these endangered animals.”

Cracking the case

Using satellite imagery of the Okavango Delta between 2015 and 2023, as well as contemporary surveys from the air, the researchers examined the links between the location of the watering holes and the dead elephants.

The team identified 20 waterholes they believe could be the cause of the elephant deaths. These waterholes had the highest average algae levels over the eight-year period, with blooms peaking in 2020.

Their analysis also revealed that the pattern of dead elephants was different than in other years, reinforcing the idea that the unusually strong blooms were responsible.

By helping lay this mystery to rest, the team have shown the importance of satellite imagery to understand similar events. The researchers hope that recent technological developments, as well as fieldwork on the ground, will help to answer some of the questions surrounding algae and their behaviour.

“Machine learning and artificial intelligence will have a big impact on studies like these in the future,” Anne says. “There’s so much data involved that these technologies will help to identify patterns that we currently can’t see.”

“It’s something that will only become more important going forward as we investigate changes in the climate, water security and pollution.”