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).