Insects in mountainous landscapes: Implications of landscape context and climate change

Abstract

Landscape context (i.e., habitat area, habitat quality and habitat fragmentation) determines the local distribution and regional survival of insect specialist herbivores. The effects of landscape context on individual species are often complex and species-specific. Furthermore, interactions with higher trophic levels, such as parasitoids, and the spatio-temporal scaledependencies of habitat parameters further complicate the investigation of species habitat requirements. Understanding the importance of landscape context is important in the setting of future landscape and climate change. For example, if sufficient habitats are not available along their migration routes, insect habitat specialists may not be able to successfully follow shifting climatic envelopes. In this thesis I addressed four intertwined questions: What are the resource requirements — habitat area, quality and fragmentation — for two specialist herbivores? How does the landscape context change with elevation? How are herbivoreparasitoid systems affected by the landscape context along elevational gradients? Do species resource requirements suggest an increased vulnerability towards temporal or spatial bottlenecks under climate change? To evaluate these questions I used a range of field and laboratory methods and statistical modelling approaches. I mapped the spatial distribution of a riparian shrub, Veronica stricta (Plantaginaceae), at different spatial scales across the Tongariro National Park, central North Island New Zealand. Patch occupancy of Trioza obscura (Hemiptera: Triozidae) and a (undescribed) gall midge (Diptera: Cecidomyiidae) were recorded in 2010 and 2011. In 2011, cecidomyiid galls were collected to investigate the cecidomyiid and its parasitoid Gastrancistrus sp. (Hymenoptera: Pteromalidae). Habitat suitability and patch-level extinctions and (re)colonisations (i.e., population dynamics) for both herbivores, T. obscura abundance, and gall midge-parasitoid proportions were predicted with generalized linear models, applying information-theoretic model selection and averaging. Overall, habitat area had the largest positive influence on insect presence and survivorship. Habitat quality (i.e., patch shading, plant size, leaf carbon/nitrogen, and vegetation cover) had a species-specific effect on the patch occupancy/abundance and population dynamics of the two herbivores, and, in interaction with elevation, had a complex effect on the gall midge-parasitoid system. In contrast, habitat fragmentation was less important in the study system. Elevation contributed positively to patch suitability and survivorship of the cecidomyiid, but decreased survivorship of T. obscura and the relative abundance of the parasitoid Gastrancistrus sp.. Spatial habitat model transferability to a regional scale was only successful for T. obscura. Altitude-related changes in the landscape matrix, such as the transition from forested to subalpine vegetation, positively affected habitat area and fragmentation, which, in turn, offset a steady decrease in host plant size (i.e., patch quality) with elevation. In summary, my study revealed that habitat area, quality and fragmentation do not necessarily follow simple linear trends along elevational gradients and that simultaneously addressing multiple hypotheses, including their interactions, can provide more ecologically meaningful insights into tritrophic systems than testing individual hypotheses separately. Although both herbivores will find suitable habitats at their current upper range limit, T. obscura will likely lose some habitat patches at lower sites following suppression of V. stricta by invasive plant species and the gall midge will likely face more patch-level extinctions at high elevations following an upwards shift of its parasitoid in future predicted climate. In conclusion, I suggest that species vulnerability to climate change in mountain environments depends on whether resource requirements allow for upward migration and suitable habitats are available in future distribution ranges.