Invasion Impact and Biotic Resistance by Invertebrate Communities
Invasive species have been recognized as one of the greatest threats to global biodiversity and can have dire economic consequences. Yet rates of invasion are increasing due to the fast and growing network of transportation across the globe. The establishment, spread and impact of invasive species are affected by environmental conditions as well as resident species. Species respond differently to the same abiotic factors and different native species can respond either positively or negatively to invasion. The interaction between invasive and resident species, as well as the effect of temperature on invasive species, has gained much attention. The synergistic effect of suboptimal temperature and biotic resistance could have a much stronger limiting or controlling effect on invasive species than either factor alone. Linepithema humile (Argentine ants) are invasive species originally from a Mediterranean climate, but successfully spreading into extra range habitats. The establishment and spread of these ants in temperate New Zealand represents an ideal model system for studying invasion biology in terms of temperature limits and biotic resistance effects. I investigated the changing distribution of the invasive species the Argentine ants over multiple years at five sites in New Zealand. To test whether their rate of spread corresponds with microclimate I investigated their fine-scare distribution patterns and evaluated the number of generations they may develop seasonally and annually in different microhabitat types. I also evaluated their impact on other arthropod species. I conducted a laboratory experiment to evaluate the effect of temperature on their aggression towards other species, walking speed, and foraging abundance. Similarly, I tested the effect of biotic resistance from other ant species (Monomorium antarcticum and Prolasius advenus) with varying colony sizes. I investigated whether there was any interactive effect of temperature and biotic resistance on the Argentine ants. The distribution of Argentine ants had declined across many invasion fronts over the past 7-8 years. They were more likely to be found in concrete, short grass and sandy habitats, which provide warm microsites. Degree-day calculations predicted that they could develop between 2.5 to 3 generations in each of the above microhabitats per year in urban and rural sites while they were predicted to be unable to develop one generation under tree habitats. In tall grass microhabitats they were predicted to develop between 1-1.5 generations per year. The Argentine ants were hypothesised to adversely affect many other arthropod species. Richness and abundance of resident beetle species were negatively correlated with the invasion of the Argentine ants. Areas invaded by the Argentine ants were also associated with a greater number of exotic beetle species, which may imply secondary invasion. Laboratory experiments showed that lowering temperatures below 17°C negatively affected the Argentine ants‟ walking speed, foraging abundance, aggression and their resource control. A high colony size of M. antarcticum (the competing ant species) affected the foraging success of Argentine ants, and the effect was stronger when coupled with unsuitable temperature (17°C and below). Therefore, Argentine ants are weak competitors at low temperature levels. The results of my thesis underline the importance of biotic and abiotic resistances, their interactive effect as well as the effect of the Argentine ants on other species. Based on climatic considerations and the habitat preferences of resident species it may be possible to predict future spread of the Argentine ants. More importantly, knowledge of microhabitat preferences and biotic resistance may help future control measures against Argentine ants based on management of vegetation structure and microhabitat availability.