Quantifying the ecological risk of exotic species–a case study using the parasitoid Meteorus pulchricornis

Abstract

Exotic species have the ability to affect a countries economy, society, and biodiversity, either positively or negatively. However, to understand their impacts, knowledge on exotic species in novel environments must improve and appropriate risk assessments be readily available. The potential for an invading species to spread depends on the traits of the species and the ability of a species to maintain constant fitness over a range of different environments. The physical environment, resource use, and species interactions are three key attributes making up a species ecological niche, and these can have a large affect on species presence, abundance, distribution, and thus the impacts of an invading species. In the first part of this thesis, three key factors (physical environment, resources and competition) of the ecological niche were examined for Meteorus pulchricornis, an exotic generalist parasitoid wasp. Interactions of the physical environment and resource availability were used as predictors of M. pulchricornis parasitism. These interactions were also examined in respect to two different spatial scales; specific sites and the individual plants within a site. This was studied across 30 sites in the Auckland region using a common tri-trophic interaction of M. pulchricornis, a Lepidopteran larvae host, the kawakawa looper moth (Cleora scriptaria), and the plant this host is commonly found on, kawakawa (Piper excelsum). Meteorus pulchricornis was present throughout the majority of the Auckland region surveyed, overall parasitizing a large percentage (19.5 %) of C. scriptaria collected. At the spatial scale of the plant, resource (caterpillar) availability and plant height had a positive effect on M. pulchricornis densities. Whereas at the site spatial scale parasitism rates declined with increasing plant richness and parasitoid densities declined with coarse woody debris. In the second part of this thesis, the scientific literature and the New Zealand Arthropod Collection were used to predict whether M. pulchricornis was competing inter-specifically with resident parasitoids for hosts. A database of 113 species was compiled with information on parasitoid species functional traits deemed important to searching and competing for hosts. Dendrograms were utilised to cluster groups of species with similar functional traits, and thus with greater potential to compete inter-specifically. Different clustering and weighing techniques, and combinations of functional traits were examined to determine the feasibility of this risk assessment framework for predicting competitive interactions. Of the different techniques and trait combinations included in the risk assessment tool, the one with the greatest predictive ability excluded taxonomic but included all life-history and morphological functional traits. Additionally, two key variables (relatedness and host overlap [i.e. resource sharing] with M. pulchricornis) were important for the grouping of species M. pulchricornis was clustered in, indicating the reliability of this framework. These two attributes are commonly used in risk assessment tools because of their assumed role in competition for resources in community ecology. The current study indicates that observing one spatial scale alone is not always sufficient when trying to determine the predictors of interconnections in community ecology and the impacts of an exotic species. Aspects of this thesis conflict with the theory that greater amounts of potential resources should support a generalist species, on the other hand, supporting the biotic resistance hypothesis, that greater plant species richness in a habitat should suppress invasion by an exotic species. Moreover, when taking into consideration a species ecological niche and its functional traits prove to have potential in enhancing risk assessment.