Loss in the USA: Key Largo Tree Cactus Eradicated by Climate Change

Scientists have documented the first local extinction of a species caused by climate change and the associated rise in sea levels. The up-to-seven-meter-tall Key Largo tree cactus (Pilosocereus millspaughii), which was found only in the Florida Keys in the USA, has disappeared.

The rare tree-like cactus species has gone extinct due to saltwater intrusion and soil erosion resulting from hurricanes. It is not only the rising sea levels but also the increasingly frequent and intense hurricanes that significantly impact coastal ecosystems like the Florida Keys. The invading saltwater deteriorates soil quality and harms or kills plants that are not salt-tolerant. Additionally, waves and storm surges contribute to soil erosion, further worsened by the loss of vegetation that usually stabilizes the soil.

From cactus forest to extinction

In 1992, an impressive cactus forest was discovered on Key Largo, consisting of over 150 tree cacti. In 2019, biologist Alan Franck from the Florida Museum of Natural History identified these imposing plants as the rare species Pilosocereus millspaughii. By that time, the species was already in steep decline, and today nothing remains of the once-impressive 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 in the entire USA in their study published in the Journal of the Botanical Research Institute of Texas. This cactus species now exists only on a few Caribbean islands, in northern Cuba, and 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 go extinct due to rising sea levels and climate change.

How the tree cacti disappeared from the Florida Keys

As early as 2011, researchers observed for the first time that saltwater was flooding the area on Key Largo. James Lange, a botanist involved in the study, found in 2015 that half of the cactus population was dead. The remains of the cacti showed signs of being eaten by animals. Lange suspects that the rise in sea levels and the accompanying saltwater flooding reduced the availability of freshwater for small mammals, causing them to eat the moisture-storing cacti. To this day, it is unclear which animal is responsible for this damage, as installed cameras did not provide any insights.

In 2017, Hurricane Irma devastated the Florida Keys and led to further saltwater flooding. The already scarce freshwater resources were further reduced, so helpers had to set up kiddie pools filled with water to prevent animals from dying of thirst. Additional storm surges and saltwater flooding in the following years further worsened the situation.

Uncertain future of the Key Largo tree cactus

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

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

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