On a wooded hillside along the banks of the Allegheny River, Gabrielle Marsden brushed up against an oblong leaf: Asimina triloba. The American pawpaw. The deciduous tree bears North America’s largest native fruit, and, as Marsden explained, supports the zebra swallowtail butterfly — a species that has all but disappeared from the Pittsburgh region. “They don’t exist here, really,” Marsden said as she trekked along the river. The closest she’s found them is in Northern West Virginia. “The most