Some Arizona border officials have welcomed President Donald Trump’s order for a military takeover at the U.S.-Mexico border.

But migrant advocates fear that by sealing hundreds of miles of border in the Southwest, the troops will effectively funnel migrants to far more dangerous crossing points. And environmentalists warn of damage to habitats that support nearly two dozen endangered species.

“Militarizing the border has historically only ramped up deaths,” said James Holeman, founder of Battalion Search and Rescue, a group of volunteers who hike through desolate regions of Arizona and New Mexico searching for remains of migrants who couldn’t survive the desert.

“You’re talking about vulnerable people that are making very deadly choices,” he said.

On April 11, Trump ordered the military to take control of the Roosevelt Reservation – a 60-foot-wide strip of federal land along the border from the Pacific Ocean to New Mexico.

Turning the border into a military base would get around the 1878 Posse Comitatus Act, which prohibits the military from engaging directly in civilian law enforcement. Migrants would be subject to military arrest for trespassing within the federal zone.

That zone and Trump’s order cover Arizona’s four border counties – Yuma, Pima, Santa Cruz and Cochise – except for a 62-mile stretch of Pima County controlled by the Tohono O’Odham Nation.

Holeman, among others, expects that gap to become an even bigger magnet for human trafficking.

The tribe’s ancestral lands span the border, and roughly 2,000 of 34,000 members live on the Mexican side.

Tribal leaders declined opportunities to discuss the situation.

Even before the military build-up to the east and west, the reservation was a hot spot for illegal crossings. The tribe’s stretch of border is relatively flat. And it’s secured with vehicle barriers and large-gapped cattle fencing that don’t impede people, because the tribe refused to allow wall construction during Trump’s first term.

The Border Patrol operates from offices just outside the reservation. Tribal police work with federal authorities when they catch migrants, Mennell said, but response times can be long.

As the military seals other parts of the border, the reservation will become a more attractive option, Holeman said, and smugglers will demand higher prices.

“Cartels have really taken over human trafficking,” he said. “Crossing here is going to be more expensive. If you have thousands and thousands of dollars, they can drive you right in.”

As of Tuesday, there were 10,281 troops assigned to the Southwest border – up from 2,500 before Trump’s order, according to U.S. Northern Command.

“I welcome it. … Anything that can reduce the impact on taxpayer dollars in this county,” said Frank Antenori, a Cochise County supervisor. “It’s about time the federal government got serious about securing the border and protecting the citizens of this county.”

Cochise, in Arizona’s southeast corner, has a population of about 124,000, and its law enforcement and health resources have been stretched thin by illegal immigration, Antenori said.

The day he was sworn in for a second term, Trump declared a national emergency at the border and ordered more military personnel and surveillance aircraft there.

He expanded the effort with the April 11 order assigning the “military missions of repelling the invasion and sealing the United States southern border from unlawful entry to maintain the sovereignty, territorial integrity, and security of the United States.”

Four days later, Interior Secretary Doug Burgum announced that his department would transfer control of 109,651 acres of federal land along the border to the Army for three years.

“The American people gave President Trump a mandate to make America safe and strong again,” he said in his announcement.

Having more troops at the border will allow faster expansion of the border wall, along with more roads, lights and surveillance systems, according to John Mennell, acting branch chief for the Tucson sector of U.S. Customs and Border Protection.

“Whatever hasn’t been done in the first term is getting done now,” he said in a phone interview.

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But there are critics in southern Arizona.

Pima County Sheriff Chris Nanos, whose department patrols Tucson and a vast stretch of borderland, said he’s concerned that the expanded troop presence - along with Trump’s immigration crackdown – will deter migrants from asking for help.

“There’s fear out there that ‘if I call 911, my family member or someone is going to be deported,’” he said.

Nanos would prefer reform of immigration policies, such as providing an easier and more orderly way to become a citizen. Although officials elsewhere say undocumented migrants squeeze their budgets, Nanos said he hasn’t seen that.

“Even if it did, isn’t that our job?” Nanos said. “We are here to protect people in this county. That’s what we’re here to do, no matter their status.”

Environmentalists are concerned about the impact the extra troops and their equipment will have on Sonoran Desert ecosystems that are home to thousands of plant and animal species, of which 23 are endangered.

The interior secretary said the troops’ presence will actually help preserve delicate habitats and sites of archaeological importance.

“High-traffic illegal crossings can lead to soil erosion, damage to fragile desert vegetation and critical wildlife habitat, loss and damage to cultural resources, increased fire risk and pollution from trash and human waste,” Burgum said when he announced the land transfer to Army control.

During Trump’s first term, the federal government installed tall fencing along roughly a third of the U.S.-Mexico border.

Before that, any barriers generally allowed animals to pass freely. Jaguars, ocelots, mountain lions, bears and other species whose ranges span the border have all been impacted, said Russ McSpadden, who leads campaigns for the Coalition for Sonoran Desert Protection.

Surveillance lights and increased vehicle patrols also disrupt species that call the region home, he said, and military vehicles will surely inflict more harm.

“The devastation was on a grand scale the first time around,” he said. “These walls are being built in really sensitive ecosystems.”

According to Defenders of Wildlife, the Sonoran pronghorn, the fastest mammal in North America, is endangered partly due to fencing. Only 160 remain in the U.S., plus 240 in Mexico.

Trump’s efforts to seal much of the border with troops comes despite big drops in illegal crossings.

Arizona has seen a huge decrease since last fall. The drop has been especially sharp since Trump took office three months ago. At the current rate of decline, the total for the fiscal year that ends Sept. 30 could be down 80% from the previous year.

Even so, Battalion Search and Rescue found nearly 20 sets of human remains last winter.

Much of the group’s work is along the “poor man’s route” through the Organ Pipe Cactus National Monument, west of the Tohono O’Odham reservation – a trek through desolate and rugged terrain that avoids populated areas but claims many lives.

“This is where people go when they have the least amount of money, the least amount of options,” Holeman said. “There is basically no water and it has the highest rate of apprehension. It has the highest risk and the lowest price.”