Four years ago, 350 elephants died near Botswana’s Okavango Delta, but the cause of their deaths was a mystery. It turns out that blue-green algae fatally poisoned them.
All the elephants died between May and June 2020. From the positioning of some of the carcasses, it was clear that the deaths were sudden. Many were face down, suggesting that they had collapsed.
Researchers quickly ruled out several causes. The elephants were too big to have starved. Nor did poachers kill them because all tusks were intact, and there were no bullet wounds. Vultures fed on the carcasses, eliminating human poison, which the scavenging birds detect.
The pandemic stopped further on-site investigations. Many suspected the elephants had died from toxic algal blooms, but they could not prove this.
A team of researchers from Botswana and the UK took up the challenge. Using satellite data, they tracked algal blooms across 3,000 waterholes in the region during 2020.
Davide Lomeo, lead author of the new study, said, “We identified 20 waterholes near fresh carcasses that experienced increased algal bloom events in 2020 compared to the previous three years combined.”
By mapping the positions of the carcasses in relation to the affected watering holes, it became clear that the two were linked. The number of dead animals hinted at the strength of the toxins. It seems that the elephants keeled over within 88 hours of drinking the contaminated water. They walked an average of 16.5km before the toxins killed them.
An elephant in a small Botswana waterhole. Photo: Jerry Kobalenko
The mass die-off sparked huge concern at the time. Botswana has one-third of all African elephants. If conditions like this had continued or recurred, it could have had a huge impact on the endangered elephant population.
Algae might sound harmless, but the waterholes hosted blooms of cyanobacteria, a group of algae that produces incredibly harmful toxins. During the dry season, most of Botswana’s elephants depend on a small number of these waterholes. They come from far and wide and have to drink there until the rains come.
Algal blooms do not occur year-round, but they were much more prevalent in 2020. The weather in Botswana shifted dramatically that year, moving from incredibly dry to incredibly wet much faster than usual.
Researchers believe that the sudden influx of rain suspended nutrients closer to the surface of the waterholes. This created the perfect conditions for these toxic blooms.
The team warns that as climate change makes Botswana hotter and drier, events such as the 2020 die-off could occur again.
