Key Largo tree cactus becomes 1st-ever US species to become extinct due to rising sea levels

A type of tropical cactus native to the Florida Keys has become the first species in U.S. history to become extinct due to rising sea levels, researchers said.

A massive stand of Key Largo tree cactus, the only one in existence in the U.S., was destroyed by rising ocean waters as ice sheets at the poles of the Earth continue to melt, according to a paper published earlier this month in the Botanical Research Institute of Texas.

The saltwater from the extra ocean water rushing into the mangrove that houses the cacti is what ultimately caused their demise and could potentially affect other plants in the future, James Lange, a research botanist at the Fairchild Tropical Botanic Garden in Miami and co-author of the study, told ABC News.

Extreme storm systems and high tides from rising seas have caused salt water intrusion and soil depletion in an isolated mangrove that housed Pilosocereus millspaughii, characterized by wooden trunks and tall columns that "flower prolifically" and smell like garlic, Lange said.

The cactus grew on a low limestone outcrop surrounded by mangroves near the shore that originally had a distinct layer of soil and organic matter that allowed the cactus and other plants to grow. Storm surge from hurricanes and "exceptionally" high tides eroded the material to near-nothing, the researchers said.

In 2011, researchers began to see saltwater flooding from king tides -- exceptionally high tides that occur during a new or full moon -- in the region, Lange said. The saltwater places stress on the plants through osmosis, by causing them to lose water through their cells, as well as by causing ionic imbalances, he added.

Salt-tolerant plants previously restricted to brackish soils beneath the mangroves slowly began creeping up, an indication that salt levels were increasing, the researchers said. By 2012, 50% of the population had died, according to the paper.

Detrimental pressure on the population that led to the ultimate demise of the species in the U.S. began in 2015, after an herbivory event by animals left the trees chewed down to the the vascular cambium. All of the fleshy parts of the trees were gone, Lange said.

Researchers still don't know what kind of animals were responsible for the damage, Lange said.

In 2017, Hurricane Irma, a Category 5 storm that decimated Southwest Florida, created a five-foot storm surge that flooded the region for days. Once the water receded, the mangrove was in such dire condition that researchers brought kiddie pools to the site to fill up with freshwater.

King tides in 2019 left large portions of the island flooded for over three months, especially the low-lying crop, according to the paper. By 2021, there were only six Key Largo tree cactus stems left, and it was clear the population was not going to survive.

Scientists made the determination in 2023 that the species was gone for good, Lange said.

"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, director of regional conservation at Fairchild Tropical Botanic Garden and lead author on a study, in a statement.

Little is known about the rare cacti that reside in Florida, Lange said. Pilosocereus millspaughii was discovered in 1992 and the documentation of the discovery is sparse since the cacti was located in a dense patch of the mangrove, dominated by tough, thorny vegetation, he said.

Another similarly named Key tree cactus, Pilosocereus robinii, is a federally endangered species that is present elsewhere in the Florida Keys and is also succumbing to some of the same environmental pressures, the researchers said. That cactus was listed as federally endangered in 1984, but its numbers decreased by 84% between 1994 and 2007.

The rapid decline of the Key Largo tree cactus has given researchers an indication of what could happen to other endemic species in the region.

Over the past 25 years, more than one in four native plant species in Florida are have been deemed critically threatened with regional extinction or are already extirpated due to habitat loss, over collecting, invasive species and other drivers of degradation, said George Gann, executive director for the Institute for Regional Conservation and co-author of the study, in a statement.

"Globally, we're in a biodiversity crisis," Lange said. "We have thousands of species, probably millions, that are, threatened with extinction."

The importance of a species is often not fully known until it is lost, Lange said. The Key Largo tree cactus is pollinated by bats, beetles and moths, and its fruit serves as a food source for birds and other small mammals.

All hope is not lost for the species. Although gone from the U.S., Pilosocereus millspaughii still grows on various islands in the Caribbean, including northern Cuba and parts of the Bahamas, and two Florida institutions -- as well as the National Seed Bank in Fort Collins, Colorado -- have hundreds of live endemic plants and seeds that they could use to start a new grove in the future, once a suitable location is determined, Lange said.

Botanists at Fairchild have tentative plans with the Florida Department of Environmental Protection to replant some in the wild, Possley said.