Report on the feasibility of assessing feral horse densities in the Australian Alps Parks using a strip-transect method

Bryan Walters, EcoPlan | January 1996

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Introduction

Feral horse abundance can be measured in three ways: as the total number of animals in a population; as the number of animals per unit area (absolute density); or by a density index where a measurable correlate of absolute density is used (relative density) (Caughley 1977).

Dung counts are more reliable indicators of animal density than visual or track counts (Seber 1982) and can provide estimates of absolute density if defecation rates are known. Dung counts have been used to provide indices of relative density, although these apply only to comparative studies between similar areas over time (Dobbie et al. 1993); or to temporally spaced surveys of the same area. For the present study the measure of relative density was chosen, where a density index of fresh dung deposits was correlated to habitat.

Home ranges of feral horse bands overlap (Duncan 1983, Dobbie et al. 1993); vary in size according to habitat (Dobbie et al. 1993); and, in arid Australia, can be as large as 88 km2 (Dobbie & Berman 1990). Within home ranges, seasonal and daily variation can be expected (Pratt et al. 1986). Feist (1971) records a home range estimate of up to 32 sq. km2 for harem bands, bachelor bands and solitary males in Wyoming and Montana. Climatically these areas are similar to conditions encountered in the Australian Alps.

Wild horses spend 50-70 per cent of the day feeding (Duncan 1985) and can tolerate a wide variety of foods and ingestion schedules (Waring 1983). Horses prefer to feed in areas with the greatest concentration of high quality food (green plant matter) and when this is sparse they search out areas with the greatest concentration of perennial herbaceous plants (green or dead) (Duncan 1983). Horses mainly eat grasses, but will ingest roots, bark, buds and fruit. Seasonal patterns in ingestive behaviour occur in most locations (Waring 1983). Horses don’t usually graze where dung occurs, but show a preference for defecating in areas containing dung (Waring 1983). Foals up to one month old may eat faecal material, usually that of their mothers, but the ingestion of faeces is rare in adults (Waring 1983). Adults may, however, eat faeces if food is scarce (Feist & McCullough 1976).