Horses (Equus caballus) were introduced to Australia in 1788 and feral populations established soon after. Australia has the largest population of feral horses in the world (~400,000 individuals). A global review revealed that the ecological impacts of feral horses include:
Damaging waterways: Trampling near streams increases run-off and reduces water quality. Feral horse grazing and trampling is linked to downstream siltation and water ponding. Trampling can increase stream depth and stream pugging.
Soil degradation: Trampling leads to increased soil compaction, soil erosion, and soil loss, and reduced water infiltration and nutrient recycling capacity.
Spreading weeds: In Australia, 156 species of non-native plants can germinate in horse dung, including 16 noxious weeds. Dung piles have greater cover of exotic species compared to surrounding areas. Trampling disturbance can also facilitate weed invasion. Exotic plants are most successful at colonising trampled areas in the Australian Alps.
Altering vegetation: Trampling in concentrated areas creates track networks, thereby reducing local plant species richness. In the Australian Alps, track networks can range from 3.4 to 5.8 km km–2. Feral horse exclusion has led to increased native plant richness and percent cover, increased vegetation biomass and height, and reduced cover of exotic species.
Effects on fauna: Globally, feral horses have been linked to reductions in the richness, abundance, or diversity of ants, reptiles, fish, small mammals, and native herbivores.
As much research compares biodiversity in the presence or absence of horses, the effects of feral horse density on native ecosystems remains largely unknown. Studies of the effects of feral horses on faunal communities are few, particularly in Australia. Further, in Australia, no studies on the effects of feral horses have undergone peer-review.