An Ancient Megadrought Turned An African Lake Into A Deadly ‘Fecal Cocktail’

Present-day Mauritius, post-fecal cocktail swamp. CREDIT: SHUTTERSTOCK
Present-day Mauritius, post-fecal cocktail swamp. CREDIT: SHUTTERSTOCK

Around 4,200 years ago, a once-pristine lake on the African island of Mauritius began transforming into a deadly swamp of poisonous algae and animal feces. The swamp had once been a source of freshwater for the island’s native species, so when it turned into a vat of poison algae and poop, it wound up entombing and killing many of the island’s dodo birds and giant tortoises.

This week, scientists discovered that horrible transformation from nice lake to deadly “fecal cocktail” was caused by a really, really intense drought.

The discovery was published this week in the May issue of The Holocene. In it, researchers from the Netherlands, Germany, and Canada analyzed fossil-rich sediment cores from beneath the area where the swamp once stood. Those cores and fossils contain scores of data that can help scientists reconstruct what an ecosystem was like in the past, said Erik de Boer, a scientist from the University of Amsterdam and one of the study’s authors.

In a blog post, de Boer explained that the megadrought on the island was triggered by the onset of an El Niño, a climactic phase that causes unusually warm conditions.


“The drought regionally induced fires on Mauritius and limited freshwater sources … by decreasing local water levels [and] increasing salt levels,” he wrote. “The excrements of the vertebrates produced hypertrophic conditions that, combined with salinization and high temperatures, created a suitable environment for toxic cyanobacteria, of which the pigments were found.”

“These factors led to a deadly cocktail, resulting in the death of 100,000s of vertebrates by intoxication, dehydration, trampling and miring,” he added.

While the drought and subsequent toxic water source wound up killing thousands of dodos and giant tortoises, however, the study noted that wasn’t the ultimate factor in their total extinction. Populations of the animals did indeed crash due to a lack of available drinking water, but as de Boer explained in an e-mail, “freshwater can always be found somewhere on the island … that would allow the island-level population to rebound when precipitation levels went back to ‘normal’.”

In other words, dodos and giant tortoises were resilient creatures, and likely could have rebounded when the El Nino passed and conditions went back to normal.

So why did the dodos and tortoises really go extinct? That honor, de Boer said, belongs to humans.

“[A]s soon as humans arrive on the island, the dodo and many of its contemporaries quickly went extinct,” he told ThinkProgress. In his blog post, he explained further: “The introduction of rats, pigs, hunting, and large-scale destruction of forest provided tough challenges that almost all of the native fauna were not prepared for.”


While the fecal cocktail swamp event was certainly unique in Mauritius, bodies of water across the world are increasingly likely to contain toxic cyanobacteria — also known as toxic algal blooms — as global climate change accelerates. According to research published in the journal Water Research, global warming and persistent droughts can “strongly affect cyanobacterial growth and bloom potentials in freshwater and marine ecosystems.” The United States is no stranger to the increase in toxic algal blooms.

But the real lesson the study provides, according to de Boer, is Mauritius-specific. As the island responds to current global warming, he said, it’s important to let current species on the extremely biodiverse island have space to respond to climactic changes, away from humans.

“I think it should be high on the political agenda, and a challenge for scientist, to find out how we can provide the threatened biodiversity of Mauritius enough space to survive the next generations,” he said.