Loss in the USA: Key Largo tree cactus wiped out by climate change

Scientists have documented the first local extinction of a species caused by climate change and the associated rise in sea level. The Key Largo tree cactus (Pilosocereus millspaughii), which can grow up to seven meters tall and occurred in the U.S. only on the Florida Keys, has disappeared.

This rare, tree-form cactus species has gone extinct due to saltwater intrusion and soil erosion as a result of hurricanes. It is not only the rising sea level, but also the hurricanes that are becoming more frequent and intense because of it, that have major impacts on coastal ecosystems like the Florida Keys. Intruding saltwater degrades soil quality and harms or kills plants that are not salt-tolerant. In addition, waves and storm surges cause soil erosion, which is further exacerbated by the loss of vegetation that stabilizes the ground.

From cactus forest to extinction

A striking cactus forest was discovered on Key Largo as recently as 1992, made up of more than 150 tree cacti towering several meters high. In 2019, biologist Alan Franck of the Florida Museum of Natural History identified these imposing plants as the rare species Pilosocereus millspaughii. By that time, the species’ extinction was already well underway, and today nothing remains of the former cactus forest.

Researchers from the Florida Museum of Natural History and the Fairchild Tropical Botanic Garden in Miami have now declared the Key Largo tree cactus extinct across the entire United States in their study, published in the Journal of the Botanical Research Institute of Texas. Worldwide, this cactus species now survives only on a few Caribbean islands, in northern Cuba, and in parts of the Bahamas.

Biologist Jennifer Possley, the study’s lead author, emphasizes that the Key Largo tree cactus is the first species in the United States to have gone extinct due to rising sea levels and climate change. A similar fate befell the Bramble Cay mosaic-tailed rat, considered the first mammal worldwide to have been wiped out by climate change.

How the tree cacti disappeared from the Florida Keys

As early as 2011, researchers observed for the first time that saltwater flooded the area on Key Largo. In 2015, botanist James Lange, who was involved in the study, found that half of the cactus population was dead. The remains of the cacti indicated that animals had been gnawing on them. Lange suspects that rising sea levels and the associated saltwater flooding reduced the availability of freshwater for small mammals, prompting them to chew on the moisture-storing cacti. To this day, it remains unclear which animal is responsible, as installed cameras provided no answers.

In 2017, Hurricane Irma devastated the Florida Keys and led to further saltwater flooding. The already scarce freshwater resources were reduced even more, so helpers had to set up paddling pools of water to save animals from dying of thirst. Additional storm surges and saltwater flooding in the years that followed worsened the situation further.

An uncertain future for the Key Largo tree cactus

In 2021, only six stunted stems remained of the once magnificent cactus forest. The researchers moved them into a greenhouse, with the plan to reintroduce them someday. However, it is unclear where this could happen, because their original site on Key Largo offers no chance of survival due to regular saltwater flooding and eroded, salty soil.

Frequent searches since then have found no naturally growing Key Largo cacti. There is also little hope that they will re-establish on their own, despite cautious plans with the Florida Department of Environmental Protection (DEP) for a small restoration project.

The Florida Keys are considered a hotspot of cactus diversity in the eastern United States, with eight recognized species, three of them endemic. Because these are low-lying, relatively isolated islands, the flora is especially threatened when high biodiversity meets vulnerability to climate-driven sea-level rise. All other cacti and rare plants growing in the region therefore face a fate similar to that of the Key Largo tree cactus.