Impact of trampling on the vegetation of subantarctic Marion Island

We assessed the impact of human trampling in three different habitats on Marion Island (46°50'S, 37°50'E). The habitats were (1) mires with wet, peaty soils and grass- and bryophyte-dominated vegetation; (2) slopes with relatively dry mineral soils, dominated by small ferns and dwarf shrub; and (3) feldmark on dry mineral soils with an open vegetation of cushion dicots and bryophytes. We examined existing walking tracks on the island. Track width (25 to 800 cm) increased with soil moisture content. Trampling reduced vegetation height, total cover, and species richness in mires and feldmark and vegetation height and herb layer cover (but not bryophyte cover or species richness) in slopes. In mires, most species were negatively affected by trampling, but in slopes trampling increased the cover of 6 out of 9 significantly affected species. The total number of species in all trampled plots in mire and feldmark communities was 10% lower, but in slopes 28% higher, than in control plots. The impact of trampling differed between growth forms. Cushion dicot, shrub, and fern covers were reduced, whereas graminoid and pleurocarpous moss covers were unaffected or increased with trampling. Trampling reduced the cover of most bryophyte species, but it did increase the cover of some. In the slope habitat, destruction by trampling of the closed herb canopy allows increased light penetration and makes the habitat more favorable for small plants such as bryophytes. We attribute the differences in how the vegetation of different habitats responds to trampling to differences in the structure of the original vegetation as well as differences in soil characteristics, especially the soil's structural stability under pressure.