As birds sing and lizards scuttle in the lush vegetation of the Tijuana River Valley, helicopters circle overhead, and Border Patrol agents on all-terrain vehicles comb the area looking to stop illegal border-crossers.

Two big metal fences and stadium lighting divide homes in Mexico from this largest intact coastal wetland in Southern California. Over the decades, fencing construction and associated roadwork have affected wildlife habitat along a 14-mile stretch between the Pacific Ocean and the Otay Mesa Port of Entry.