Endangered Species Act at 50: Is it facing its own extinction?

Biologist Ashley Wilson carefully disentangled a bat from netting above a tree-lined river and examined the wriggling, furry mammal in her headlamp’s glow. “Another big brown,” she said with a sigh.

It was a common type, one of many Ms. Wilson and colleagues had snagged on summer nights in the southern Michigan countryside. They were looking for increasingly scarce Indiana and northern long-eared bats, which historically migrated there for the birthing season, sheltering behind peeling bark of dead trees.

The scientists had yet to spot either species this year as they embarked on a netting mission.