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.