Removing Rats with Care

Invasive species cause much damage to native biota, and eradicating these invasives is often a difficult time-consuming and expensive process that involves careful planning and monitoring of sympatric species. Gregg Howald of Island Conservation, Canada, and colleagues at the University of California, Santa Cruz, eradicated black rats (Rattus rattus) on Anacapa island, off the coast of California, while simultaneously monitoring the impact of these efforts on native fauna between 2001 and 2008.

Pellets containing the poison Brodifacoum were air-dropped during the dry season, in a staggered fashion, between 2001-2002, to cover Anacapa Island. The island has a native population of deer mice, and care was taken not to eradicate this species. Before air-dropping pellets, specific species, like deer mice and raptors, that were thought to be potentially at risk from the poison were captured, and a certain part of the local population was held in captivity. Several environmental variables (soil, ocean samples) and animal populations, including the target species, were monitored closely before and after the application of the poison. Rats were completely eradicated by the procedure and monitoring in the year found no more rats.

However, while most other native species were not adversely affected, the poison decimated populations of deer mice, raptors and small birds. Rufus-crowned sparrows were most affected, and were not sighted by the authors even 6-7 years after application. The captive-held populations of deer mice and raptors were released one year after application, and numbers are now seen to be similar to pre-application levels. Following successful eradication of black rats from the island, positive effects were seen on murrulets and other seabirds, including reduced predation on eggs and sightings of a one additional species of seabird. The authors stress the importance of holding captive populations of natives species during large-scale eradication efforts.

Further reading
Howald G, Donlan CJ , Faulkner KR, Gellerman SOH, Croll DA and Tershy BR. 2009. Eradication of black rats Rattus rattus from Anacapa Island. Oryx 44(1): 30–40.

R Nandini is a Research Associate at the National Institute of Advanced Studies, Indian Institute of Science campus, Bangalore 560012. Mail at

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2010 Mar