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

Abstract

Feral horses (Equus caballus) in Australia are a growing problem despite implementation of management strategies. The increasing number of feral horses within the Tuan and Toolara State Forest (TTSF), a coniferous plantation on the Sunshine Coast, Queensland, and particularly near major public roads, has been recognised as a problem in the last decade. Desensitised to road traffic, horses are a serious distraction for motorists, and have potential to cause serious animal-vehicle accidents.The primary objective of the study was to characterise the essential demographics of the TTSF feral horses, so that the most suitable methods for long-term population management could be determined. The study was conducted from 2012 – 2014 and determined social structure, reproductive performance and survival, and population growth rate. The estimation of population distribution, abundance and density together with assessment of habitat use and home range size were also addressed.The studied population did not show significant annual variation in demographics, which is consistent with measurements in other unmanaged feral horse populations in Australia and abroad. The overall size and age composition of social groups remained stable during the study period. The average harem size was estimated as 4.95 (95%CI 4.53-5.41), and usually consisted of 1 stallion, 2 - 3 adult females, and 2 immature offspring (≤ 3 years of age). Males not associated with harems consisted of adult (≥ 3 years old) and sub-adult (2 years old) individuals, either forming groups of 2 – 6 or living by themselves. The population showed a stable age distribution, with adult horses constituting the largest group (68%; n = 247). The sex ratio of adult female to male horses was nearly equal (0.99:1.00).The mean annual fecundity was 0.23 ± 0.07 SD and was comparable to those obtained for other feral horse populations where the environment imposed nutritional limitations. Adult females were observed to foal on average every second year. The overall nutritional status of the population expressed as body condition score (BCS; 0 - 5) was 2.55 ± 0.51 SD with adult females having poorer scores than other age and gender groups. Survival estimates were consistently high (0.92 – 0.95) across all age groups. The average annual finite rate of population increase (λ) for the three years of the study was 1.088 which was lower than the maximum reported for populations living in the most favourable conditions. Elasticity analysis demonstrated that the TTSF population growth rate was almost seven times more sensitive to changes in adult survival compared with juvenile survival, and almost twice more sensitive than changes in fecundity.Assessment of the population distribution based on a strip transect survey using horse dung on forest tracks indicated that the majority of the forestry was occupied by horses, with the highest population density being located in the central region of the plantation. Abundance and density of horses were estimated by distance line transect survey of dung counts, dung disappearance rate of 444 (± 150.7 SD) days and individual defecation rates of 7.97 (± 8.74 SD) over 24 hours. The analysis indicated that the TTSF was occupied by 1321 horses (95%CI 940 - 1965), which corresponded to a density of 1.8 horses/km2.Habitat use quantified by visual detection of horses, dung counts, and Global Positioning System (GPS) tracking, identified a preference for open habitats of young pine forest and harvested areas, and avoidance of closed canopy habitats of juvenile and mature pine. Home range size determined by GPS tracking was greater than that measured by direct observation (mean ± SD; 16.90 ± 9.11 km2 and 6.71 ± 3.30 km2 respectively). All harems were loyal to their home ranges; harem members occupied one core area more than other parts of their range, and home ranges highly overlapped.The findings of this thesis confirm that there is a gradual increase in feral horse distribution and population size within the TTSF. It is evident that the design and implementation of a program to manage feral horses in the TTSF will need to consider a combination of approaches, which will need to satisfy a number of criteria including efficacy, cost effectiveness, occupational safety, environmental impacts and animal welfare. Continued monitoring of the population growth is essential to measure the effectiveness of chosen management strategies.