Invasion patterns of non-native ants in natural ecosystems in warm, temperate New Zealand

Non-native species have the ability to negatively impact ecosystems, and the recipient biodiversity they may invade. However, they must first go through a series of abiotic and biotic filters that limit their ability to spread once established, which ultimately influences their distribution across different habitats. By understanding which habitats are most vulnerable to invasion, pest managers can prioritise their surveillance areas to focus on those most at risk. We assessed temporal and spatial invasion by ants within forested and non-forested ecosystems using a replicated sample design based on data collected in 2005 and by repeating the sampling 10 years later. We predicted that forested ecosystems would be more resistant than non-forested ecosystems to invasion, likely due to unfavourable habitat factors that limit ant establishment and spread, even after a 10-year period. In both sampling years, non-forested ecosystems were more susceptible to invasion by non-native ants, with ants being found at both the edge and in the interior of these habitats. Conversely, in forested ecosystems, non-native ants were concentrated on the forest edges. Although there was invasion of non-native ant species further into some non-forested ecosystems over the 10-year period, spread did not penetrate beyond the edge in forest ecosystems suggesting that habitat characteristics are important in determining non-native ant spread. We recommend prioritising surveillance and management of pest ants in non-forested ecosystems rather than forested ecosystems in New Zealand. Our findings highlight the need for future work on the mechanisms of spread of non-native ants in non-forested ecosystems.