As politicians in Washington quarrel over funding for border security, a protest rally is planned for Saturday at the Texas wildlife centre that is set to be one of the first locations of Donald Trump’s wall.

This year marks the 75th anniversary of the Santa Ana national wildlife refuge, a 2,088-acre site by the Rio Grande river that is festooned with Spanish moss and a haven for migratory birds, butterflies and ocelots. But campaigners fear the sanctuary will be wrecked if the government builds a giant barrier through it.

“It’s one of the most biodiverse refuges in the United States,” said Scott Nichol of the Sierra Club, an environmental advocacy organisation that is one of the groups holding the event in a field adjacent to the refuge.

Ocelots – small, spotted wild cats – are endangered in the US because of habitat destruction and Nichol fears that a wall would trap them, not only restricting their movement but killing them if the area floods during a storm. “That basically turns the refuge into a death trap, the waters rise and the animals all drown,” he said.

The possible 150ft “enforcement zone” around a barrier would also be disastrous for wildlife, he said, causing vegetation to be cleared and replaced by gravel paths used by US border patrol vehicles and brightly lit at all hours.

The Rio Grande valley, home to about 1.4 million people, is by far the busiest section of the south-west frontier in terms of migrant apprehensions by the border patrol, which indicates that it is also the most popular region for unauthorised crossings.

In south Texas, the Rio Grande river makes for a natural border, but some parts are also closed off by levees and more than 50 miles of fencing, which often lies far north of the official demarcation between the US and Mexico.

The Trump administration intends to expand and fortify that barrier network, with Santa Ana acting as a “pilot project” that would see an 18ft bollard fence, built on top of a concrete wall, to replace an existing levee.

But for its most ambitious plans, the government must obtain land itself in some places, and taking private property, through a process known as eminent domain, can be time-consuming and costly for the government. Efrén Olivares of the Texas Civil Rights Project, who is scheduled to speak at the rally, said that of more than 300 cases filed that relate to existing fencing, about 70 have been pending for a decade.

The privately run National Butterfly Center, roughly 20 miles west of Santa Ana, also abuts the Rio Grande. It is suing the Department of Homeland Security, alleging that last summer the government clandestinely began preparing to construct a wall through the centre, in violation of property rights and environmental assessment obligations.

Building on its own land is a far simpler process for the federal government, though, potentially giving Trump a quick victory as the president seeks to show he is making good on a core campaign promise.

“If they had the money for it they technically, legally, could start doing anything because the land already belonged to the federal government, so if they want to destroy a natural reserve they could waive all the laws,” Olivares said.

Citing a need to expedite its mission of enhancing border security, the Department of Homeland Security can assert the right to bypass legal requirements such as conducting environmental impact assessments. It used this for barrier projects near San Diego last year, and near El Paso this week.

In addition to environmental worries, some Texas activists are worried about the practical and psychological impacts of an increasingly militarised border. They are also disturbed that border funding is being used in Washington as a bargaining chip in the battle over the future of the Daca programme, created by Barack Obama to protect hundreds of thousands of undocumented young people.