Ground-based rodent control in a remote Hawaiian rainforest on Maui.

Effective control of introduced mammalian predators is essential to the recovery of native bird species in Hawai‘i.

Between August 1996 and December 2004, introduced rodents were controlled within three home ranges of the Po‘ouli

Melamprosops phaeosoma, a critically endangered Hawaiian honeycreeper. Rats were controlled using a combination

of ground-based rodenticide (0.005% diphacinone) application and snap traps. Beginning in August 2001, we monitored

the effectiveness of these rodent control efforts. Relative abundances of Black Rats Rattus rattus and Polynesian Rats

R. exulans were measured in each of five snap-trapping grids seven times over a 35-month period. Rat populations

decreased inside of the rodent control areas, but control effectiveness differed between rat species. During the first

year of monitoring, target control levels for R. rattus were consistently achieved in only one of the rodent control areas.

Control techniques were refined in areas failing to meet targets. Subsequently, we achieved target control levels for R.

rattus more consistently in all three rodent control areas. However, relative abundances of R. exulans did not differ

between rodent control and reference areas, indicating that our rodent control techniques were insufficient to reduce

population levels of this species. These findings signify a need for further improvement of rodent control methods in

Hawai‘i, especially for Polynesian Rats, and demonstrate the critical importance of periodic monitoring of the response

of rodent populations to management. In the future, managers may need to design rodent control operations targeting

R. rattus and R. exulans independently to achieve best results.