Environmental gradients shift the direction of the relationship between native and alien plant species richness

Aim To assess how environmental, biotic and anthropogenic factors shape nativealien plant species richness relationships across a heterogeneous landscape. Location Banks Peninsula, New Zealand. Methods We integrated a comprehensive floristic survey of over 1200 systematically located 6 x 6 m plots, with corresponding climate, environmental and anthropogenic data. General linear models examined variation in native and alien plant species richness across the entire landscape, between native- and alien-dominated plots, and within separate elevational bands. Results Across all plots, there was a significant negative correlation between native and alien species richness, but this relationship differed within subsets of the data: the correlation was positive in alien-dominated plots but negative in native-dominated plots. Within separate elevational bands, native and alien species richness were positively correlated at lower elevations, but negatively correlated at higher elevations. Alien species richness tended to be high across the elevation gradient but peaked in warmer, mid- to low-elevation sites, while native species richness increased linearly with elevation. The negative relationship between native and alien species richness in native-dominated communities reflected a land-use gradient with low native and high alien richness in more heavily modified native-dominated vegetation. In contrast, native and alien richness were positively correlated in very heavily modified alien-dominated plots, most likely due to covariation along a gradient of management intensity. Main conclusions Both positive and negative nativealien richness relationships can occur across the same landscape, depending on the plant community and the underlying human and environmental gradients examined. Human habitat modification, which is often confounded with environmental variation, can result in high alien and low native species richness in areas still dominated by native species. In the most heavily human modified areas, dominated by alien species, both native and alien species may be responding to similar underlying gradients.