A rat gnaws at a flag-topped wax block on Alaska's Rat Island in August 2007.
Bite marks in the paraffin-and-peanut butter blocks help researchers and conservationists determine how many rats are in an area—and whether a poisoning program has been effective, according to Carolyn Kurle, a researcher at the University of California, Santa Cruz.
While some animal rights activists oppose rat-eradication projects, conservationists say rat removal is the only way some seabirds and other island species can survive.
"The eradication of invasive species from islands—especially rodents—has heralded a new era in conservation management globally," said Alan Saunders, director of the cooperative islands initiative.
"Spectacular ecological responses have been measured following rat eradications."