NATURE NOTES:
Cacti Count: Taking Stock of Big Bend National Park’s Endangered Plants

photograph by Whitney Behr. Carolyn Whiting – Big Bend’s new park botanist – and her collaborators walked miles of desert to learn about the populations sizes of the park’s endangered plants.

In 1894, a 22-year-old Texas Ranger named Everett Townsend was tracking stolen mules when he arrived at the South Rim of the Chisos Mountains. He looked out from forested heights upon a panorama of mountains, mesas and desert badlands, to the Rio Grande and its canyons. The future “Father of Big Bend National Park” was, he said, “overpoweringly impressed”: Here was something rare and special, to be preserved and shared. 

Townsend’s insight – which helped lead to the park’s creation 50 years later – was sound indeed. Big Bend is a place of wonders. That includes a number of rare and endangered plants – some found nowhere else in the country, or nowhere else on Earth.

Carolyn Whiting is Big Bend’s park botanist.

“I think there’s always something to discover in Big Bend National Park, as a visitor.” Whiting said. “You can take a drive and watch the scenery change, or you can get out of your car and you can find little cacti growing along the side of the road. You don’t have to go far from the trail to get a taste of the huge diversity of habitat and scenery that Big Bend has to offer.” 

Whiting became park botanist in October. It’s an ideal fit. While settling into the job, she’s also completing a PhD dissertation – on the park’s threatened and endangered plants. Four species here are federally protected under the Endangered Species Act.

Three are cacti. And one has a particular distinction.

“I like it because it only grows in Big Bend National Park,” Whiting said. “It feels really special when you’re looking at it – this is the only place in the world where you get to see this species. And it’s also very pretty.”

The Chisos hedgehog – Echinocereus chisoensis – has familiar local kin. Claret cup cacti and strawberry pitaya are of the same genus. And like the Texas rainbow cactus, the Chisos hedgehog usually has a single stem. But its big pink flower, which blooms in March, sets it apart. And it’s found only in a park habitat called “desert pavement.” In these lowland areas scoured and leveled by wind, the Chisos hedgehog grows in the shade of a creosote or other “nurse plant.”

The bunched cory cactus was listed as threatened in 1979. Its blooms too are pink. But this baseball-sized cactus grows on rough limestone outcrops. It’s found in the park, across the river in Coahuila – and downstream in the Rio Grande’s Lower Canyons.

Then there’s Lloyd’s mariposa cactus. Like the cory cactus, it grows on limestone exposures. But it’s found more widely, both within the park and beyond.

“It’s very cute,” Whiting said. “It’s small, about the size of a golfball. Its spines are so dense – that’s one of the distinguishing features of it – you can’t really see the stem of the plant because there are so many spines.”

The most recent addition to the list is Guadalupe fescue. In the U.S., this grass is found only in a single canyon in the Chisos. It received federal protection in 2017.

But as with the three cacti, park staff have long known the fescue was rare. Indeed, park staffers – including Whiting’s predecessor, Joe Sirotnak – have monitored the four plants for decades.

They did it using a system of demographic plots. These plots were placed in dense stands of the rare species – so that many plants could be tracked over time. The method yielded important insights – including into the plants’ lifespans and reproduction rates. But it left a central question unanswered.

“And this is the question we’re concerned with with endangered species,” Whiting said. “How many are there? And are we losing them or gaining them? It was a question that was worthy of a pretty narrow focus.”

Whiting took that question up in her PhD research.

She started by mapping the habitat where the species occur. Then she sampled that habitat evenly, to get an unbiased estimate of plant densities.

It required extensive fieldwork. Whiting and her collaborators walked miles of Big Bend deserts and canyons in “transects,” documenting each plant they found. Flowering plants are easier to identify, and Whiting timed the sampling to blooms. 

By multiplying density by total habitat, Whiting is generating population estimates for the four plants. Her statistical work is ongoing. But the preliminary findings have raised estimates for Guadalupe fescue from a few hundred to more than a thousand plants. And the Lloyd’s mariposa cactus appears to be more abundant than once thought. 

The work is revealing how rare the plants actually are. And Whiting’s research will also provide a “baseline” for future monitoring, to track whether plant populations are stable or declining.

“As we’re learning more about these plants,” Whiting said, “we might find evidence that it’s time to have a hard look and see whether they need to be delisted or not. And keeping track of them over time is certainly important as we have climate change and more extreme droughts and changes in precipitation patterns.”

Protecting the four endangered plants is just one aspect of Whiting’s work as park botanist. Controlling invasive species is another priority. And in many ways, these four plants are just the tip of the botanical iceberg here. 

Diversity is part of the spell Big Bend casts. From river to summit, from tawny volcanic badlands to hidden springs in shaded canyons, the park contains worlds upon worlds. Each of those worlds sustains a unique suite of living things. 

In those diverse niches, there are likely as-yet unheralded plants that are just as rare as those with official recognition, Whiting said.

“I absolutely think we have other plants in the park that are just as worthy of this kind of attention,” she said. “It concerns me that we’re not getting them that attention, I’d hate to see species blink out, when we could have been paying closer attention to them. We have huge diversity in the park. The list is long of plants that are interesting.” 

photograph by Carolyn Whiting. Golfball-sized and densely covered in spines, the Lloyd’s mariposa cactus is one of four threatened or endangered plants found in Big Bend National Park.