Feral goat eradications on islands


Feral goats Capra hircus, particularly where introduced to oceanic islands, may have a devastating impact through overgrazing, leading to habitat degradation and biodiversity loss. Removing non-native goats from islands may thus be desirable. A review of feral goat eradication campaigns on islands was undertaken to access approaches and successes of eradication attempts.

Available material on feral goat eradication campaigns on islands was reviewed, to assess methods used, successes, and challenges to help facilitate future island goat removal. Data covering most islands were compiled using published and gray literature and personal communications with researchers and conservationists.

Goats have been eradicated from at least 120 islands (ranging in area from 1 to 132,867 ha, totaling approx. 567,000 ha). Of these, 60% (72) were documented in published, 14% (17) in the grey literature (unpublished reports) and 26% (31) other sources. However, 84% of published and grey literature lack sufficient detail to adequately assess eradication efficacy (e.g. methods, timing, number of goats removed; or costs of eradication programmes), thus are of little use in developing and guiding future removals. About 70% of known eradications took place between 1960 and 2003.

The four largest goat eradications (island size and animals removed) have been on Lana'i (Hawaii, 36,100 ha), San Clemente (California, USA 14,800 ha), Pinta (Galápagos 5,940 ha), and Raoul (New Zealand, 2,943 ha). A number of unsuccessful eradication attempts have been documented, failing primarily due to a lack of political will, inappropriate methods, or the failure to detect the last remaining animals.

Hunting (e.g. using hunting dogs (20 campaigns), shooting from a helicopter (13 campaigns), and the use of ‘Judas goats’ (see below)) is the commonest eradication method. Poisoning, trapping, biocontrol (releasing dogs), habitat alteration via fire, and live removal (on nine islands) have also been used. Although often not necessary on smaller islands, multiple methods and specialized techniques are required to achieve success on larger islands with large goat populations e.g. recently on Isabela and Santiago islands (Galápagos) eradication techniques included hunting by helicopter, use of specially trained hunting dogs, GPS (global positioning system) and GIS (geographic information system) technology, and Judas goat techniques. Such techniques allow for eradications on larger islands within a shorter time period and with increased cost-effectiveness.

The use of specially trained goat-hunting dogs increases the ability to detect and kill goats at low densities and in heavily vegetated areas. Although expensive, hunting by helicopter is very effective when removing goats at high densities in open areas, and appears vital on larger islands.

A common reason for reported unsuccessful eradications was the failure to detect and remove the last surviving goats or inappropriate hunting methods. By exploiting their gregarious nature, the ‘Judas goat method’ can be very successfully in detecting goats at low densities, and subsequently as a cost-effective monitoring tool to confirm eradication. Radio-telemetry collars are fitted to goats which are released and allowed to seek out others; Judas goats are radio-tracked on foot or by helicopter, and accompanying goats are shot. This method has been successful in a number of eradications, e.g. San Clemente island where more than 29,000 goats were removed, but combined trapping and helicopter shooting failed to remove the last individuals; Judas goats finally allowed removal of the last 263 individuals. Recently developed hormone therapy and sterilization techniques should further improve the efficacy of Judas goats.

Given the clear biodiversity benefits, the authors conclude that feral goat populations should be routinely removed from islands where feasible.

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