Did toxic algae kill hundreds of elephants in Botswana?

An investigation into the sudden deaths of at least 350 elephants in Botswana in 2020 has revealed the cause was almost certainly a “toxic brew” of open water tainted by a species of cyanobacteria that released cyanotoxins, essentially contaminating the elephants’ watering holes.

According to researchers, approximately 20 watering holes in Botswana’s Okavango Delta had been contaminated across roughly 6,000sq km (2,316 square miles).

What is cyanobacteria and how does it harm elephants?

Although not all cyanobacteria, commonly referred to as blue-green algae, is toxic, some cyanobacteria can produce a type of deadly algal blooms (HABs) in standing water. This is the type which was discovered in the investigation carried out by researchers at King’s College London.

The study showed that the African elephants (Loxodonta africana) died in May and June 2020 after drinking from water holes contaminated with these toxic algal blooms.

“Scientists believe that the production of cyanotoxins is related to certain environmental triggers, for example, sudden rise in water temperature, nutrient loading, salinity,” Davide Lomeo, Earth observation scientist at King’s College London, a collaborator with Plymouth Marine Laboratory and the Natural History Museum in London, and lead researcher in the recent study, told Al Jazeera.

How did the elephant deaths come to light?

In early to mid-2020, a series of routine aerial surveys conducted by helicopter by the conservation organisation, Elephants Without Borders, revealed multiple elephant carcasses scattered across the landscape of the Ngamiland district of northern Botswana.

The aerial survey showed 161 elephant carcasses and 222 sets of bones, while also counting 2,682 live elephants throughout the eastern region of the Okavango Panhandle. In addition, the distance between the dead elephants indicated the deaths had been sudden, rather than gradual.

“The strong clustering of carcasses also suggests that the event was sudden, with limited dispersal of elephants prior to death,” the authors of the study said.

How did researchers identify toxic algae as the cause of death?

Before researchers confirmed it was toxic algae which killed the elephants, they had to rule out several other probable causes.

“Although this area is a known poaching hotspot in Botswana, this was ruled out since elephant carcasses were found with tusks intact,” the authors of the study said.

Other initial theories included virulent and bacterial causes, such as encephalomyocarditis virus or anthrax, but the evidence taken from the field – such as the age of the dead elephants and the absence of any clinical signs of disease, meant the researchers ruled these out as the cause.

The distribution of carcasses and bones suggested a unique “spatial pattern”, which indicated that localised factors may have played a role in the mass die-off. This led to further exploration of specific environmental and ecological conditions in the affected areas.