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18 November 2016 Estimating distribution and abundance of feral horses (Equus caballus) in a coniferous plantation in Australia, using line-transect surveys of dung
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Context. Feral horses are a growing problem in Australia, despite implementation of management strategies. The incidence of horse sightings and horse-associated vehicle collisions within the Tuan and Toolara State Forest (TTSF), a coniferous plantation in south-eastern Queensland, has increased in the past decade, indicating an increase in population and the necessity to undertake an accurate appraisal of population abundance.

Aim. To determine the feasibility of using dung counts, defecation and dung-decay rates to determine the distribution and abundance of feral horse population in a managed coniferous plantation where dense vegetation prohibits the use of visual-based animal detection methods.

Methods. Population distribution was assessed by dung count incorporated into a vehicle strip-transect survey on 582 km of forest tracks. Population abundance was estimated from dung density, the mean defecation frequency of feral horses, and the mean number of days required for dung to decay. Dung density was obtained from on-ground distance line-transect survey that sampled all representative plantation habitats and consisted of 111 transects totalling 44.3 km.

Key results. The strip-transect survey clearly showed that although feral horses were dispersed across the plantation, the distribution was uneven, with the central region of the plantation being the most heavily populated. The combination of dung counts, defecation rate (mean ± s.d., 7.97 ± 8.74) over 24 h and dung-decay rate (444 ± 150.7 days) provided an estimation of the density of feral horses in various habitats and indicated that the plantation was occupied by 1321 (95% CI 940–1965) horses, which corresponded to an average density of 1.8 horses km–2. The method clearly identified variations in horse abundance among the various habitats within the surveyed areas. Open habitats, created following harvesting, showed higher occurrence rates, whereas the habitats of mature forest were scantily occupied.

Key conclusions. Dung counts are a simple, effective and practical technique that can provide information on distribution and abundance of feral horse population in densely forested habitats where visual-based techniques are not applicable. However, unbiased and precise defecation and decay rates must be estimated.

Implications. The study validated the use of dung counts to provide information on feral horse distribution and abundance in densely forested environments where direct methods of census may be difficult to obtain. The methods are applicable to a range of ecosystems, but defecation and dung-decay rates must be determined separately for each ecosystem.

© CSIRO 2016
Magdalena A. Zabek, David M. Berman, Simon Blomberg, and John Wright "Estimating distribution and abundance of feral horses (Equus caballus) in a coniferous plantation in Australia, using line-transect surveys of dung," Wildlife Research 43(7), 604-614, (18 November 2016).
Received: 6 August 2015; Accepted: 1 October 2016; Published: 18 November 2016

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