The eradication of ungulates (sheep and goats) from Dirk Hartog Island, Shark Bay World Heritage Area, Australia

Abstract

The eradication of ungulates from offshore islands has now become achievable for island managers, with the size and complexity of an island no longer a major impediment to the desired outcome. Here, we report on a whole-of-island eradication campaign of ungulates (sheep Ovis aries and goats Capra hircus) from the semi-arid Dirk Hartog Island (63,300 ha) off the western Australian coast. The motive behind this campaign was to contribute towards the ecological restoration of this former grazing lease. From 2005, a concerted effort to remove ungulates began with regular destocking, and from 2010 a methodical aerial and ground shooting campaign was undertaken. Long-term commitments of funding and departmental support, staff with diverse skills, and an advisory network of professional people, have been critical components to this large-scale exercise. From 2005 to 2017 a total of 16,318 ungulates (5185 sheep and 11,133 goats) were removed from Dirk Hartog Island: 6839 by mustering, 2422 by ground shooting, 7040 by aerial shooting, and finally 17 by follow-up aerial monitoring and ground shooting. The island was declared free of ungulates in November 2017. To determine the success of the whole-of-island eradication campaign, multiple methods were adopted to locate remaining animals: use of ‘Judas’ goats, monitoring by motion-sensor cameras at water sources and across the island, and recording of tracks and fresh scats to locate any remaining animals. We estimated the likelihood that sheep and goats have been successfully eradicated from the island is 99.9% and 96.9%, respectively. The total cost (AUS$) of the aerial component of the eradication was $1,055,184, an average of $150/goat or $16/ha. The monitoring phase of the campaign (aerial detection and ground shoot) cost the least in terms of actual expenses (approximately $187,000) but the most is terms of cost per remaining goat (approximately $14,400). Ecosystem recovery following the eradication is already apparent with increased vegetation cover and reduced erosion. We conclude with some shared lessons that may assist similar large-scale eradication campaigns of islands. To date, Dirk Hartog Island is the largest island in the world where whole-of-island goat (and sheep) eradication has been achieved.