Eight of the 11 surviving races of Galápagos giant tortoise Geochelone elephantopus are threatened due to small populations and predation and/or competition by introduced mammals. Systematic hunting has been effective in reducing numbers of feral goats Capra hircus and pigs Sus scrofa; goats have been eradicated on several smaller islands. Hunting has been ineffective against feral dogs Canus familiaris particularly on larger more vegetated islands. This paper evaluates the success of three methods employed to conserve giant tortoises: 1) control of introduced mammal populations; 2) captive-raising of young for restocking; and 3) protection of nests using rock corrals to prevent predation by pigs or dogs. The latter is summarized here.
From the mid-1960s onwards various trials using rock corrals constructed around nests were undertaken. Corrals were constructed of large lava rocks (1.5-2 m in diameter; 1 m high). Corrals of these dimensions do not block solar radiation from reaching the soil surface throughout most of the daylight hours.
Corralling against pigs: Corralling was initially applied on a limited scale during the nesting seasons of 1964/65-1969/70: 10-25 nests protected each year of the Santa Cruz race porteri; none of these were destroyed by pigs.
Beginning in the 1970/71 nesting season, corrals were used more extensively. Major nesting areas of porteri, vicina and darwini populations were visited every 1-4 weeks when females were laying (June-December). Some nests were destroyed by pigs as in scattered sites outside of core areas, or found by pigs between visits, however, most were protected; 262 nests of these races were protected in the 1970/71 and 1971/72 nesting seasons. Some unprotected nests left unprotected as no pig sign was evident, were marked but not protected, and subsequently monitored.
Corralling against dogs: Four chathamensis nests were protected with corrals in 1971.
Success against pigs: Corrals resulted in 99.6 % success in preventing nest destruction. Only one of 262 corralled nests was predated, this onSanta Cruz in 1970/71 (n=46 nests). No nests were predated on San Salvador 1971/72 (n=115), San Pedro area 1970/71 (n=38), Cerro Azul 1971/72 (n=31) or Isabela 1971/72 (n=32).
Few unprotected nests survived the incubation period (3-8 months) without being destroyed by pigs, e.g. on Santa Cruz as no pig sign was evident, 29 nests were marked but left unprotected in 1970. Between the sixth and seventh month visits, 23 were destroyed, signs indicating a single pair of pigs to be responsible. Subsequently all nests in this area were corralled; none of the 90 protected in the 1970/71 and 1971/72 nesting seasons were damaged.
Success against dogs: The corrals were ineffective against dogs; they jumped into the corrals and destroyed the nests. Other nest protection methods will be tested e.g. wire mesh cages.
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