The United States has experienced its first local extinction of a species directly attributed to sea level rise. The Key Largo tree cactus (Pilosocereus millspaughii) once thrived in a single population discovered in 1992 on the Florida Keys. Despite efforts to protect it, this species has been eradicated from its natural habitat in the U.S., marking a significant milestone in the ongoing battle against climate change.
The Key Largo tree cactus faced several environmental challenges, including saltwater intrusion, soil depletion from hurricanes and high tides, and herbivory by local mammals. These factors combined to significantly reduce its population from a thriving stand of about 150 stems to just six ailing fragments by 2021. Researchers salvaged these remnants for off-site cultivation to ensure their survival.
According to Jennifer Possley, director of regional conservation at Fairchild Tropical Botanic Garden, the decline of the Key Largo tree cactus could serve as an indicator for how other low-lying coastal plants will respond to climate change. This sentiment reflects a growing concern among environmental scientists about the broader implications of rising sea levels on biodiversity.
The Key Largo tree cactus was often confused with the Key tree cactus (Pilosocereus robinii), a federally endangered species also found in the Florida Keys. Both species share similar characteristics, such as their perpendicular growth and cream-colored flowers. However, distinct differences, such as the long, woolly hairs at the base of the Key Largo tree cactus flowers and longer spines, led researchers to identify it as a separate species.
By 2019, it was confirmed that the Key Largo population was the only known instance of Pilosocereus millspaughii in the U.S. Unfortunately, it faced similar environmental pressures as its relative, which has also seen significant population declines due to habitat loss and human activities. Efforts to monitor and conserve these species have been ongoing, with researchers at Fairchild Tropical Botanic Garden leading the charge since 2007.
Despite significant conservation efforts, including off-site cultivation and seed banking, the Key Largo tree cactus could not withstand the compounded environmental stressors. Researchers have initiated robust conservation collections, growing potted cacti at a facility in Coral Gables, Florida, and banking seeds for long-term preservation.
Possley and her team at Fairchild Tropical Botanic Garden have plans to replant the Key Largo tree cactus in the wild in collaboration with the Florida Department of Environmental Protection. However, Possley cautions that this may be a temporary solution rather than a long-term fix. Suitable habitats for these cacti are rapidly disappearing, complicating efforts to reintroduce and sustain populations in their natural environments.
The extinction of the Key Largo tree cactus highlights the complex and interconnected nature of climate change impacts. Study co-author George Gann, executive director for the Institute for Regional Conservation, emphasized that the loss of biodiversity is becoming increasingly evident, with more than one in four native plant species in South Florida critically threatened or already extinct. The knowledge gained from studying and conserving the Key Largo tree cactus will be instrumental in protecting other endangered plant species in the region and beyond.
