Assessing risk to native ecosystems: using exotic ants as a model

Abstract

Invasive species are widely recognised as drivers of ecological change. As such, understanding the processes that facilitate invasion success, and the potential mechanisms and magnitude of impact, are critical for effective biosecurity management. Predicting risk associated with exotic species, so that management actions can be justified, is a major challenge faced by pest managers. In this thesis, I present a conceptual framework for assessing ecological risk in natural ecosystems developed from natural hazard (earthquake) risk assessment frameworks, and apply this using exotic ants as a model. To assess the vulnerability of different ecosystems to exotic ant invasion, I replicated a sampling study conducted across the Auckland region in 2004, allowing a temporal aspect to be included in the analyses. I found open-canopy ecosystems, consisting of short stature vegetation, to be most vulnerable to exotic ant invasion with several exotic ant species found to be ubiquitous throughout. In comparison, the same species were generally limited to the edges of closed-canopy forest ecosystems, even after a 10-year period. I then focused on dietary impact-related research within open-canopy ecosystems. I assessed competition between native and invasive ants using isotopic niche overlap and co-occurrence patterns, as well as employing DNA metabarcoding, to understand what invertebrate taxa are most at risk via predation. Furthermore, I investigated the potential disruption of floral visitor networks by exotic ants through conducting surveys across the flowering plant community and using focal observations to determine whether ants may be competitively excluding important pollinators. I found trophic position varied between native and invasive ant species; although two invasive species represented the two lowest trophic positions, most other species reflected trophic positions similar to predators. There was evidence of competitive exclusion between a native and invasive ant species, as revealed through high isotope niche overlap and negative co-occurrence patterns. Exotic ants were found to be common floral visitors, although this varied spatially. The presence of exotic ants on flowers was associated with a significant, negative response by important insect pollinators. Overall, my research greatly contributes to the ecological risk assessment posed by exotic species in native ecosystems and contributes to the impact-based research of invasive ants.