Sea level rise claims its first coastal plant victim in the U.S.

The Key Largo tree cactus, the only population of its kind in the US, has been recently lost due to rising sea levels, marking a significant ecological event. 

The species, known as Pilosocereus millspaughii, continues to grow on some Caribbean islands but had a single population in Florida, discovered in 1992. 

Bellwether for coastal plants

Over time, saltwater intrusion, hurricanes, soil depletion, and mammalian herbivores significantly pressured the Key Largo tree cactus population in Florida. 

By 2021, the once-thriving group of around 150 stems had dwindled to just six, which researchers salvaged for cultivation.

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

Identifying the Key Largo tree cactus 

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

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

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

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

Habitat destruction from human development 

The decline of the Key Largo cactus mirrors the environmental challenges faced by the Key tree cactus. Once abundant, the Key tree cactus’s numbers have plummeted due to habitat destruction from human development. 

Botanist John Small observed 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 federally endangered in 1984, the Key tree cactus population continued to decrease, dropping by 84% between 1994 and 2007. Researchers from Fairchild have been monitoring these populations annually since 2007. 

A study linked higher salt levels in the soil beneath dead cacti to increased mortality following storm surges, highlighting the impact of rising salinity.

Herbivores and extreme weather

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

Efforts to identify the herbivores using cameras were unsuccessful, but the damage continued. By 2017, Hurricane Irma caused a five-foot storm surge, flooding large parts of Key Largo and severely damaging the cactus habitat.

Key Largo tree cactus stems

In 2021, only six Key Largo tree cactus stems remained. Recognizing the imminent extinction of this population, researchers allowed the plants to flower and fruit before salvaging the remaining green material for cultivation in greenhouses. 

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

Protecting the Key tree cactus 

While reintroduction efforts have helped sustain the Key tree cactus in Florida, finding suitable habitats for replanting is challenging. 

Possley noted that environments suitable for tree cacti, such as the fringe between mangroves and upland hammocks, are rapidly disappearing.

On the front lines of biodiversity loss 

The decline of the Key Largo tree cactus serves as a stark reminder of the broader impacts of climate change. 

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

Over the past 25 years, more than one-in-four native plant species in South Florida have faced regional extinction or severe threats due to habitat loss, over-collecting, and invasive species. More than 50 species have already vanished, including four global extinctions.

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

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

Image Credit: Photo courtesy of Susan Kolterman

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