Restoration of insular ecosystems: control of feral goats on Aldabra Atoll, Republic of Seychelles
Published source details
Rainbolt R.E. & Coblentz B.E. (1999) Restoration of insular ecosystems: control of feral goats on Aldabra Atoll, Republic of Seychelles. Biological Invasions, 1, 363-375.
Published source details Rainbolt R.E. & Coblentz B.E. (1999) Restoration of insular ecosystems: control of feral goats on Aldabra Atoll, Republic of Seychelles. Biological Invasions, 1, 363-375.
Feral goats Capra hircus introduced to Aldabra Atoll (Seychelles), due to their grazing and browsing was impacting upon the vegetation, with implications for other native endemic biota. This prompted a limited control program in 1987 and 1988 to assess the potential for eradicating the goats. Following this, in 1993-1995 eradication was attempted; summarized here are the techniques employed, focusing on the effectiveness of ‘Judas goats’ to assist in goat location. Judas goats are used to help locate (due to their natural propensity to join their conspecifics) other goats, which as densities become lower and remaining individuals become increasingly wary, have proven effective in other eradication campaigns.
Study area: Eradications, assited by using Judas goats, were undertaken on Ile Picard and Ile Malabar (both with small remnant feral goat populations of <20 individuals each) and Grande Terre (population about 1,000 goats) in the Aldabra Atoll (9º24’S, 46º20’E).
Hunting methods: During October 1993 to May 1994 and November 1994 to May 1995, goats were killed by two hunters using rifles. Hunting on foot was the most practical method due for example, to the relatively small size of the islands, long distance from the Seychelles ‘mainland’ thus prohibiting use of helicopters.
At the start, 28 goats were chased and captured by hand, fitted with radio-transmitters and released; these were used as ‘Judas goats’.
It was not known at what stage using Judas goats would be more efficient than traditional hunting (i.e. tracking and shooting without their use). It was also unknown whether Judas goats would be more efficient at finding conspecifics if they were captured and released in the same area (‘resident’), or transported to an unfamiliar release site (‘non-resident’ goats); therefore an attempt was made to test this on Grande Terre by moving a number of Judas goats from there capture localities to opposite sides (east or west) of the island. Judas goats which travel larger distances and have larger home ranges, might also locate more conspecifics, therefore home range sizes of Judas goats and distances travelled to locate conspecifics were recorded using radio-telemetry.
The goats on Ile Picard (n=13) and Ile Malabar (n=19) were eradicated during the first season. On Grande Terre, 798 goats were killed. Hunting assisted by Judas goats became increasingly important over time; 18.0% (n=85) of goats were killed in the presence of Judas goats in the first season compared to 42.3% (n=126) in the second season. The overall kill rate for was almost twice that for Judas goat hunting (0.61 goats killed/h) than traditional hunting (0.32 goats killed/h). The home range size of each Judas goat and the number of goats killed in association with it was positively related.
Due to logistical difficulties and goat density differences between east and west Grande Terre, no conclusion could be made as to whether resident or non-resident goats were more effective. Continually shooting near Judas goats caused some to become increasingly wary and more difficult to use towards the end of the project when most needed. The 6–7 months between hunting seasons did not decrease wariness of Judas goats; most gave birth during this period or soon after the second season began, and became even more wary. A total of 84 goats were estimated to remain on Grande Terre at the end of the program.
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