Factors Promoting Coexistence between Endemic Ants and Invasive Wasps
Authors:   , Duthie Catherine 
Published: November 2021

<p>Invasive animals can alter the community composition of native ecosystems by means of competition and predation. In this study I investigated the factors that may facilitate coexistence between endemic ants and invasive wasps. Previous research has shown that entire communities can be impacted by invasions. Endemic species subject to pressure from invasive species may undergo a niche shift to enable coexistence and minimise the impact of this pressure. The invertebrate community composition of Nothofagus forests in the South Island of New Zealand has been altered by predation from Invasive Vespula wasps. Ants and wasps in this ecosystem coexist on the same trophic level; they simultaneously fill multiple trophic roles as primary predators, secondary predators, and primary consumers. The outcome of competition between species such as ants and wasps is not easy to predict, and may vary in different communities and with different densities of competitors. In this dissertation I aimed to determine the extent to which competition occurs between native ants and invasive Vespula wasps, and to investigate the impacts of invasion on the native invertebrate community. I quantified the invertebrate community composition of Nothofagus forests and then experimentally reduced wasp numbers to investigate any changes as a result of a reduction in predation or competition. The observed changes in community composition were as a result of differing abundances of taxonomic groups within my study sites. In order to more robustly determine the community effects of wasp removal it may be necessary to reduce wasp numbers by up to 90% for many years. Even under these conditions, species that are particularly vulnerable to wasp predation or competition may have already been permanently excluded from this system. I then investigated temporal niche shifts by native ants when faced with reduced competition for food resources from invasive wasps. There was an increase in the numbers of ants foraging on honeydew when I experimentally reduced wasp numbers. This increase may be due to increases in both the quantity and quality of the available honeydew. When densities of wasps were substantially reduced there was a difference in the foraging abundances of ants and wasps; however, there was no change in the overall temporal foraging pattern of ants. Isotope ratios and consequently trophic levels of native competitors may change in response to the removal of an invasive species. To test this I examined changes in isotope ratios as a result of removal of wasps. The observed changes in the trophic levels of both ants and wasps appear to be a result of natural seasonal variation in consumption related to the nutritional requirements of the colony. Finally, I examined behavioral interactions between native ants and invasive wasps during foraging. This study has indicated that wasps may find and access resources more readily when ants are present. Native ants may facilitate foraging by wasps, as demonstrated by the increase in wasp numbers when foraging in the presence of ants. Additionally, the impact of competition between wasps and ants is likely to be density dependant. Co-occurrence between endemic and invasive competitors is possible through two important mechanisms, niche separation and behavioural adaptations. Native ants in this system are able to forage in different temporal niches than invasive wasps, and their dominant behaviour serves to diminish competitive interactions. These findings have implications for the ecology of these forests in understanding the considerable impact that invasive species may have on native ecosystems and particularly those species which have similar resource requirements.</p>