First local US extinction to sea level rise

The Key Largo tree cactus population in the Florida Keys has been lost to sea level rise.

Tall cactus key largo
Tall Key Largo tree cactus. Credit: Susan Kolterman.

While the cactus still grows on a few scattered islands in the Caribbean, researchers warn that the loss of the local US population is a harbinger of what is to come if global warming  due to human activity is not reversed.

“Unfortunately, the Key Largo tree cactus may be a bellwether for how other low-lying coastal plants will respond to climate change,” says Jennifer Possley, director of regional conservation at Fairchild Tropical Botanic Garden in Florida.

Possley is the lead author on a study about the cactus population’s decline published in the Journal of the Botanical Research Institute of Texas.

Key Largo tree cactus (Pilosocereus millspaughii) can grow to about 6m tall. It has cream-coloured flowers which smell like garlic. The flowers reflect in moonlight, attracting bat pollinators. Its red fruit attract birds and mammals.

The cactus population has been under significant pressure from predation and the changing environment.

Rising seas have seen salt water intrude into the plant’s habitat. High tides and hurricanes have led to increased soil erosion.

By 2021, the only surviving stand of the Key Largo tree cactus – which was once thriving with about 150 stems – had been reduced to ailing fragments. The remaining stems have been salvaged by researchers for off-site cultivation.

Conservationists rescuing dying cactus
Staff from Fairchild and the Florida Department of Environmental Protection removed all remaining green material of the Key Largo tree cactus in 2021 after it became clear the population was not going to survive. Credit: Jennifer Possley.

The Florida Keys population was first discovered in 1992 and confirmed in 2019 as the only example of the cactus in the US.

The similarly named Key tree cactus (Pilosocereus robinii) is also very similar looking to P. millspaughii. In fact, P. millspaughii was originally thought to be a unique population of the Key tree cactus. But the similarities don’t stop there.

P. robinii was federally listed as endangered in 1984. Its numbers fell between 1994 and 2007 by 84%.

Conservation efforts for the tree cactus species have been ongoing since 2007 but increased soil salinity in the limestone outcrop where the Key Largo tree cactus grew would have eventually killed the population off. The process was hastened by large storms including the category 5 Hurricane Irma which swept across the region in 2017.

“We have tentative plans with the Florida Department of Environmental Protection to replant some in the wild,” Possley says. 

Key largo tree cactus flower
Key Largo tree cactus flower. Credit: Susan Kolterman.

Similar efforts have helped keep the Key tree cactus in Florida. “The amount of reintroduced material of this species is already more than the amount of wild material that’s left,” says Possley.

But such efforts, Possley warns, are only temporary solutions as the degradation continues of the fragile mangrove habitats on which the cactus species rely.

“We are on the front lines of biodiversity loss,” says study co-author George Gann, executive director for the Institute for Regional Conservation. “Our research in South Florida over the past 25 years shows that more than 1 in 4 native plant species are critically threatened with regional extinction or are already extirpated due to habitat loss, over collecting, invasive species and other drivers of degradation.

“More than 50 are already gone, including 4 global extinctions.”

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