The Sub-Lethal and Density-Dependent Effects of an Invasive Wasp on an Endemic Ant

Abstract

The detrimental effects of invasive species on recipient taxa are most frequently reported in terms of displacement or extinction. However, there may be other less obvious effects. For example, a reduction in resource availability mediated by a novel competitor may affect growth and development in recipient species. The cost of aggressive competitive interactions with invasives may promote niche shifts or altered spatial distribution to minimise competition, and intense predation by exotic species may result in a reduced effective population size with a concomitant reduction in genetic diversity. In this thesis I examined the sub-lethal effects of varying densities of the invasive competitor and predator Vespula vulgaris on the morphology, behaviour, population genetics and spatial distribution of the New Zealand endemic ant Prolasius advenus. The restriction of food resources can result in reduced worker size and altered scaling relationships of adult body parts in ants. Measurements of nine morphological characters from P. advenus worker ants collected from sites of varying wasp density revealed that workers were significantly smaller where there were more wasps. I also found evidence of allometric scaling relationships among body parts, which varied between areas of high and low wasp density. In particular the scaling relationships between the abdominal segments and overall size were found to be weaker where there were more wasps. This scaling variation resulted in ant workers with proportionally smaller abdomens in areas where wasps were most abundant, which might reduce their resource gathering and defensive efficiency. I tested the hypothesis that tasks performed by workers would be size correlated. In the presence of high densities of wasps, honeydew collection and brood care was undertaken by larger workers, whereas foraging in leaf litter was undertaken by smaller workers. In contrast, no relationship was found between task and worker size where wasp densities were lower. I suggest that in addition to increased efficiency in the collection of liquid carbohydrate resources larger workers may also serve a defensive role. I also tested the hypothesis that where the exclusively diurnal wasps were most abundant P. advenus workers would minimise potential competitive interactions by foraging predominantly at night. However, P. advenus were found to forage as much or more by day in the presence of high densities of wasps and to forage more nocturnally where wasps were sparse. Increasing the number of foragers by day may improve P. advenus’ competitive ability with wasps, but has the potential to come at the cost of other nest functions, energetic gains and increased worker mortality. Competitive mechanisms might influence ant population genetics, but wasps have also been shown to prey on dispersing ant queens. Analysis of microsatellite DNA markers revealed evidence of a genetic bottleneck in P. advenus populations where invasive predatory wasps have persisted at high population densities for more than twenty years. Ant populations at all wasp densities displayed significant heterozygote deficit, which may indicate that even at comparatively low densities predation by wasps has a detrimental effect on P. advenus genetic diversity. Alternatively, the observed heterozygote deficit may be the result of limited dispersal, population substructure or the peculiarities of the ant species‟ mating system. An examination of the broader population genetic structure of P. advenus from the seven sites sampled indicated that they were derived from two distinct populations. However, no individual assigned entirely to either population suggesting either some remaining admixture between the two populations or that they are derived from a common ancestral population. Finally I examined the effects of increasing wasp density, honeydew availability and habitat diversity on the distribution of P. advenus. Prolasius advenus nests tended to be smaller and populations displayed less variation in nest size where wasp densities were highest, which could indicate reduced longevity. Nest distribution was overdispersed relative to both con-and hetero-specific neighbours in high wasp density sites, but apparently randomly distributed in low wasp density sites. Both wasp and ant nest densities were greatest where honeydew was most abundant and plant diversity was lowest. These results suggest that intra- and inter-specific competition in combination with resource and nest site availability influences P. advenus nest distribution. Competition and predation by invasive species may influence recipient species‟ morphology, behaviour, genetic diversity and spatial distribution over a comparatively short period of time. These effects are much more subtle than under situations of direct predation. Such effects may not be immediately obvious in terms of abundance, but have potential long-term implications for the fitness and persistence of P. advenus in the presence of high densities of wasps.