Customs and Border Protection will soon finish installing 50 wildlife passages across 63 miles of recently-completed border wall in southern Arizona in an attempt to allow more small animals to move across the border. The openings, which some have likened to “doggy doors,” are flush with the ground and the dimensions of a standard sheet of paper—eight and a half inches wide and 11 inches tall.

The agency says the openings will make it easier for small animals to get through the wall, and that it plans to install more in the future. But scientists and environmentalists tell National Geographic that these openings are too small and too far apart to have a significant impact.

Nearly 400 miles of border wall have