The first dead elephants were discovered in May 2020. By July of that year, more than 350 endangered animals were found dead in a remote part of Botswana. Global concern quickly escalated as veterinarians on the scene eliminated the usual suspects. There were no signs of starvation, infection or natural anthrax, and the giant mammals had intact teeth, eliminating the possibility of poaching. Some of the elephants were found face down, indicating a sudden collapse.

That leaves only one prime suspect: toxic cyanobacteria, also known as blue-green algae. This was confirmed four years later in a new study from King’s College London. Analysis of satellite data by geographer David Lomeo and colleagues shows that toxic algal blooms broke out in water bodies near the Okavango Delta during the same period, almost certainly poisoning African bush elephants (Loxodonta africana).

“We found 20 wells near fresh carcasses where algal blooms increased in 2020 compared to the previous three years combined. These wells also showed the highest average algal biomass for the period 2015-2023.” explains Lomeo.

Researchers examined the spatial relationship between 3,389 bodies of water in the Okavango Delta and the location of dead elephants.

“Algal blooms are routinely observed by satellite, but these data are not often used to study mass die-offs,” says Lomeo. “As the scale of elephant deaths from unknown causes is much greater than anything seen before, it is important to use all sources of evidence to investigate what is happening to these endangered animals.”

The team also found that elephant carcasses were more dispersed than the historical pattern of death, marked by elephant bones following the chronology of poisoning. The animals likely died within 88 hours of exposure, allowing them to travel a certain distance from toxic bodies of water before dying.

“Our results suggest that the likely source of cyanotoxin exposure is seasonal, predominantly rain-fed water bodies rather than permanent water bodies (lakes, rivers, and lagoons) in the Panhandle,” the team wrote in their paper.

“Cyanobacteria are often found in large numbers in turbid, stagnant, nutrient-rich water, and some blooming species can cause harm through toxin production.”

Botswana, a landlocked southern African country, is home to the world’s largest elephant population, numbering over 130,000. However, the number of these magnificent animals is decreasing all over the world. This is one of the largest known mass die-offs of any large mammal in recent history, but it is far from an isolated incident.

Last year alone, 94,000 birds died on a lake in California due to a climate-induced botulism outbreak; extreme heat caused more than 100 monkeys to fall from trees to their deaths in Mexico; Thousands of eels found dead in New Zealand streams; and Australian bats died of mass starvation.

Although mass fatalities are often reported in isolation, they are becoming increasingly common, and those listed above likely have a common cause. From elephants in Botswana to the worst mass coral bleaching in Australian waters, our wildlife is under siege from the natural disasters we are causing due to climate change. Blooms of toxic or oxygen-absorbing microbes, including cyanobacteria, are occurring more frequently in both freshwater and saltwater lakes due to warming conditions. These same conditions physiologically stress wildlife and make them more susceptible to disease.

“This event highlights the alarming trend of climate-driven disease outbreaks affecting large ungulates,” the researchers explain, “and reflects the broader, devastating effects of climate change on biodiversity and ecosystem health.”

Despite this dramatic increase in deaths around us, leaders and industries that could do something about it are making the situation worse by failing to reduce fossil fuel emissions.

“South Africa is predicted to become drier and hotter due to climate change,” says Lomeo. “As a result, water bodies in this region are likely to become drier for many months of the year. Our findings point to potential negative impacts on the quantity and quality of water, as well as potentially devastating consequences for animals.” This study was published on: Total Environmental Science.