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

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.

现有科研成果往往证实，需对野生马种群实施管控，但尚未明确达成环境保护目标所需的管控强度。为更有效地管控野生马的环境影响，我们首先需要厘清野生马种群密度与环境影响之间的关联。我们在澳大利亚阿尔卑斯山的两个研究区域——博贡高平原（Bogong High Plains, BHP）与维多利亚州东部阿尔卑斯山（Eastern Victorian Alps, EVA）——记录了植被与土壤扰动情况，以及该扰动潜在成因的相关迹象。我们构建了密度-影响函数，以协助管理者确定管控项目所需的野生马种群密度目标。在博贡高平原区域，仅检测到极少量野生马影响迹象，99%的样带长度未观测到野生马造成的干扰。与之形成鲜明对比的是，维多利亚州东部阿尔卑斯山的野生马种群规模更大、密度更高，其野生马相关干扰程度显著更强。但即便如此，该区域仍有超过83%的步行样带长度未受野生马干扰。我们检测到野生马干扰的阈值约为每公顷250堆马粪。超过该阈值后，种群密度的小幅提升会导致干扰程度成比例地大幅增加。在此情境下，仅需开展相对少量的种群管控工作，即可大幅降低野生马的直接干扰。但若种群密度低于该阈值，由于低密度管控存在诸多难度，即便投入更多成本与管控精力，也难以使本已处于较低水平的直接干扰出现明显改善。相较于野生马，鹿、野猪、火灾与人类活动相关的综合干扰程度更为显著。若不辅以针对其他干扰因子的管控措施，仅通过管控野生马以降低其直接干扰，大概率无法达成预期的环境保护效益。