Study confirms massive elephant die-off in Botswana was caused by toxic algal bloom due to climate change
Study confirms massive elephant die-off in Botswana was caused by toxic
algal bloom due to climate change
Cyril Zenda, Down to Earth
December 2, 2024
Some four years after the mysterious death of nearly 400 African elephants
in Botswana’s Okavango Delta, remote sensing and spatial analysis suggest
that the massive die-off was a result of drinking from stagnant water holes
(pans) where toxic algae populations had exploded due to climate change.
Until now, there was no evidence to support the widely debated idea that
poisoning was the cause of these deaths in May-June of 2020.
This new study led by King’s College London has provided additional
evidence to support the findings that elephants died from drinking water
poisoned by an explosion of toxic algae populations in the water pans due
to the effects of climate change.
The climate crisis is increasing the intensity and severity of harmful
algal blooms. After drinking the affected water pans, elephants of all ages
were spotted walking in circles before collapsing and dying. This baffled
scientists, with many theories circulating about the cause of death,
including cyanide poisoning or some unknown infections.
‘Increased Harmful Cyanobacteria Bloom Risks’
The 11-member team of international researchers stated their analysis shows
animals were very likely poisoned by stagnant watering pans where toxic
blooms of blue-green algae, or cyanobacteria, had developed after a very
wet year followed a very dry one.
“The drastic changes in ecohydrological and climate conditions,
particularly in the shift between 2019 (low rainfall and high temperatures)
and 2020 (high rainfall and lower temperature), underscore the likelihood
for altered water quality and increased harmful cyanobacteria bloom risks,”
the researchers say in their findings published in the journal Science of
The Total Environment.
“These environmental dynamics, occurring in tandem with the carcass spatial
clustering, provide compelling evidence of water quality deterioration as a
possible contributing factor in the mass die-off event.”
The mass elephant deaths are cause for great concern to the global
conservation stakeholders as the African elephant population has fallen
drastically in the past half century. Already threatened by poaching,
climate change-including deaths such as these mass-die offs add further
pressure to the pachyderms’ fast dwindling population.
“This unprecedented die-off within the largest remaining population of a
threatened signature megafauna underlines the escalating concerns
surrounding the impact of drought and climate change on the Okavango Delta,
one of the most important ecosystems in the world,” the researchers
concluded. “Globally, this event underscores the alarming trend of sudden,
climate-induced diseases affecting large ungulates, reflecting the broader,
devastating impacts of climate change on biodiversity and ecosystem health.
Satellite Data and Spatial Analysis
Lead researcher, Davide Lomeo, a researcher in the Department of Geography
at King’s College London and co-supervised by Plymouth Marine Laboratory
(PML) and the Natural History Museum says by combining satellite data and
spatial analysis, the team was able to trace the link between 20 most
contaminated of the nearly 3,400 waterholes in the area and the locations
of deceased elephants to conclude that they were the sources of the deaths.
Their analysis pointed to waterholes near the carcasses having heightened
algal levels and repeated bloom events in 2020 compared to previous years,
especially during the period coinciding with the mass die offs.
“We identified 20 waterholes near fresh carcasses that experienced
increased algal bloom events in 2020 compared to the previous three years
combined. These waterholes also exhibited the highest average algal biomass
of the period 2015 – 2023,” said Lomeo. He said these findings point to
heightened risk and likelihood of the presence of algal toxins in these
waterholes.
After drinking the contaminated water, the elephants were estimated to have
walked an average of 16.5 km from the toxic waterholes and died within some
88 hours of exposure.
The team concluded that the shift from a very dry 2019, a driest year in
decades, to an extremely wet 2020 could have led to a resuspension of
significant amounts of sediments and nutrients from the ground, promoting
the unprecedented algal growth.
The research also showed that decayed elephant carcasses were more spread
out across the landscape than fresh carcasses, indicating that the die-off
in 2020 was different from ordinary elephant mortality patterns.
Not only elephants died. It is believed that thousands of other smaller
animals also died, but usually these are disposed of by carnivorous
scavengers, leaving no carcasses, unlike in the case of elephants.
Toxins produced by the algae growing in watering holes was one suspected
cause of the elephants’ deaths, though evidence had remained inconclusive,
in part because the event took place in the midst of the COVID-19 pandemic
when movements were restricted, preventing the timely collection of samples.
The deaths of 25 elephants in neighbouring Zimbabwe from septicaemia in the
same year had appeared to cast some doubt on algal toxins being the cause
of the deaths of the elephants in northern Botswana.
Need for Water Quality Surveillance
At 132,000, Botswana is home to a third of all African elephants, followed
by Zimbabwe with about 100,000 of the jumbos. Four other southern African
countries also have sizable elephant numbers. The findings are a cause of
concern as the region is experiencing another round of severe drought the
end of which might result in a repeat of the 2019-20 conditions that led to
the mass-die off of the animals.
“Southern Africa is projected to become drier and hotter under climate
changes, and as a result, waterholes across this region will likely be
drier for more months of the year. Our findings point to the potential
negative effects on water quantity and quality, and the catastrophic
repercussions on animals, this could have,” Lomeo said.
He said their work, conducted in collaboration with the local authorities
in Botswana, underscores the severe ecological consequences of toxic algal
proliferation, emphasising the critical need for comprehensive water
quality surveillance across all waterbodies, including the smallest ones.
“The research demonstrates the effectiveness of satellite-based detection
in identifying diverse sources of contamination, reinforcing the importance
of expanding Earth observation applications to enable swift intervention
when similar environmental threats emerge.”
The study involved researchers from the University of Botswana, the Natural
History Museum, London, Queen’s University Belfast, and the Plymouth Marine
Laboratory (PML).