MANAUS, Brazil — Crouched over the leaf litter, where dry leaves accumulate on the forest floor, a researcher tries to capture a distinct croak using a directional microphone. Identifying the sound of a small frog is often one of the conclusive proofs that a new species has been found. It’s nighttime. He wears long clothing as protection against mosquitoes and ants, and boots to keep his feet dry. Finding amphibians in the Amazon doesn’t require…