Local vs regional factors as determinants of the invasibility of indigenous forest fragments by alien plant species

Both local and regional filters can determine the invasion of alien species into native plant communities. However, their relative importance is essentially unknown. We used plot data from fragments of indigenous forests in southeastern New Zealand to infer which factors are important in explaining invasibility, measured as alien species richness. Twenty-eight predictor variables comprising both local factors (stand structure and soil) and regional ones (climate and land cover) were assessed. Reduction or increase in deviance in linear models was assessed, both individually and with a forward and backward stepwise variable selection procedure using the Akaike information criterion (AIC). We found that higher alien species richness was mainly associated with forest fragments of small area in warm and dry climates and where there were only small areas of surrounding indigenous forest. Local soil and stand structure variables had considerably smaller effects on alien species richness than the regional land cover and climate variables. Alien species richness showed no relationship with native species richness. We conclude that in the forest fragments investigated here, of the variables included in the analyses, regional land cover and climate variables are potentially important drivers for alien species richness at plot level. This has implications for projections of alien species spread in the future under different climate change and land use scenarios.