|Abstract: ||Nature is intrinsically complex; yet ecological research is steadily gaining insight into the relationships between abiotic and biotic conditions. This thesis seeks to examine these interactions; between the causal drivers of global environmental change (GEC) and between biotic units in which that change is manifested. Concern is mounting over the ecological surprises that multiple GEC drivers can exert on biota when they act simultaneously. Studying the greatest current threat to biodiversity, habitat modification, and the greatest potential future threat, climate change, I investigate the nature of these interactions. I determine whether they are of a synergistic nature, whereby one driver is exacerbated by another; or of an antagonistic nature, whereby the effect is reduced. These effects are quantified on two measures of biotic change: abundance and species interactions in a tri-trophic forest study system in New Zealand.
Taking a range of methodological approaches I utilise: field observation analysis, in-situ experiments and ex-situ experiments to investigate aspects of the umbrella research question. Throughout this research I study a tri-trophic biotic system in which to investigate broad ecological trends: an understory shrub, Macropiper excelsum; its herbivorous moth, Cleora scriptaria; and the herbivore’s endoparasitoids, Aleiodes declanae and Meteorus pulchricornis. Three principal findings emerge from this investigation. Firstly, although climate and habitat fragmentation exhibit a mixture of effects on biota, when they interact, the net effects are all seemingly negative: abundance is reduced and the frequency of biotic interactions decrease. Secondly, these drivers combine in a non-additive fashion depending upon the trophic level and biotic measure assessed: antagonistically interacting to impact the plant species and species interactions and synergistically interacting to impact the insect species and abundance. Thirdly, the influence of interspecific relationships on dynamics within this study system is ostensibly comparable to the impact of anthropogenic pressures. This research suggests that, where possible, it is vitally important to investigate all known simultaneous drivers of change, to specify which taxonomic unit is being studied and to integrate biotic interactions when predicting the impacts on biodiversity in a changing world.|