|dc.description.abstract||As plants and insect herbivores represent major components of eukaryote biodiversity, they form the basis of virtually all terrestrial food webs. Despite this, host plants, dietary breadth and species interactions of many insects are unknown or poorly understood. Identifying and quantifying these complex interactions among insects and their hosts is challenging but is fundamental to our understanding of ecosystem structure and function. DNA barcoding is becoming an accessible tool for ecologists and has the potential to revolutionise our understanding of food webs. There is an urgent need to address species interactions, particularly with the rapid habitat degradation occurring in some of the most biodiverse habitats on Earth.
This thesis investigates the ecology of a large and relatively under-studied insect Order, the Orthoptera. The challenge of studying the interactions of these insects and their host plants in natural environments is addressed by examining the use of DNA barcoding and metabarcoding. Orthoptera were sampled in a range of habitat types in sub-tropical southeast Queensland, Australia, and tropical Sabah, Malaysian Borneo, so that community and dietary analyses could be carried out. I addressed the following aims: 1) to compare the utility of simple DNA barcoding and metabarcoding for herbivorous insect food web analysis, 2) to assess dietary breadth and overlap among different orthopteran species sharing the same habitat type, in an Australian subtropical eucalypt woodland, 3) to describe the effects on the composition of orthopteran assemblages of landscape-level habitat change (logging of primary forests and conversion to oil palm (Elaeis guineensis) plantations) in the southeast Asian tropics, 4) to compare the diet of one group of orthopterans (the Tetrigidae) across different habitat components within anthropogenically modified Malaysian tropical rainforest.
DNA barcoding using Sanger sequencing and DNA metabarcoding using an Illumina Miseq were trialled to determine how appropriate they were for the use of gut content analysis using the orthopteran specimens collected from southeast Queensland. The use of two plant gene markers was also assessed. From these results, the diets of two abundant species Merrinella sp. and Methiolopsis sp., occupying the same habitat, have been described and their dietary overlap measured. Orthoptera were also collected from primary rainforest, logged forest and oil palm plantations in Sabah, Malaysia, in order to compare the assemblages across the landscape. Additionally, dietary analysis using DNA metabarcoding was carried out on one family, Tetrigidae, to assess dietary differences with anthropogenic habitat change.
My research presents detailed methodology for the use of two DNA barcoding methods for measuring herbivorous insect diets. Our results found that although the Sanger sequencing method is useful for single species (unmixed) samples, the metabarcoding approach is more appropriate when sampling species with unknown dietary breadths. There remain limitations within this field in terms of accuracy of taxonomic identifications, but nevertheless, DNA metabarcoding is a useful method that is changing our understanding of food web ecology.
Using DNA metabarcoding, we confirmed that dietary overlap was low across Methiolopsis sp. and Merrinella sp. but also that they both had very broad diets. These results suggest that competition for resources between these two species is low. This could change, however, if plant availability is altered in response to environmental change.
Orthoptera assemblages were affected by habitat degradation (through logging) and the conversion of forest to oil palm across a landscape in Sabah, Malaysia. The greatest difference in species composition was between plantations and primary forests. Logged forests had the lowest species richness of the three habitats. The assemblage of species in palm plantations was clearly different from those elsewhere. These findings support other studies on different arthropod taxa and conclude that the plantation habitats are unsuitable for many forest-dwelling species. Additionally, diets of the Tetrigidae differed across these habitat types, with species in plantations feeding on a much higher proportion of mosses than those in forests.
Overall, this thesis describes and tests molecular methodology that can be used in future food web studies to measure dietary composition, breadth and overlap of insects. I show how habitat degradation can impact orthopteran community composition and highlight that Tetrigidae are an important family in tropical habitats. My research has addressed a gap in our understanding of the community and feeding ecology of Orthoptera and can be applied to other taxa to monitor responses of biodiversity in a changing world.||