Sea level rise causes local extinction of rare cactus, an omen for conservation efforts

As out-of-control heat cooks our planet, shattering temperature records for the last 13 consecutive months, it melts polar ice and raises sea levels. Most people are familiar with how this works, but we typically only think of the consequences of sea level rise as threatening to drown Miami, for example. But extinction is also a factor, as evidenced by recent news that the United States has suffered its first-ever local extinction as a result of sea levels rising. The victim is the iconic 20-foot tall Key Largo tree cactus (Pilosocereus millspaughii).

The prickly behemoth — encased in light green skin and sporting cream-colored, garlic scented flowers — was first discovered in 1992, according to the study published in the Journal of the Botanical Research Institute of Texas. From an initial population of more than 150 stems in the Florida Keys, the once-thriving Key Largo tree cacti began dwindling due to salt water intrusion, soil depletion, intensified hurricanes and unusually high tides, all of which are either caused or exacerbated by global warming. As a result, the Key Largo tree cactus is now locally extinct from both the Florida Keys and the United States as a whole, though other related cacti are indigenous to Cuba and the Bahamas.

The Key Largo tree cactus was initially found growing in the United States in 1992 at a single site. That population has since been lost to a combination of rising sea levels and increasingly intense storms. (Photo courtesy of Susan Kolterman)

Even worse, the scientists who chronicled the Key Largo tree cactus' extinction are now predicting that this is only foreshadowing future local extinctions as part of the ongoing sixth mass extinction driven by human activity.

A 2023 study published in the journal Proceedings of the National Academy of Sciences found that humans have caused as many extinctions over the past 500 years as would have occurred without them over 18,000 years. Similarly, a 2021 study in the journal Communications Earth & Environment reported that the average predicted extinction rate for freshwater animals and plants today is three orders of magnitude higher than it was 66 million years ago, when the dinosaurs famously went extinct during the so-called Cretaceous–Paleogene extinction event.

One of the primary culprits behind these mass extinctions is climate change, in which the planet overheats because humans burn fossil fuels that pump greenhouse gases into the atmosphere. Among other things, climate change melts Earth's ice caps and thus causes sea levels to rise.

As for the Key Largo tree cactus, "This is the first of many losses likely to be suffered in the coming decades," research botanist Jimmy Lange told Salon. "Much of the unique Keys flora are found at relatively low elevations, even for the Keys. This situation is confounded by decades of habitat loss, degradation and other factors like invasive species that threaten the integrity of these terrestrial ecosystems. Luckily, there are so many great land managers, NGOs and [government] agencies working together to preserve these unique habitats and the natural resources they support."

Though this extinction is ominous for other flora and fauna in ecologically precarious regions, Lange also notes that the Key Largo tree cactus has intrinsic value of its own. Its loss in the United States is irreplaceable.

Staff from Fairchild and the Florida Department of Environmental Protection removed all remaining green material in 2021 after it became clear the population was not going to survive. (Photo courtesy of Jennifer Possley)

"I remember the first time I hiked out through the mangrove swamp and into the dense brush where this population occurred and being amazed by the sheer size of the stand," Lange said. "Dozens of large stems towering above the shrub canopy, covered in large tufts of hair. It was quite the site to behold. It speaks to the treasure that is the Florida Keys flora, where so many unique plants, many at the northern extent of their tropical range, make a home on the rocky substrate formed by ancient coral reefs during periods of much higher sea level."

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Lange added, "Apart from species like this that are found in other parts of the tropics, many plants and animals are endemic (found nowhere else) to the Keys."

This is not to say that the Key Largo tree cactus is incapable of a comeback in the United States. Quite to the contrary, Jennifer Possley — the director of regional conservation at the Fairchild Tropical Botanic Garden — hopes that it can be revived.

“We have tentative plans with the Florida Department of Environmental Protection to replant some in the wild,” Possley said in a statement. The conservationists became aware of the need for major restoration efforts over the past decade, as one natural event after another took a recorded toll on the cactus population. This included Hurricane Irma, a category 5 storm that created a 5-foot surge as it swept across southern Florida in 2017. Because the highest point on Key Largo is only 15 feet above sea level, large portions of the island were flooded for days after the storm. Exceptionally high king tides in 2019 likewise flooded large portions of the island for months, and by 2021 only six Key Largo tree cactus stems remained alive.

To salvage the plants that at this point were obviously doomed, a team of scientists allowed the plants to flower and fruit before saving as much green material as possible, to replant it in greenhouses or outdoor controlled settings. So while technically the plant isn't extinct on the same level as a Tyrannosaurus rex, it is still severe and can negatively impact ecosystems.

These are the patchwork efforts made by local scientists to protect their ecosystems from global warming — but they know that, while they may win a few battles, they are losing the larger war.

"People should get out and experience the unique ecosystems of the Florida Keys while they can," Lange warned. As the study notes in its abstract, "The other cacti in the region, and all rare plants in the Florida Keys, are threatened with a similar fate."