Estimating the density of free-ranging wild horses in rugged gorges using a photographic mark - recapture technique

Abstract

Estimating the density of large, feral species such as wild horses at landscape scales can present a logistical hurdle for wildlife managers attempting to set density-based management targets. We undertook aerial surveys of wild horses by using a helicopter in Guy Fawkes River National Park in north-eastern New South Wales across 3 years to determine whether meaningful density estimates could be obtained efficiently by a mark-recapture technique based on recognition of individual horses. Horse groups photographed from the air on the first of two surveys conducted each year were 'marked' on the basis of a unique combination of colours and natural markings, and 'recaptured' if they were photographed and identified on the second survey. Population size was estimated with the program MARK using a range of population estimators; however, because horses appeared to be evading detection on the second survey of each year, we chose a final estimation model that accounted for detection shyness in the study species. In 2005, the density estimate was 3.8 horses per km 2 (upper and lower 95% CL = 3.5-5.7 horses per km 2 ). Following horse control in these catchments, the estimate in 2007 was 2.3 horses per km 2 (upper and lower 95% CL = 2.1-3.4 horses per km 2 ), and this change in density can be accounted for by the known number of horses removed from the survey area between survey periods. Overall, the technique proved useful for estimating densities of wild horses in deeply dissected gorge country where other estimation techniques (such as line transects) are not practical; however, low recapture rates in one of the years of the study shows that the technique may not always be applicable. Our technique should also be suitable for surveying other large mammals with broad ranges in open environments, provided recognition of individuals from unique marks is possible.

Understanding population dynamics of feral horses in the Tuan and Toolara State Forest for successful long-term population management

“…In open habitats, fleeing horses frequently stopped within visual range and could still be identified, but in dense habitats movement of horses away from the observer resulted in loss of visual contact. Shyness was suggested to be the reason for the lower detection rates of feral horses in the markrecapture study by Vernes et al (2009). Drive counts of species such as deer in forested habitats were similarly influenced by the flight response elicited by an approaching vehicle (Hemami et al 2007;Borkowski et al 2011 other areas within the plantation.…”

“…It is not possible to determine the accuracy of each technique because actual population size was not known. It is known, however, that direct sighting methods are less precise, because not all animals are detected equally (Seber 1992), especially in heavily vegetated habitats (Bayliss and Yeomans 1989a;Gill et al 1996;Stephens et al 2006;Vernes et al 2009) where observations are fewer resulting in an underestimation of population parameters (Otis et al 1978;Crane et al 1997;Stephens et al 2006). Therefore, it is likely that mark-recapture underestimated horse abundance in the SA.…”

“…This movement was partially reversed in spring, when some harems and bachelors returned to previously occupied habitats. Human activity is known to influence wild herbivore distribution and density at a landscape scale (Kuiters et al 1996;Vernes et al 2009). In particular, iatrogenic changes in distribution and availability of forested habitats have the greatest consequences on habitat use by forest ungulates (Putman 1986;Gill et al 1996).…”

“…While feral horse population dynamics have been studied in a range of environments outside Australia Scorolli and Lopez Cazorla 2010a), most Australian studies have focused on social organisation, estimating population abundance and assessing environmental and economic impacts (Hoffmann 1983;Walter and Hone 2003;Dawson et al 2006;Vernes et al 2009). Little is known about population dynamics with regards to reproductive performance (fecundity), mortality and the rate of population change.…”

“…Although Australia has the world's largest population of feral horses (Dobbie et al 1993;Dawson et al 2006), there have been few studies documenting feral horse demographics. The most recent Australian studies have focussed on population distribution, density and abundance Vernes et al 2009), habitat use , horse impact on the environment , and population dynamics ; all were conducted in natural ecosystems within national parks. The only study on social organisation of feral horses in Australia was performed in the Northern Territory in the late 1980s .…”