The Key Largo tree cactus (Pilosocereus millspaughii) has become the first Florida species to become locally extinct due to the climate crisis, according to researchers from the Fairchild Tropical Botanic Garden and the Florida Museum of Natural History.

The cactus can still be found in parts of the Bahamas, northern Cuba and a few Caribbean islands, a press release from the Florida Museum of Natural History said.

The Key Largo tree cactus population in the United States was made up of just one stand found in the Florida Keys, which had been monitored on and off since being discovered in 1992.

“Unfortunately, the Key Largo tree cactus may be a bellwether for how other low-lying coastal plants will respond to climate change,” said Jennifer Possley, lead author of the study and Fairchild Tropical Botanic Garden’s director of regional conservation, in the press release.

Saltwater inundation from rising sea levels, soil depletion due to high tides and hurricanes and mammal populations eating the plants had been putting pressure on the remaining cacti.

A once-thriving population of approximately 150 stems had by 2021 been reduced to six unhealthy fragments. The researcher team salvaged the ailing fragments to be cultivated at another location to ensure their survival.

The team initially thought the Florida Keys’ Key Largo tree cactus population, discovered in a remote mangrove forest, to be part of an isolated population of Key tree cactus (Pilosocereus robinii), an endangered species found elsewhere in the Keys.

The cacti both have stems that can grow to more than 20 feet, garlic-scented flowers that attract bat pollinators and reflect moonlight, and red and purple fruit enjoyed by mammals and birds.

However, there were also differences, noticed by Alan Franck, Florida Museum of Natural History’s herbarium collection manager, who suspected the Key Largo cactus to be a different species.

“The most striking difference is the tuft of long, woolly hairs at the base of the flowers and fruits,” said Franck in the press release.

Additionally, Key Largo tree cactus spines are twice as long as those of the Key tree cactus.

In 2019, Franck confirmed the population on Key Largo as the only known occurrence of the species in the U.S.

The Key tree cactus had been succumbing to environmental pressures for a long time and was federally listed as endangered in 1984. From 1994 to 2007, its abundance decreased by 84 percent.

The Key tree cactus “was for a long time very abundant [on Key West]… In recent years, with the destruction of the hammock for securing firewood and for developing building sites, this interesting cactus has become scarce, until at present it is on the verge of extermination in its natural habitat,” botanist John Small noted in 1917.

In 2007, Fairchild researchers began working alongside local land managers to monitor all tree cactus populations each year.

One of their studies revealed higher salt levels in the soil of dead cacti for years after a Lower Keys storm surge event.

The researchers began collecting both tree cactus species for conservation, and they are now grown in Coral Gables. Wild and cultivated seeds are also conserved in a seed bank.

“We noticed the first big problem in 2015,” said James Lange, co-author of the study and a Fairchild research botanist. “In 2011, we started seeing saltwater flooding from king tides in the area. That limits the amount of freshwater available to small mammals and might be related to why the herbivores targeted this cactus, but we can’t say for sure. We’d never seen cactus herbivory like this anywhere in the Lower Keys, where flooding has tended to be less extensive.”

When it was clear the Key Largo tree cactus population in Florida would not survive, the researchers replanted its remaining green material in greenhouses or controlled outdoor settings, but there are no known naturally growing plants remaining in the U.S.

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

The Key Largo tree cactus’ decline and necessary removal has been an indicator of what researchers can expect the future to look like for such species on a warming planet: a complicated series of intertwined events putting pressure on already stressed species.

“We are on the front lines of biodiversity loss,” said George Gann, study co-author and the Institute for Regional Conservation’s executive director, in the press release. “Our research in South Florida over the past 25 years shows that more than one-in-four 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 four global extinctions.”

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