DENVER (AP) -- About this time every year, southeastern Colorado towns like La Junta see more visitors looking to glimpse saucer-sized Texas brown tarantulas as they scurry out of their burrows and across fields and roads in search of a mate.
The eight-legged spectacle might not last long: Scientists say climate change, theft and human development are cutting into tarantulas' population, with fewer emerging year after year.
"It's a noticeable numbers game," said Ryan Jones, a research associate at the Denver Museum of Nature of Science. "If you compare pictures, you can tell that there are just fewer males out looking for love."
If the tarantulas in southeastern Colorado die out, they'll start a domino effect in the ecosystem, entomologist Maia Holmes said, likely allowing their prey to multiply and the creatures that eat them to starve.
"Once we cross that line we can't go back," Holmes said.
Scientists don't have precise population data because there isn't a dedicated group focused on the tarantulas in the region, but Jones, who is working on his PhD in program integrative and systems biology at the University of Colorado Denver, said people who live in that part of the state call and email the museum every year -- people who used to have a hard time driving around without squishing tarantulas on the road.
Compare that to what a former colleague of Holmes, who is the director of Colorado State University's Bug Zoo, told her. Twenty years ago, he said he'd see hundreds around La Junta on a given day but "now, if you drive around all night you'd maybe see 20 or 30," she said.
The shriveling tarantula population acts as an early indicator that climate change is worsening in Colorado, Holmes and Jones said. Indeed, Holmes said, the largest single factor for fewer tarantulas is climate change because warming temperatures and less rainfall make for a less hospitable environment.
Studies suggest arachnids are particularly susceptible to changing temperatures and weather patterns, which could also cause different species to grow larger or even become more aggressive, according to Sergio Henriques, co-chair of the International Union for Conservation of Nature's Spider and Scorpion Specialist Group.
Aphonopelma hentzi, which is the species found in southeastern Colorado, live as far south as Texas and as far west as California. But since habitats vary widely across the American South and Southwest, Holmes and Jones said it's difficult to know whether populations will shrink everywhere else.
Here, at least, diseases that can kill tarantulas survive better in warmer weather, and hot days can trigger the arachnids to come out of their burrows too early, which burns their much-needed energy reserves. Or they'll emerge too late, leaving them unable to prepare their burrows properly for winter, Holmes said.
"And no water means no plants and if there's no plants, there's no prey," Holmes said. "They can starve. They can die of dehydration."
Every decade, National Weather Service staff in Pueblo calculate 30-year averages for temperatures and rainfall in the region, NWS meteorologist Eric Petersen said.
In 2020, they recorded a 1.3-degree annual temperature increase and rainfall decrease of about half an inch, indicating a warming and drying trend for the region. Typically, Petersen said, 30-year average temperatures change by about a tenth of a degree.
"So 1.3 degrees is a fairly large change," Petersen said. "And we live in a climate where we only get about 12 to 14 inches of rain a year, so half an inch is a fairly substantial drop."
Tarantulas take eight years to mature, Holmes said, so scientists can also take into account fires, droughts and hot years that will kill more adolescents than normal.
"Eight years ago was a really bad fire year, a really bad drought year," she said.
Steve Keefer has lived in southeast Colorado for about 30 years, and said he's seen a lot of tarantulas in his time. The district wildlife manager for Colorado Parks and Wildlife said he hasn't noticed fewer tarantulas as much as they're emerging later in the year and in different locations. At least one person told him they had seen tarantulas as far north as Colorado Springs.
Typically, Keefer said, they'd show up around Labor Day, and infrequently in late August. Lately, it's been late September and early October, which he said would coincide with reports of warmer weather.
-- Arachno-mania and sprawl
Here's where humans come in: The more we build, the more we cut into the habitat in which tarantulas live and burrow, Holmes said. And especially since reports of the tarantulas' presence spread across the country in 2019, there's been an uptick of people scooping them up and taking them home, either to keep as pets or to illegally sell on the black market.
Taking tarantulas and keeping them as pets isn't technically illegal but selling them is, Keefer said. And if you do keep them, the males (who are the majority of tarantulas out in the open) won't live long.
While Holmes estimated that if the tarantulas go extinct in southeast Colorado, grasshoppers could multiply faster and devastate local plantlife. Wasps that feed on the tarantulas could also die out, leaving birds, reptiles and mammals like coyotes to go hungry -- and potentially starve.
But just how drastic the ripple effect could be isn't clear, she said.
"That's the scary thing, we don't know. That's what's terrifying," Holmes said. "Oftentimes we don't fully understand these components of the ecosystem until they're gone and we start to see things collapse."
"Extinction is forever," she added.