The Diet and Nutritional Ecology of the Auckland Tree Wētā Hemideina thoracica
Brown, M. (2013). The Diet and Nutritional Ecology of the Auckland Tree Wētā Hemideina thoracica (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/8771
Permanent Research Commons link: http://hdl.handle.net/10289/8771
Insects are the main consumers of primary production in many terrestrial ecosystems, they are involved in the upwards migration of energy to higher trophic levels, and are often extremely important in structuring their ecosystems. Despite the many important ways that insects influence their ecosystems, many of the most basic aspects of the functional ecology of many groups of insects have not been investigated, and a great deal remains to be learned about what roles they play in the structure and functioning of their ecosystems. This study focuses on the Auckland tree wētā Hemideina thoracica (Orthoptera: Anostostomatidae, White, 1842), the most widespread species of tree wētā in the North Island of New Zealand, and one of the least studied species in the genus Hemideina. Faecal fragment analysis revealed that the diet of a population of Hemideina thoracica from a mixed podocarp/broadleaf forest was composed primarily of the leaves, fruits, and seeds, of the native plants Prumnopitys taxifolia, Podocarpus totara, Dacrycarpus dacrydioides, Kunzia ericoides, Melicytus sp., Pennantia corymbosa, and Coprosma rotundifolia, but the consumption of other invertebrates was also common, and formed a smaller component of the overall diet. Hemideina thoracica were also shown to feed selectively while foraging. It was determined that some plants were eaten readily by the wētā when encountered, while other plants that were equally abundant were either not consumed, or were consumed far less than would be predicted by their availability. Performing a solvent extraction, using hexane, revealed that the concentration of lipids and oils in the leaves of preferred plant species was higher than in the leaves of non-preferred species, and Hemideina thoracica may preferentially consume plants that have high lipid concentrations in their leaves. It also appeared probable that they forage on the forest floor more frequently than has been previously recognised. A nutritional analysis performed in captivity revealed that H. thoracica are capable of balancing their consumption of the macronutrients protein and carbohydrates to construct an optimal diet from one or more sub-optimal sources, and strongly regulate their consumption of protein. When presented with the opportunity to construct their own diet, they consumed significantly more carbohydrates than protein, and constructed a diet with a mean protein to carbohydrate ratio of 27:73. They digested carbohydrates more efficiently than protein, but converted the protein that they consumed into biomass with a very high level of conversion efficiency. These results supported the results of the fragment analysis by demonstrating that the natural, optimal diet of H. thoracica is most likely composed of carbohydrate rich foods, but they may also be naturally omnivorous, as they utilise protein very efficiently. The third, and final experiment, investigated whether H. thoracica feed on fruit in a manner that may facilitate seed dispersal for native plants in the wild. This experiment involved feeding fruit from three different native plants, Coprosma repens, Cordyline australis, and Fuchsia procumbens to a group of 40 H. thoracica, and revealed that H. thoracica frequently consume the flesh of the berry from around the seed, without ingesting the seeds themselves. When the seeds were consumed, 100 % of them were destroyed by the wētās digestive process, even in the case of very small and numerous seeds, like those of F. procumbens. It is therefore highly unlikely that H. thoracica is a seed dispersing plant-mutualist, as the seeds are either left where the fruit had fallen, or are destroyed. The combined results of these studies demonstrated that Hemideina thoracica is an omnivorous polyphage, and their wild diet is composed primarily of leaves, fruits, and seeds, and the concentration of lipids and oils in the leaves appears to be an important cue in determining the palatability of different species. They are also naturally omnivorous, and protein derived from eating other insects is a common component of their overall diet They have a well developed ability to balance their consumption of protein and carbohydrates, and self-select a carbohydrate rich diet in captivity. It is also extremely unlikely that that H. thoracica act as seed dispersers for native, fruit producing plants.
University of Waikato
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