Native species
Elevation was negatively correlated with native species richness at both
grains on Marion Island. High-altitude environments, both on Marion
Island and elsewhere, typically harbour fewer native species and are
unable to support non-native species that originated from warmer
climates (Chown et al., 2013, Lembrechts et al., 2016). Furthermore, the
windy conditions prevalent in these high-altitude areas may limit some
species (Momberg et al., 2021). Additionally, the soils at higher
altitudes are shallower (or absent) and generally more nutrient poor on
this geologically young volcanic island, due to lower biotic inputs
(Haussmann et al., 2013).
The effect of elevation on richness was dependent on the presence of the
keystone cushion plant, Azorella selago , at the large grain.
Cushion plants are common in cold and windy regions, where they modify
microhabitats to the benefit of other species growing within them (le
Roux and McGeoch, 2010, Badano et al., 2010, Van der Merwe et al., 2021,
Reid et al., 2010). At low altitudes (c . < 250 m
a.s.l.), richness was higher where A. selago occurs. Conversely,
at high altitudes where A. selago is often the dominant or even
only plant species, fewer species occurred in the presence than the
absence of this species. The results of our study align with those of
Raath-Krüger et al. (2019), who observed a positive impact of A.
selago on the occurrence and cover of some native species at low
altitudes. This suggests that, as environmental stressors become
increasingly severe, biotic interactions become less important and
environmental factors emerge as primary limiting factors (e.g.
Raath-Krüger et al., 2019, Louthan et al., 2018).
Higher native richness in north-facing slopes could be due to that
aspect receiving the most potential direct sunlight (Måren et al.,
2015). Higher richness on warmer north-facing surfaces is likely
necessitated by the high precipitation levels on Marion Island
(c . 2000 mm annually, (le Roux and McGeoch, 2008)). In more arid
regions where soil moisture is limiting, higher incident radiation can
result in lower soil moisture and, as a result, lower richness
(Najafifar et al. 2019). These findings further support the water-energy
hypothesis, indicating the importance of energy over water in moist high
latitude and high altitude areas (Hawkins et al., 2003, Hufnagel and
Mics, 2022).
Most vascular plants’ ecological limits are influenced by water
availability; even in wet environments like Marion Island, plant
distributions can be affected by water (le Roux et al., 2013a). As such,
in our study, plots with high TWI at small grain, and plots closer to
drainage lines (i.e., wetter plots) at both grains, contained more
native species. Although TWI is typically only modestly correlated with
actual soil moisture, it is a good proxy in the absence of field
measurements (le Roux et al., 2013a, Riihimäki et al., 2021). Similarly,
distance to nearest drainage is a useful proxy for plant-available water
because the draining potential of the soil is a function of vertical
rise and horizontal flow (Marshall et al., 1996). Our study, therefore,
underscores the importance of soil moisture for plant species richness,
which has sometimes been considered less essential in cold and wet
environments.
Native richness was higher for both grains on steeper slopes. High
environmental heterogeneity, as found on steeper slopes, can increase
habitat complexity. Complex habitats offer a variety of microhabitats,
refuges, and opportunities for species to specialise in different
ecological roles or exploit specific resources, resulting in higher
species richness (Tuomisto et al., 2017, Stein et al., 2014, Kreft and
Jetz, 2007, Stein and Kreft, 2015).