Effects of Feral Horses on Vegetation of Sable Island, Nova Scotia

Résumé

To provide necessary information for the management of biodiversity on Sable Island, Nova Scotia, we studied the effects of feral horses on vegetation using exclosures and ancillary observations. Nine plant communities inside and outside of six exclosures were compared using various vegetation parameters and Mann-Whitney tests to evaluate the significance of differences. The most important findings were as follows: (1) effects of horses were greatest in the Marram (Ammophila breviligulata) grassland and much less in the communities that were not dominated by Marram Grass; (2) effects on Marram grassland varied substantially among sites; (3) the cover of standing litter of herbaceous plants was on average of 9.3 times greater inside exclosures in grassland habitats; (4) the cover of living foliage was usually higher inside exclosures, but not all differences were significant; (5) species richness and species diversity were not substantially affected; (6) the average cover of Marram Grass, the most abundant plant and a key sand binder on the island, was greater inside exclosures in six of seven study sites, significantly so in three of them; and (7) there were inconsistent differences in cover of other species at different sites. Wetland habitats cover a relatively small portion of Sable Island, but they support much of the plant biodiversity. There is evidence of strong but variable effects of horses on wetland vegetation. “Horse lawns” are littoral habitats dominated by Agrostis stolonifera (Carpet Bentgrass) and other low-growing plants. The lawn habitats represent less than 1% of the island’s vegetation, and their presence is believed to be due to grazing and trampling by horses.