Drivers of alien species richness
At large grains, alien species richness declined with increasing elevation, but absence of A. selago led to higher alien species richness compared to its presence at low elevation (with this effect reversed at higher elevations) (Figure 4, Table 1c). Furthermore, distance to nearest drainage positively influenced, whilst northness negatively influenced, alien species richness at large grains (Figure 4, Table 1c). Alien species richness in grey-bedded ash geology was significantly higher than in other geologies (Figure 4, Tables 1c, A5). However, since there were very few plots in this geology (N = 5), the effect of geology on alien species richness can be considered negligible.
At small grain, the presence of A. selago , northness and elevation negatively affected alien species richness whilst distance to nearest drainage positively affected alien species richness (Figure 4, Table 1c).
When analyses were repeated for only plots with at least one alien species (i.e., alien species richness ≥ 1), only northness was a significant driver at large grain, having a negative impact on alien species richness. At small grain, A. selago and northness became unimportant when alien richness was at least one, while elevation and distance to the nearest drainage exhibited similar effects as those observed in the main dataset (Table A6).
Both elevation and temperature negatively affected alien Δ9-1, and the rare, grey-bedded ash geology had a significantly higher alien Δ9-1 for the full dataset (Figure 4, Table 1d).
Elevation had a similar effect on alien Δ9-1 in the subset of the data with at least one alien species present in each plot (Table A6). Distance to nearest drainage had a negative effect on alien Δ9-1 in the dataset of plots with at least one alien species present, despite not having an effect in analyses of the full dataset (Table A6).
Table 1: Results from simultaneous auto-regressive models with a Poisson distribution showing the predictors of vascular plant species richness at the large (9 m2) and small (1 m2) grain sizes and at the difference in richness between the two grain sizes (Δ9-1) for both native and alien species.