Alien species
Alien plant richness, which is considerably lower than native plant richness on Marion Island at the scales studied here, was partly affected by energy availability. Positive correlations between northness and alien species richness at both grains provide additional evidence of the importance of sunlight and warmth availability for plant growth (Måren et al., 2015). Furthermore, alien species richness was higher in low elevation areas, characterised by warmer temperatures, nutrient-rich conditions, and pronounced animal and human activities (Haussmann et al., 2013, le Roux et al., 2013b, Greve et al., 2017). However, richness increased with distance to drainage areas, likely due to the island’s high precipitation levels being sufficient to support the island’s alien species.
Biotic interactions seem to play a minor role in alien species richness on the island. At the large grain, alien species appeared to compete with A. selago at low elevations but were facilitated at high elevations. However, with alien species rare at high elevations, there is much uncertainty around the facilitative effects of A. selagoon alien species at high altitudes. Studies in similar climates have shown cushion plants facilitating non-native species at higher elevations where they would not be able to occur in the absence of this interaction (e.g., Badano et al., 2015, Cavieres et al., 2008, Arredondo-Núñez et al., 2009).
Most alien species on Marion Island still have localised distributions, suggesting they are still in a lag phase and could potentially extend their ranges in future (Greve et al., 2017, Crooks et al., 1999, le Roux et al., 2013b). If this is the case for many of the non-native species, then the patterns of alien richness observed in this study could be expected to change in the future, particularly as warming temperatures may remove some climatic barriers to the spread of localised alien species.