Arachnid ecology in New Zealand, exploring unknown and poorly understood factors.
Spiders (Araneae) are an important ecological component of most terrestrial ecosystems. Comparatively, New Zealand spider ecology is poorly understood, particularly with regards to how spiders have been affected by the influx of new organisms entering an otherwise isolated habitat. This thesis aimed to observe the impact of vegetation structural complexity and mammalian predation on coastal Otago spider abundance and diversity, focusing in particular on the seasonal loss of vegetation complexity on introduced deciduous trees and the impact of introduced rodents. In addition, in order to non-destructively sample spiders in a predator-free ecosanctuary, a novel no-kill trap was designed and tested.The native New Zealand green mistletoe (Ileostylus micranthus) provides a continuous area of evergreen foliage on introduced deciduous trees throughout the winter. Recording invertebrate numbers from mistletoe samples and comparing them to host plant samples provided insight into how invertebrate populations react to loss of structural complexity on introduced deciduous trees during leaf fall. There was no significant difference between mistletoe and host plant invertebrate populations during the summer, but during the winter, invertebrate numbers on deciduous hosts were significantly lower in comparison to evergreen hosts and to mistletoes on either plant host type. A novel, non-kill spider trap was developed to assist with sampling in environmentally sensitive areas, such as ecosanctuaries. Dubbed the Sanctum Aranearum, the Sanctum utilizes structural complexity to attract spiders and other invertebrates to settle within the trap, which can then be readily collected without killing by-catch. The Sanctum was constructed from a white cuboid bucket with windows cut near the surface to allow invertebrates to enter. The internal structure included large and small plastic tubes and wire imbedded in sand. The Sanctum was tested against pitfall traps as a close analogue to compare capture results and found that the Sanctum was able to capture a higher abundance of invertebrates and a greater diversity of arachnids. The impact of predation by introduced rodents was assessed by sampling spiders at the Orokonui Ecosanctuary, near Dunedin, Otago, while it underwent a rodent elimination program. This elimination programme provided an optimal chance to observe how spider populations would be affected by the removal of rodents from a habitat. Sancta were utilized due to the sensitive nature of the ecosanctuary. A nearby Queen Elizabeth II National Trust conservation area with comparable vegetation was used as a control site to compare spider populations, as rodents were known to be present. Analyses were unable to separate the impact of rodents from potential seasonal effects on spider populations. It was noted that larger native spiders were more abundant in Orokonui than at the control site, but lack of historical data made any conclusions purely speculative. This thesis has made a novel contribution in terms of understanding the role of mistletoes as over-winter refugia for invertebrates. The findings in Chapter 2 have a multitude of implications regarding how urban and agricultural invertebrate populations survive winter with the introduction and proliferation of many deciduous plants to New Zealand. Both beneficial and pest species could potentially utilize mistletoes and further research is required. The novel Sanctum Aranearum presented in Chapter 3 and successfully deployed in an ecosanctuary in Chapter 4 is a valuable addition to arachnological methods and further work to explore the potential field uses for it are already being planned.
Advisor: Lord, Janice; Barratt, Barbara
Degree Name: Master of Science
Degree Discipline: Botany
Publisher: University of Otago
Keywords: New Zealand; Araneae; Spider; ecology; mistletoe; ecosanctuary
Research Type: Thesis