Abstract
Biological invasions by non-native plant species and their impacts on native biodiversity and ecosystems are a crucial global environmental challenge. Aotearoa New Zealand is a susceptible and highly invaded landscape with over half of its naturalised plant species being non-native. Understanding and assessing spatial patterns of current and likely future invasions is therefore crucial. At global and regional scales, climate is a fundamental driver of spatial patterns of species distributions. Identifying areas of similar climate is accordingly a first towards assessing potential invasion risk, with more climatically similar regions being more likely to exchange species than less similar regions. This research aims to assess the invasion risk across New Zealand, specifically in its three main grassland ecosystems (alpine, tall tussock, and low-producing grasslands) using climate matching. An analysis of the climatic niches of the four main exotic plant life forms (trees, woody/shrub, herbaceous, and graminoid) present in New Zealand found a high level of niche overlap. This indicates that species from all life forms have invaded similar climatic conditions in New Zealand. In addition to the three grassland types, the climatic conditions in low-producing grassland showed the highest overlap with the exotic species' climatic niches. This indicated the climatic conditions and ecosystems of low-producing grasslands offer the highest potential for invasive plant species compared to other grassland areas in New Zealand. A species-level niche overlap analysis of six invasive graminoid species already established in New Zealand showed a low degree of overlap between each species’ native and invaded New Zealand climatic niche, indicating that some form of niche shift might have occurred during the invasion process. When extending this analysis to include all invaded locations globally, niche overlap between the native and the invaded niche was higher but did not show evidence of full niche conservatism. The results from this research can help to inform management efforts in controlling the spread of invasive species within New Zealand and in identifying areas globally that might act as potential source locations for new invasive species in New Zealand.