Use of density‐impact functions to inform and improve the environmental outcomes of feral horse management

Abstract

The available science often demonstrates the need for feral horse population control but not the degree of control required to achieve environmental conservation objectives. To better manage the influence of feral horses, we must first understand the relationship between feral horse density and environmental impact. We recorded vegetation and soil disturbance, and the sign of potential causes of this impact in two parts of the Australian Alps, the Bogong high plains (BHP) and the Eastern Victorian Alps (EVA). We calculated density‐impact functions to assist managers with determining feral horse density targets for control programmes. Minimal sign of feral horse impact was detected on the BHP, with no impact of feral horses observed along 99% of the length of transects. In contrast, impacts assigned to feral horses were significantly higher in the EVA, where a larger, higher‐density population of feral horses existed. However, greater than 83% of the walked transect length was still undisturbed by feral horses in the EVA. We detected a threshold of horse impact at ~ 250 horse faecal piles per ha. Above this threshold, a slight increase in horse density resulted in a disproportionately large increase in impact. In this context, a relatively small population control effort may substantially reduce direct horse impact. But where horse densities exist below this threshold, considerably more expense and control effort (resulting from the difficulties related to control at low density) is likely to make very little difference to an already low level of direct impact. The combined impacts associated with the sign of deer, feral pigs, fire and humans were large compared to that of feral horses. Management of feral horses to reduce their direct impact is unlikely to be beneficial without complementary management to reduce the effects of these other agents of impact.

The Ecological Separation of Deer and Domestic, Feral and Native Mammals in Tropical Northern Australia—A Review

“…Taking feral horses as an example, a common theme is the desire to maintain horse densities where their ecological damage is minimised and their ecological, economic and cultural benefits are maximised [ 75 ]. Research to achieve this balance for most introduced species in Australia has received little attention [ 75 , 76 ].…”

“…Taking feral horses as an example, a common theme is the desire to maintain horse densities where their ecological damage is minimised and their ecological, economic and cultural benefits are maximised [ 75 ]. Research to achieve this balance for most introduced species in Australia has received little attention [ 75 , 76 ]. Successful management requires the determination of a threshold level of population or density below which the impact is benign or acceptable, based on structured measurement of detrimental and beneficial impacts.…”