The Key Largo tree cactus, the only population of its species in the U.S., has recently disappeared due to rising sea levels, marking a major ecological event.

The species known as Pilosocereus millspaughiistill grows on some Caribbean islands, but had one population in Florida, discovered in 1992.

Belwether for coastal plants

Over time, saltwater infiltration, hurricanes, soil depletion, and herbivorous mammals have severely depleted Florida’s Key Largo tree cactus population.

In 2021, of the once flourishing cluster of about 150 stems, only six remained. These were saved by researchers for cultivation.

“Unfortunately, the Key Largo tree cactus may be a harbinger of how other low-growing coastal plants will respond to climate change,” said lead author Jennifer Possley, regional conservation director at Fairchild Tropical Botanic Garden.

Key Largo Tree Cactus Identification

Initially, researchers were uncertain about the cactus’s identity and confused it with the closely related Key tree cactus (Pilosocereus robinii), an endangered species found elsewhere in the Keys.

The two species share similarities, such as their vertical stems and garlic-scented flowers, but differ in important characteristics.

“The most noticeable difference is the tuft of long, woolly hairs at the base of the flowers and fruits,” said Alan Franck, herbarium collection manager at the Florida Museum of Natural History.

These unique features, along with longer spines, led to confirmation that the Key Largo cactus was indeed Pilosocereus millspaughiithe first of its kind in the US.

Habitat destruction due to human development

The decline of the Key Largo cactus reflects the environmental challenges facing the Key tree cactus. The Key tree cactus was once abundant, but its numbers have declined sharply due to habitat destruction by human development.

Botanist John Small noted in 1917 that the species had become scarce in its natural habitat due to deforestation and land development.

Impact of rising salinity

Despite being listed as a federal endangered species in 1984, the Key tree cactus population has continued to decline, declining 84 percent between 1994 and 2007. Fairchild researchers have been monitoring these populations annually since 2007.

A study has found that higher soil salinity levels under dead cacti lead to higher mortality after storm surges, underscoring the impact of rising salinity.

Herbivores and extreme weather

The Key Largo tree cactus faced similar threats, exacerbated by herbivore consumption and extreme weather events. In 2015, James Lange and his team observed significant herbivory, likely due to the shortage of freshwater caused by king tides.

Attempts to identify the herbivores using cameras were unsuccessful, but the damage continued. In 2017, Hurricane Irma brought a five-foot storm surge that flooded large parts of Key Largo and severely damaged cactus habitat.

Trunks of Key Largo Cactus Trees

By 2021, there were only six stands of Key Largo tree cactus left. Recognizing the impending extinction of this population, researchers allowed the plants to flower and fruit before saving the remaining green material for greenhouse cultivation.

“We have preliminary plans with the Florida Department of Environmental Protection to replant some of them in the wild,” Possley said.

Protection of the key cactus

Although reintroduction efforts have contributed to the survival of the Key tree cactus in Florida, finding suitable habitat for replanting is a challenge.

Possley noted that habitats suitable for tree cacti, such as the edges between mangrove forests and highland hammocks, are rapidly disappearing.

At the frontlines of biodiversity loss

The decline of the Key Largo tree cactus is a painful reminder of the broader impacts of climate change.

“We are on the front lines of biodiversity loss,” said George Gann, director of the Institute for Regional Conservation.

In the past 25 years, more than one in four native plant species in South Florida has faced regional extinction or serious threats from habitat loss, overcollection, and invasive species. More than 50 species have already disappeared, including four global extinctions.

This study, published in the Journal of the Botanical Research Institute of Texasunderlines the urgency of addressing the complex and multifaceted challenges posed by climate change and environmental degradation.

As researchers strive to conserve the remaining tree cacti, their work highlights the urgent need for proactive conservation measures to protect biodiversity in a rapidly changing world.

Image Credits: Photo courtesy of Susan Kolterman

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