Eradication of Norway Rats for Recovery of Seabird Habitat on Langara Island, British Columbia

Abstract

Introduced Rattus norvegicus (Norway rats) caused the decline of Synthliboramphus antiquus (ancient murrelets) and other seabirds breeding on Langara Island (approximately 3,100 ha), British Columbia. Using funds from the litigation settlement following the Nestucca oil spill, Environment Canada eradicated Norway rats using a technique developed in New Zealand which involved dispensing wax baits containing the anticoagulant brodifacoum at 50 ppm from fixed bait stations. Bait stations were placed every 75 to 100 m on a grid over the entire island (1 station/ha). Rats removed bait for 26 days, after which crews placed baits in protective plastic bags in each bait station. Stations loaded with baits were left on the island and rechecked four times over 2 years, after which bait stations and remaining bait were removed. The eradication succeeded. No signs of rats have been detected on Langara Island and its associated islands since January 1996. No rats were trapped during 1,700 trap‐nights following the poison campaign. Incisor marks of rats were not found on apples or oil‐dipped chew‐sticks. Corvus corax (common ravens) likely suffered greater than 50% mortality from the eradication after apparently gaining access to the poison directly from bait stations and from scavenging rat carcasses. A monitoring and response system is being developed in conjunction with current users of the islands. The success on Langara Island demonstrates how the technique proven on small New Zealand islands of less than 300 ha can be effectively extrapolated to much larger islands.

Mammal invaders on islands: impact, control and control impact

“…For example, it was admitted as late as 1976 that islands of more than 1 ha could not be freed of rats (Atkinson, 2001). However, improved protocols led to their eradication from the 3253 ha Langara Island (Bertram & Nagorsen, 1995;Drever & Harestad, 1998;Taylor et al, 2000). This was made possible by both the availability of the 'one shot' anticoagulants in baits that are attractive to rodents and the development of a successful technique of dispensing poison baits from fixed stations spaced such that at least one station lies within the home range of each rat (Taylor & Thomas, 1989;Taylor & Thomas, 1993;Clout, 1999).…”

Eradications of invasive mammals from islands: why, where, how and what next?

“…These have all been the target of previous eradication programmes (Courchamp et al 2003;Nogales et al 2004;Campbell and Donlan 2005). Eradications of invasive mammals from islands are therefore not only feasible, but highly beneficial for both seabirds and other components of the ecosystem, including terrestrial birds, lizards, amphibians, native mammals, invertebrates and plant communities, and, on inhabited islands, agricultural productivity (Newman 1994;Towns and Broome 2003;Croll et al 2005).…”

“…The complex issues involved with these ambitious projects should not, however, distract from the realisation that many, much smaller islands are sufficiently isolated, particularly given the limited swimming ability of rats and mice, that if human activities can be controlled appropriately and the future risk of re-invasion reduced to an acceptable level, eradication can be achieved with modest financial investment. Introduced vertebrates vary enormously in the level of effort required for eradication, but in general, ungulates, cats and rats are easier to eradicate than rabbits, mice, mongooses, mustelids and birds (Courchamp et al 2003;Towns and Broome 2003;Martins et al 2006). Knowledge of rodent eradication in particular is high and growing.…”

“…Fortunately, the capacity to remove introduced vertebrates has increased enormously in recent decades because of improvement in GPS technology to assist bait distribution, the development of more effective poisons and bait delivery systems, the use of helicopters, trained dogs and Judas animals (an individual fitted with a transmitter and released that leads hunters to a remaining wild herd), improved risk management, and modelling of optimal strategies for eliminating the last few animals (Towns and Broome 2003;Morrison et al 2007). The largest islands cleared to date of the most injurious of the various introduced mammals are indicated in Table 2.…”

“…However, that the project was even attempted not only underlines both a highly laudable financial commitment, but a striking change in attitude in the last decade to the scale of eradications that are considered technically possible. The latter results largely from pioneering efforts in New Zealand since the 1970s, and the development of an expertise base that has since become a global export (Towns and Broome 2003;Rauzon 2007).…”