In some habitats, hooves of free-roaming horses compact the soil, and when the soil is compacted, air spaces are minimized, leaving nowhere for water to collect. When this occurs, soil in areas where horses are prevalent has a water penetration resistance over 15 times higher than that in areas without horses. Trampling also causes soil erosion and damages vegetation, and because the soil cannot hold water, plant regrowth is hindered. Horse trampling also has the potential to damage waterways and bog habitats. Trampling near streams increases runoff, reducing the quality of the water and causing harm to the ecosystem of the waterway. Horse excrement carcasses that result when feral horses perish add to the negative environmental impact of feral horses in Australia.