Population dynamics of feral horses (Equus caballus) in an exotic coniferous plantation in Australia

Abstract

Context Understanding population dynamics of invasive species is crucial for the development of management strategies. Feral horses (Equus caballus) are a growing problem in the Tuan–Toolara State Forest (TTSF), a coniferous plantation in south-eastern Queensland, Australia. Aim The population dynamics of the TTSF feral horses was not known. Therefore, the study was designed to characterise the major vital parameters of this population and, using these data, develop a long-term management plan. Methods The study was conducted over 3 years (2011–14) involving 522 individually identified horses. Foaling rates were used to calculate fecundity. Body fat distribution was estimated using body condition score (BCS), which reflects the nutritional, metabolic and general health of individual animals. Multi state mark–capture population models were used to estimate age-specific survival, and the Leslie age-structured projection matrix model was used to determine the annual rate at which the population increased. Key results The mean annual fecundity was low (0.23 ± 0.07 s.d.). The mean BCS of the population was mid-range (2.55 ± 0.51 s.d.) with adult females having lower 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 3 years of the study was 1.09. Sensitivity analysis demonstrated that the population growth rate was almost seven times more sensitive to changes in adult survival compared with juvenile survival, and almost twice as sensitive to changes in fecundity. Key conclusions Population dynamics of the TTSF feral horses were comparable to other feral horse populations similarly challenged by environmental nutritional limitations. Implications Defining population dynamics of the TTSF feral horses permits the formulation of management goals that can be audited and adapted as required. The most effective strategy for controlling population growth in the TTSF would involve the continuing removal of substantial numbers of adult females or manipulation of survival and/or fecundity. As selective removal will likely alter the adult sex ratio and age structure of the population, ongoing assessment is necessary to minimise adverse welfare outcomes.

Feral Horse Population Model and Body Condition: Useful Management Tools in Tornquist Park, Argentina?

“…As a consequence, the demographic parameters of the population increased and resulted in a moderate finite growth rate (λ = 1.10), almost doubling the population growth rate observed during 1995-2002 (λ = 1.06). This value is similar (λ = 1.06-1.10) to what has been reported in food-limited feral horse populations (Linklater et al 2004, Dawson and Hone 2012, Richards et al 2014, Zabek et al 2016) but lower than observed in other populations (λ = 1.15-1.30; Eberhardt et al 1982, Garrott and Taylor 1990, Grange et al 2009 Elasticity analysis of demographic parameters estimates their relative contribution to the population growth rate and potential targets for management (Caswell 2000, De Kroon et al 2000. The value of elasticity of adult female survival in the study population was higher than elasticity of survival of foal, yearling, and juvenile females, and also higher than adult female fecundity.…”

“…Long‐term studies (i.e., ≥5 yrs) of individually identified feral horses are scarce (Berger 1986, Garrott and Taylor 1990, Goodloe et al 2000, Grange et al 2009, Richard et al 2014). Matrix population models from feral horse demographic data and elasticity analyses have been conducted on data from New Zealand (Linklater et al 2004), Australia (Zabek et al 2016), and Canada (Richard et al 2014), but none of these studies validated their population size projections with independent data from another study period.…”

Use of Remote Camera Traps to Evaluate Animal-Based Welfare Indicators in Individual Free-Roaming Wild Horses

“…Understanding population dynamics and the processes that influence them is critical for management of populations. In Australia, there are few demographic studies on wild horses [5,10,16], particularly within woodland habitats [17,18]. In south-east Australia, although free-roaming horses commonly reside in woodland habitats (eucalyptus forests) and undulating and often steep terrain, population and demographic data for horses in these habitats are lacking.…”

“…The importance of developing scientific methods for capturing data to enable the assessment of the welfare of free-roaming wild animals has recently been highlighted [1]. Many studies have evaluated wild horse behaviours, time budgets, home ranges, body condition scores and social organisation, e.g., [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], but to date, an extensive range of welfare indicators has apparently not been assessed.…”

“…Current knowledge of wild horse ecology and behaviour has historically relied on direct observations of horses, e.g., [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The disadvantages of direct observations for assessing indicators of welfare include practical limitations on the number of observations of individual animals that can be made, and usually the need to observe them from long distances.…”