Sulphur butterflies glide across Zorrillo Canyon, hundreds of them, moving back and forth against the cerulean sky. It’s nothing short of a fairy wonderland for the scientists below.

At least, that’s how Jon Rebman, curator of botany at the San Diego Natural History Museum, describes it as he begins the hike into the canyon on a late October morning.

Rebman is four days into a weeklong expedition in the Sierra de las Cacachilas mountain range in Baja Sur, just outside of La Paz, part of a collaborative effort with Mexican and U.S. scientists to track the region’s biodiversity.

He and his botany team — along with dozens of other scientists from the NAT and Mexico studying mammals, insects and reptiles — have explored the nearby mountaintops and canyon washes, all while camping at Rancho Cacachilas, an eco-tourism destination the museum partners with to conduct its research.

For many of the scientists, it’s a return; this year’s expedition serves as a follow-up to an investigation into the Sierra launched more than a decade ago in 2013. Until that first expedition, little was known about Las Cacachilas’ biodiversity and opportunity for study.

“It was like a black hole,” Rebman says. “This entire Sierra had almost no historical collection.”

Today, they’re expanding on that knowledge, and on their work to document all of the area’s biodiversity. Their goals are twofold: They want to identify the species for the sake of science. But their research is also part of a larger effort to understand how Rancho Cacachilas’ farming practices and land and water management are impacting the area.

Rebman is perhaps one of the most qualified people to hike with in Baja — that is, if the goal is to learn the most information possible about the region’s flora. “I’m usually botanizing all the time in my head,” as he puts it.

The result is a nearly encyclopedic knowledge curated over more than 30 years of research on the peninsula.

Rebman is a go-to authority on Baja and Southern California flora, authoring books and dozens of papers in his years in the field. He’s played a key role in expanding the NAT’s Baja herbarium since landing there in 1996; there are now more than 55,000 plant specimens in its Baja collection. Five species are named for him — soon six — and he’s described more than 30 to science.

In Zorrillo Canyon, he is eager to share as he hikes slowly, pausing every few yards to offer his team facts about surrounding plants and quiz his mentee, Abraham Sanchez, a 27-year-old master’s student in taxonomy. Palo blanco, hierba de indio, an endemic cotton plant — the list goes on.

“You want to hear a big pollination story?” Rebman asks three hours in, launching into a tale of how wasps pollinate rock fig trees, like the one nestled among granite boulders above. He acknowledges his loquacious tendencies. (“Mucho blah blah blah,” he says, apologizing with a chuckle.) But his insight is part of the point of the expedition.

So is the collaboration between scientists. Sharing meals at camp, researchers excitedly tell each other about their findings. It’s not uncommon for one to casually pick up a tarantula found near the morning coffee or hold a rosy boa after dinner.

There are missed opportunities, too — the shocked faces of herpetologists who weren’t there to see a snake, a cape stripe racer, devouring a lizard under the shade of a tree.

But then there are the simpler moments: scientists kneeling in the dirt and overturning rocks to spot bugs, calling to a small owl in the quiet of the night or frolicking with nets to capture winged insects.

As Rebman and his group continue up the trail, a voice exclaims from across the canyon: “I caught my first butterfly!”

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The scientists are exploring a section of the Sierra that lies within Rancho Cacachilas, a tourism destination and series of ranches founded by Christy Walton — of the Walmart family — around 2011.

The ranch is 40 minutes southeast of La Paz, the capital of Baja Sur, in a region famed for its adventure tourism. La Paz is known for its beaches, snorkeling, fishing and boat tours out to Isla Espiritu Santo, a UNESCO World Heritage Site. But its nearby mountains, less of a draw, are surrounded by cattle ranches and open space, the fishing villages a distant vista to the east.

With this expedition, the first in a three-year project, scientists have been invited to Rancho Cacachilas to assess how the flora and fauna are responding to the ranch’s land management practices.

Rancho Cacachilas is a working ranch, growing vegetables and raising goats. Nearly 1,400 tourists come each year to enjoy its guided mule rides, mountain biking excursions and artisanal goat cheese workshops. But it also aims to serve as a model for sustainable agriculture and ranching, a longstanding livelihood in the area, explains Jose Manuel, the ranch’s general manager.

“The idea is to keep the people on the ground, give them opportunities and give them more ways to work and to preserve those traditions,” he says.

It’s research the area needs: Decades of cattle overgrazing have led to erosion and degradation of the watershed in Las Cacachilas, and the area struggles with water scarcity. While ample rain decorated the hillsides in shades of green this fall, that’s not always the case.

As the scientists conduct their research, they are camping at Rancho Cacachilas — glamping, really. There are clean bathrooms with warm water and bucket showers. Three meals a day are served — gourmet, farm-to-table dishes including mole, tacos and pozole, complete with fresh cheese, vegetables and dessert after every dinner.

The comfort is welcome after long days hiking in 85-degree heat and humidity. The scientists rise around 5:30 a.m., even before the stunning sunrise over the Sea of Cortez, and tuck into their tents around 9 or 10 p.m.

Many are used to the days of field work. Rebman arrived before the other scientists — driving nearly 1,000 miles from San Diego, mostly listening to 80s music — and will stay on a few days longer to do additional research.

The NAT has been conducting research in Baja for over a century, with naturalists visiting the peninsula to document everything from its cacti to marine life.

More recently, in addition to their work in Las Cacachilas, scientists have worked on a red-legged frog breeding program in the Sierra San Pedro Martir, in northern Baja, and helped organize a program to bring more women into botany.

Rebman has been coming to Baja for more than 30 years and even lived here for a time. He’s created strong friendships with the scientists, and his passion for the plants and the land is infectious.

He warns he will use the word “endemic” countless times — referring to the term to describe species that only exist in a certain place. More than a quarter of all the plant species found across the Baja California peninsula are found exclusively in the region, with five endemic to Las Cacachilas.

But as he sees it, despite the hundreds of miles from San Diego, Baja Sur is simply the furthest end of a connected natural gradient, separated only by a political border.

“That is the original California,” he said in his office the week before the expedition.

“It is like this continuous grade of things all the way down the peninsula,” he adds. “A lot of it comes from our area and trickles down.”

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Rebman can talk about anything — his music interests and family, love of the iNaturalist app, even the Barbie movie or how he no longer considers himself a romantic. But most conversations come back to plants.

It wasn’t always thus. Growing up in Illinois, the only plants he learned about were corn and beans. Instead, he dreamed of being a country veterinarian. He even lived in a dog kennel for a time, where he was paid to let the dogs out.

That changed in college.

“I realized, ‘Oh, my God, people don’t know that much about the diversity of plants,’” he said. “So I started learning that diversity and have never stopped.”

Rebman gradually made his way to southwestern desert landscapes — first Arizona, then on to Ensenada on a Fulbright. He eventually arrived in San Diego, a jumping point to the flora in Baja he’d come to love so dearly.

He’s a specialist in prickly-pear and cholla cactus species. But as a field biologist, he actively collects or documents nearly every plant he sets his sights on. He has more than 37,000 specimens in his personal collection.

In the years following the NAT’s first expedition to Las Cacachilas, Rebman and his team ultimately recorded 500 plant species in the Sierra. But for Rebman, one of the goals of this trip is to identify more species and ultimately publish a scientific paper on the findings.

To Sula Vanderplank, a British botanist who lives and works in Loreto, about 150 miles north, Rebman is an “olympian” in the field.

Vanderplank is a research associate for the NAT and a leader in land conservation for Pronatura Noroeste, an environmental organization in northwestern Mexico and one of the NAT’s partners on the expedition. The museum also worked with universities in Baja California, the Biological Research Centre of the North-West and other groups to make the expedition happen.

Vanderplank, who has dedicated her work to the peninsula for 20 years, says the partnerships have helped bring more people into the research, with longtime experts like Rebman serving as a guide.

“I feel like it used to be the elite club,” she said. “What I love now is that it’s more young people, more students, more up-and-coming researchers … a really nice change over time.”

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There’s no competition among the NAT scientists and their disciplines, Rebman says — well, except for a potential rivalry with Shahan Derkarabetian, the curator of invertebrate zoology at the museum, over how many species both scientists can identify.

Derkarabetian makes a distinct foil to Rebman’s style. Rebman barely lets a moment pass between sentences; Derkarabetian is quieter, offering information only when it’s asked of him.

He finds peace in the field, overturning stones and trudging up leaf-covered hillsides in pursuit of the smallest of insects — “looking for anything that looks like an arachnid, and any other weirdos.”

“Rock piles are my favorite thing,” he says, off the trail in Zorrillo Canyon.

That morning, he spots a tailless whip scorpion and an unnamed millipede, among other species familiar and unknown, capturing some to bring back and simply observing others.

But Derkarabetian is here on a mission to find the Kevonones mexicanus, a rare arachnid that is known only from a specimen described in 1898 in Cabo San Lucas with a simple drawing.

Scientists collected one other specimen in 2016 during the first expedition to the Sierra. No others have been found since.

“If it’s there, I’ll find it,” he says.

He still hasn’t found it, but he’s undeterred. For Derkarabetian, it’s about the act of discovery, a love of scouring for unique insects that he’s held since he was about 9 years old growing up in Rancho Cucamonga.

Later that night, a black witch — a moth about the size of a small human hand — lands on a light trap the team set up to attract the insects. Once he spots it, Derkarabetian quietly exclaims, “Oh my goodness, oh my goodness.”

It’s not the first time he’s seen one, but the thrill hasn’t waned.

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Over the course of the expedition, there’s a look toward the future — not only of the land and its plants and animals — but of who will carry on the veteran scientists’ research, like Rebman and Derkarabetian’s, in the decades to come.

In recent years Rebman has lost his parents, his sister and two brothers-in-law. He knows he won’t live forever, and in his garrulousness, he’s conveying to younger scientists an important message: This will one day be in your hands.

“Who’s gonna be able to assess diversity loss?” he wondered. “If you can’t recognize everything, how do you know what you’re missing?”

On the final night of the expedition, the scientists gather at a community event in El Sargento, about a 15-mile drive east from Rancho Cacachilas. Each team has a table set up with photos and live specimens, showing the public what they’ve found in the nearby wilderness.

Beyond inspiring the next generation of curious minds, it’s clear the work these scientists are doing is fueling them now.

As the evening comes to a close, Derkarabetian heads out, tired. He hasn’t found the rare arachnid he was hoping for — the discovery that could bridge the gap between his research and that of scientists over a century ago.

But this wasn’t the worst thing.

“I’ve had species that have taken me years,” he says, a glint of excitement flashing in his eyes. “I like a challenge.”

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