Invasion dynamics of social wasps on New Zealand's offshore islands

Abstract

Invasive species can radically alter the ecosystems they invade. Understanding characteristics that drive the invasiveness of species and the invasibility of ecosystems is important for developing management strategies. Social insects are highly successful invaders and their social lifestyle renders them particularly challenging to control. In this study, I explore the invasion dynamics and impacts of four species of introduced social Vespula and Polistes on New Zealand’s offshore islands, many of which are the last refuges for endemic species. First, I compared the genetic diversity and structuring of Vespula germanica and V. vulgaris between their native and invaded ranges, including six New Zealand offshore islands. Results show that low genetic diversity in the invaded range does not impede the invasion success of both Vespula species, and confirm the UK as the origin of New Zealand populations. Second, I quantified the abundance of Vespula and Polistes on 36 offshore islands and modelled them against biogeographic, biotic and anthropogenic factors. Social wasps occurred on 97% of islands surveyed. Strong island isolation and absence of human settlement were associated with low abundance of Vespula, and canopy cover was negatively correlated with the abundance of both Vespula and Polistes. This highlights the importance of protecting and restoring native forest on islands. While population genetics could not explain the invasiveness of Vespula in New Zealand, biogeography and anthropogenic impact could predict the abundance of Vespula and Polistes. Third, I investigated whether the four invasive wasp species partition resources on Ahuahu (Great Mercury Island) to facilitate coexistence. I refined a method for DNA metabarcoding of faecal samples, and revealed spatial, temporal and diet partitioning among the four wasp species. The coexistence of four invasive wasp species on Ahuahu probably amplified their ecological impact. Because they are generalist predators, they affected the native invertebrate fauna on multiple trophic levels Given their extraordinary invasion success and negative ecological impacts on island ecosystems, decision makers in New Zealand should include vespid wasps in island conservation strategies. Finally, as social insects are invading new regions at a rapid pace, progressing our understanding of their invasion dynamics and developing management strategies is imperative.