Evaluating alternative rodenticides for island conservation: roof rat eradication from the San Jorge Islands, Mexico

Summary

Black rats Rattus rattus were introduced to the San Jorge Islands (Gulf of California), Mexico in the mid-1800s during guano mining operations. Introduced rats Rattus spp. are well-known drivers of extinction and endangerment of island flora and fauna. The use of the toxin brodifacoum to eradicate rats for the purposes of nature conservation has been successfully undertaken on numerous small islands. However, brodifacoum is toxic to other animals in addition to rats and its use may therefore not always be desirable unless appropriate, sometimes costly, mitigation measures are undertaken.

As part of a regional conservation programme, an experimental opportunity arose to investigate the effectiveness of three rodenticides in the eradication of introduced black rats on the San Jorge Islands. Brodifacoum was used on the larger island, while two less toxic rodenticides, diphacinone and cholecalciferol, where used on the two nearby, smaller islands.

Study site: The San Jorge Islands lie in the Gulf of California, 41 km from Puerto Penasco, Sonora, Mexico. This island group comprises Main Island (14 ha) with two smaller islands, North Island (lying to the northwest), and South Island (lying to the southeast) each about 5 ha in area. The two smaller islands are connected to the main island during maximum spring low tides by a narrow, approx. 200 m long, isthmus. The islands are arid, steep and rocky with no terrestrial plants.

Introduced black rats Rattus rattus were present on all three islands prior to the eradication, and there are no native terrestrial vertebrates. The endangered, fish-eating bat Myotis vivesi occurs and is known to be vulnerable to rat predation. The islands are important for breeding seabirds including: brown booby Sula leucogaster, double-crested cormorant Phalacrocorax auritius, Heerman's gull Larus heermanni and red-billed tropicbird Phaethon aetherus. Craveri's murrelet Synthliboramphus craveri, previously present, were probably extirpated due to rat predation.

Baiting methodology: Rat eradication began in August 2000 and was timed to minimise disturbance of nesting seabirds and California sea lions Zalophus californianus. Bait stations were constructed from 50-cm lengths of 10 cm diameter plastic pipe and placed on the islands in a 25 × 25 m grid, with bait placed in the centre of the pipe. Rats could enter and remove bait from either of the open ends. Stations were placed out two days prior to bait deployment so that rats could 'acclimatise' to their presence. In areas inaccessible due to cliffs, bait was broadcast by hand. Three rodenticides were used, one on each island:

Main Island - Brodifacoum (50 ppm, Final® Blox™ Bell Laboratories, Madison, Wisconsin, USA) - 95 bait stations.

North island - Cholecalciferol (50 ppm, Ditrac® Blox™ Bell Laboratories, Madison) - 16 bait stations.

South Island - Diphacinone (750 ppm, Quintox® Bell Laboratories) - 17 bait stations.

Brodifacoum and diphacinone bait were supplied as 20 g extruded cereal wax blocks, while cholecalciferol bait was in cereal pellet form in 10 g packages. Over one month (August–September 2000) bait stations were monitored and bait replenished daily for the first 5–10 days and once a week thereafter. Subsequently stations were checked five times over the next two years (October 2000, January 2001, April 2001, July 2001 and March 2002). In addition to monitoring bait uptake from the bait stations, snap-traps and indicator blocks were used to monitor rat presence three days prior to, during and after baiting.

Trapping: Trapping indicated presence of rats on all three islands, with trap success three days prior to poison baiting between 38 and 62% (Main Island 38 traps, North Island 20 traps, South Island 20 traps).

Bait stations: Rat activity at the bait stations and poison used is summarised in Table 1 (attached). Rats began consuming bait within 3-4 days of it being placed in the bait stations.

On Main Island, brodifacoum bait uptake peaked (at 66%) between days five and 10, and ceased after 24 days. The bait uptake was similar to other rat eradication campaigns, showing a single pulse uptake event with a lag time of a few days.

On North Island, cholecalciferol bait uptake peaked after only two days (27%) and declined consistently thereafter.

On South Island, diphacinone bait uptake peaked around day five and ceased on day 10.

Eradication success: Rats were successfully eradicated from all three islands. Monitoring during the month after baiting showed no sign of rat presence on any of the islands. During visits in the following two years (October 2000, January 2001, April 2001, July 2001 and March 2002) no rats or rat sign were noted on any of the islands, with a combination of snap-trapping and indicator blocks being used for detection (Main Island: 340 trap nights/916 indicator block nights; South Island: 163/536; North Island 140/430).

Possibility of inter-island movement: Given that rats on San Jorge were observed feeding on the intertidal zone and rats are known to expand their ranges when territories become unoccupied, there is a possibility that rats may have moved between islands during the eradication programme. However this is considered unlikely for several reasons. Baiting took place during a neap tidal cycle (therefore there was no land bridge available to cross). If inter-island movement did occur (via swimming), it would probably have only involved a small number of rats - a translocation study of radio-collared rats on Anacapa Island (similar to San Jorge) to adjacent islets revealed no inter-island movements. Lastly, the different bait uptake patterns suggested that the rodenticide effects were different, thus indicating that a real comparison was achieved.

Conclusions: The San Jorge islands have little in the way of food resources outside the seabird breeding season and this undoubtedly assisted in the rat eradication. Regardless, all three rodenticides tested in this study proved effective in eradicating ship rats. This eradication programme therefore shows that, at least in some cases, poisons less toxic than brodifacoum can be effective when attempting to eradicate rats from islands.


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