The United States has experienced its first known local extinction due to sea level rise, with the loss of the Key Largo tree cactus.
This rare cactus, scientifically known as Pilosocereus millspaughii, was found only in the Florida Keys but still grows on some Caribbean islands like northern Cuba and parts of the Bahamas.
In the U.S., this cactus was discovered in 1992 in the Florida Keys and monitored periodically.
Rising sea levels, saltwater intrusion, soil erosion from hurricanes and high tides, and animals eating the plants all contributed to its decline.
By 2021, a once-thriving population of about 150 stems was reduced to six struggling fragments. Researchers salvaged these remaining pieces for cultivation elsewhere to ensure their survival.
Jennifer Possley, director of regional conservation at Fairchild Tropical Botanic Garden and lead author of a study published in the Journal of the Botanical Research Institute of Texas, said, “The Key Largo tree cactus may be a warning for how other low-lying coastal plants will respond to climate change.”
The study also looked at two closely related cacti, including the Key tree cactus (Pilosocereus robinii), a federally endangered species found elsewhere in the Florida Keys.
Both cacti have similar appearances, with tall stems and cream-colored flowers that attract bat pollinators, and bright fruit that draws birds and mammals.
However, the Key Largo tree cactus has long, woolly hairs at the base of its flowers and fruits and longer spines than the Key tree cactus.
Alan Franck, herbarium collection manager at the Florida Museum of Natural History, identified the Key Largo tree cactus as a distinct species in 2019.
By then, the cactus was already suffering from environmental pressures similar to those affecting the Key tree cactus, whose numbers have plummeted due to human activities.
In 1917, botanist John Small noted that the Key tree cactus was once abundant on Key West but had become scarce due to habitat destruction. Listed as federally endangered in 1984, its population decreased by 84% between 1994 and 2007.
Since 2007, Fairchild researchers have monitored all tree cactus populations annually, working with local land managers.
A Fairchild-led study found higher salt levels in soil beneath dead cacti compared to living ones, linking cactus mortality to increased salinity following storm surges. Conservation efforts include growing potted cacti and banking seeds for long-term preservation.
In 2021, Fairchild and the Florida Department of Environmental Protection removed all remaining green material from the Key Largo tree cactus site after realizing the population would not survive. The cactus grew on a low limestone outcrop surrounded by mangroves, but hurricanes and high tides eroded the soil, and salt-tolerant plants began encroaching.
In 2015, researchers noticed severe damage from animals eating the cactus, likely because saltwater flooding from king tides limited freshwater availability. Despite setting up cameras, they couldn’t identify the culprits. When they returned the following year, half of the remaining population had died. In 2017, Hurricane Irma’s storm surge further devastated the habitat.
By 2021, only six cactus stems remained. The researchers allowed the plants to flower and fruit before salvaging the remaining green material for cultivation in greenhouses. There are currently no naturally growing Key Largo cacti in the U.S.
Possley mentioned tentative plans with the Florida Department of Environmental Protection to replant some cacti in the wild, similar to efforts that have helped the Key tree cactus survive. However, suitable environments for these cacti are disappearing, making it challenging to find new locations for reintroduction.
The decline of the Key Largo tree cactus highlights the messy and complex impact of climate change on species already under stress. “We are on the front lines of biodiversity loss,” said study co-author George Gann, executive director for the Institute for Regional Conservation. “Our research shows that more than one in four native plant species in South Florida are critically threatened with regional extinction due to habitat loss, over-collecting, invasive species, and other factors. Over 50 species are already gone, including four global extinctions.”
The story of the Key Largo tree cactus serves as a stark reminder of the urgent need to address climate change and protect vulnerable species from its devastating effects.
