Abstract
Habitat fragmentation and the introduction of non-native mammalian predators have caused many endemic New Zealand invertebrates to become vulnerable to extinction. However, many of these taxa have been critically understudied, and little can be done to conserve these species without detailed information about their behaviour and life histories. One such group is the stag beetle genus Geodorcus. Like most stag beetle genera, Geodorcus has striking sexual dimorphism, where the males have heavily exaggerated mandibles that they use in contests over mating opportunities. Most of the species in Geodorcus have heavily restricted distributions and are vulnerable to habitat destruction, illegal collection and non-native predators. In response, Geodorcus is considered critically threatened and is protected under Schedule 7 of the Wildlife Act of 1953. The only widespread species in this genus is the Helm’s stag beetle, Geodorcus helmsi. The research in this thesis is limited to G. helmsi, but this research may also be useful as a model, to help understand and conserve the more vulnerable species.
In Chapter 2, I used linear and geometric morphometric analyses on wild populations and museum specimens from Stewart Island to assess whether the males of G. helmsi have alternative male morphs. In Chapter 3, I used traditional and contemporary morphometric analyses to determine whether there are morphological differences between different populations of G. helmsi. In Chapter 4, I assessed the effect that rats have had on a population of G. helmsi, by comparing mandible lengths and head widths of the remains of preyed upon specimens to live specimens in a natural experiment. In Chapter 5, I identified the type and density of sensilla on the legs, maxillary palps and all antennae on male and female G. helmsi with a scanning electron microscope, to determine whether either sex uses pheromones to attract conspecifics. Finally, in Chapter 6, I attempted to determine what factors affect the outcome of competitive interactions between male G. helmsi and whether they use any assessment strategies during competitive interactions.
The culmination of all chapters in this thesis allows us to make five broad conclusions. (1) Within a single population, there is no evidence for alternative male morphs, but a gradual shift between smaller and larger individuals. (2) The morphology of G. helmsi subtly shifts across different populations in line with what is found in genetic analyses of this species. (3) Rats have had a profound effect on the sizes of beetles that can be found in wild populations on Stewart Island. Larger males appear to be more affected than smaller males in areas with predators, causing a potential limit to the size beetles can grow without being a detriment to survival. (4) There is evidence from SEM examinations that indicates that chemical communication is used by both sexes and that males are more perceptive to chemical stimuli. (5) Larger males are more likely to win in aggressive interactions, but no assessment strategy could be identified.
This research sheds new light on various aspects of the ecology and life history of G. helmsi. I was able to see that G. helmsi is a good candidate to explore the effects of limited gene flow in an increasingly fragmented distribution. Finally, these results show that the various threats (non-native predators, illegal collection, etc.) to G. helmsi can very likely be extended to the other species of flightless Lucanidae.