Native forest invasion by exotic plants can have cascading negative effects. Native plant species richness has been hypothesised to be valuable by providing resistance to exotic plant invasions (the “biotic resistance” hypothesis). However, studies of the native-exotic richness relationship found opposite results at different spatial scales, termed an “invasion paradox”. It has been hypothesised that at small scales, biotic resistance generates a negative native richness-exotic richness relationship, whereas at larger scales, habitat heterogeneity drives a predominantly positive relationship.
We studied factors influencing exotic plant invasions and the native-exotic richness relationship in New Zealand’s native forests, including adjacent land cover types, climatic conditions, species interactions and spatial scale. We analysed data from the National Vegetation Survey 2002-2007, the New Zealand Land Cover Database 2008-2009 and NIWA climate data. We predicted exotic richness from native richness at different tiers (below 30 cm and above 5 m height, respectively), adjacent non-native land cover types (exotic grassland, exotic forest and disturbed land), plant competition (tree basal area and native species ground cover), mean temperature and rainfall as fixed effects. This analysis was repeated at six spatial scales (20 m plots to 256 km square groups of plots). Habitat heterogeneity across scales was measured using the variance of climatic conditions among scale groups of plots.
Results/Conclusions
Adjacent non-native land cover types, especially exotic grassland, significantly increased exotic richness across all spatial scales. As expected, habitat heterogeneity was greater at larger scales. The relationship between native tree richness and exotic richness was negative at small scales (<32 km square groups) and positive at large scales (>64 km square groups) showing the “invasion paradox” for the first time within a single dataset. Interestingly, tree basal area and native species ground cover had a negative relationship with exotic richness which became non-significant at larger scales.
These results showed that nearby seed sources play an important role in exotic species invasion at all scales, which needs to be taken into consideration in studies such as this. Competitive interactions between native and exotic plant species underly native forest resistance to invasions, as shown through effects of native tree species diversity, tree basal area, and native ground cover, supporting the biotic resistance hypothesis. Although habitat heterogeneity means we cannot detect biotic resistance at large spatial scales, biotic resistance will still be acting in each plot across whole landscapes and is therefore a valuable ecosystem service.