The influence of feral horses dung piles on surrounding vegetation.

The effect of grazing by large herbivores on grassland communities has been extensively studied, however less is known about changes associated with sites of high-intensity activity, such as dust bowls, tracks, urine patches and dung piles, that might induce disproportionate impacts to the directly affected areas and the surrounding vegetation. This paper explores the changes associated with exotic feral horse dung piles in relicts of natural grasslands in Argentine Pampas. We expected greater changes in the composition of plant communities and a greater facilitation effect on the establishment of invasive alien plants adjacent to the dung piles. Characteristics of the vegetation surrounding dung piles were recorded in 10025 cm plots located 0–1.5 m from the edge of the piles. We compared the immediate surroundings with reference plots at 5 m. The diversity of plant species increased as distance from the edge increased from 0–1.5 m; however, species richness was significantly higher in the first meter next to dung piles than 5 m away. Percentage cover of bare ground decreased further from the dung piles at both scales. Percentage cover of woody plants was greater at greater distances from the manure, whereas percentage cover of exotic plants was significantly higher next to the dung piles. The reported changes could be related to concentration gradients of nutrients liberated from the manure and/or to behaviour patterns of the horses, which may avoid grazing in the immediate surroundings of dung piles. These changes result in invasion windows facilitating the establishment and subsequent dispersal of exotic plant species in grasslands.