Conservation action in the Galapagos: Feral pig (Sus Scrofa) eradication from Santiago Island

Summary

Santiago is a protected island situated in the centre of the Galapagos archipelago (Ecuador). Among the endemic fauna on the island are a number of birds, the rice rat Nesoryzomys swarthi and five reptiles, including the lava lizard Microlophus albemarlensis and the giant Galapagos tortoise Geochelone elephantopus. Introduced mammals include pigs Sus scrofa, goats Capra hircus, donkeys Equus asinus, ship rats Rattus rattus and house mice Mus musculus.

Likely to have been introduced shortly after 1835, pigs were reported to be numerous by 1875. As well as eating native plants they consume invertebrates, eggs and hatchlings of Galapagos tortoises and green turtles Chelonia mydas, lava lizards and Galapagos petrels Pterodroma phaeopygia, as well as other native vertebrates. It was decided that a pig eradication programme should be attempted as it was apparent that they were major drivers of vertebrate extinctions and ecosystem change on Santiago.

Initial pig control (1968-1985): Attempts to control pigs on Santiago (58,465 ha) began in 1968 with sporadic hunting and use of traps and snares. After 1974, hunting effort was recorded and between 250-500 hunter-days/year were expended over a 10 year period (up to 1985). Hunters used 0.22 calibre rifles and non-specialist dogs to assist in location of pigs.

Poisoning-baiting in conjunction with hunting (1985-1995): Eradication effort increased in 1985 to around 1,500 hunter-days/year alongside a poisoning campaign from 1985 to 1989. In 1989 hunting effort was doubled but subsequently receeded to previous levels in 1991-1994. Hunting effort once again increased in 1995, again using dogs to help locate pigs. In conjunction, 1080 poisoning was also carried out using goat carcasses as bait; carcasses were placed along trails in October 1995, December 1995 and January 1996.

Eradication methodology (1998-2000): In 1998 a more organized methodology was adopted with the island divided into blocks. Teams of 12-15 hunters covered each block with 1-2 dogs per hunter. The dogs were not specialized 'pig dogs' and sometimes chased goats and other animals. A GPS was used to document daily block coverage and locations of pigs. Additional trails were cut ito improve access through dense vegetation.

A revised poisoning campaign was implemented. Spot baiting with antiemetics along with 1080 poison (230 mg/kg bait) was used. The anti emetics - Metoclopramide (20 mg/kg bait) and atropine sulphate (8 mg/kg bait)- delay vomiting and therefore increase intake of poison once the bait has been eaten. The dosage accorded to LD99 of 2.3 mg/kg body weight. Baits were poisoned chunks of meat or carcasses along with non-poisoned attractant carcasses. These were hidden under vegetation to avoid poisoning of Galapagos hawks Buteo galapagoensis.

In 1999 warfarin baits were used in an attempt to acheive greater efficiency. Importantly pigs were not 'bait shy' (as perhaps thay had become to 10800) with warfarin baits. The dosage accorded to LD90 of 12 mg/kg body weight. This dosage also reduced risks to non-target Galapagos hawks and dogs as the dosage needed to kill these animals is far greater than that used in bait to kill pigs.

It was found that at this dosage most pigs killed by baits were smaller individuals. Therefore a dosage of over 20 mg/kg body weight was administered in order to kill larger pigs. The bait remained highly effective for about eight days but kill rates decreased markedly after 12 days as the bait decomposed.

Monitoring: Intensive monitoring was carried out over a four month period from June to September 2000. An estimated 2,414 man-hours and 1,298 dog-hours were expended during this time, and 1,128 toxic and non-toxic baits were laid out in order to detect the presence of any remaining pigs.

Success of pig eradication: Monitoring in 2000 showed that the pig eradication had been acheived. Over 18,800 pigs were removed since attempts at control began in 1968. A summary of pig removal over these years is given below.

1968 – 1985: Pig eradication attempts were uncoordinated and the scale ineffective, with no evidence of a reduction in the pig population.

1985 – 1990: The poison campaign was fairly effective. 50 mg of sodium monoflouroacetate (1080) was added to each bait. Initially goat meat and sea turtle eggs were used (the latter use was stopped for obvious conservation concerns) of which 57% of baited items were consumed. In 1989, using 1080 poison without anti emetics - 1,896 pigs were removed, in 1990 with the same baiting effort, 523 pigs were removed.

The hunting effort was much better organized and recorded, and a greater success was acheived.

1995 – 1998: The poisoning campaign using goat meat bait was relatively unsuccessful with only 15% of the total baits being consumed. 1080 became more limited in effectiveness, perhaps as pigs were vomiting up poisoned bait before sufficent poison had been ingested, and also because they had becom 1080 'shy' i.e. they could detect and were avoiding bait laced with 1080. Most eradication in these years was a result of increased hunting effort.

1998 – 2000: The combination of using anti emetics and 1080, and the introduction of warfarin as poison (used extensively in place of 1080) greatly increased efficiency. Hunting effectiveness also increased due to the network of trails allowing access to previously inaccessible areas of the island.

By mid 1999, pig numbers had been massively reduced as a result of the baiting programme and greater hunting coverage and effort. Numbers of pigs had been reduced to such an extent that hunters could identify individual signs of the few remainig animals. The last pig to be shot was in April 2000 and the last eradicated using baiting in October 2000 (on the seventh monitoring trip). Futher monitoring after this confirmed that the pig eradication had been successful.

Non-target species affected: The only impacts on non-target species were of deaths of non-native ship rat R.rattus through 1080 poisoning. No native rats were thought to have been affected as they were locally distributed and bait-stations were some distance from areas that they occupied. Likewise no raptors were affected by the poisoning campaign.


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