Leiopelma hochstetteri Fitzinger 1861 (Anura: Leiopelmatidae) habitat ecology in the Waitakere Ranges, New Zealand

Abstract

Declines and extinctions of amphibian populations are a global dilemma with complex local causes, which should be viewed in the context of a much larger biodiversity crisis. As other animal groups, amphibians with restricted distributions, such as island endemics, are thought to be more vulnerable to environmental change and susceptible to population declines. In the New Zealand archipelago, the only four native species of frogs (Leiopelma hochstetteri, L. archeyi, L. hamiltoni and L. pakeka) are classified as threatened. In particular Leiopelma hochstetteri, the most widespread and abundant endemic frog species in New Zealand, now survives only in spatially fragmented populations as a result of direct or indirect human activity. Hence, it is recognised as threatened and fully protected by legislation. In the last fifty years, some L. hochstetteri populations have been studied, providing descriptive information, which may be used to assess the current status (increasing, stable or declining) of previously or never monitored populations. This thesis examines the diet and trophic level, the effects ship rats (Rattus rattus) as well as the distribution and abundance of L. hochstetteri on a habitat-use context, to provide a basis for evaluating conceivable decline-agents, and to establish a platform to design directed conservation strategies. The Waitakere Ranges are considered a Leiopelma hochstetteri conservation management unit, on which L. hochstetteri has been previously studied. This area consists of a series of hills that run roughly north–south, which are mostly covered in regenerating indigenous vegetation. Today, 60% of the Waitakere Ranges fall within a Regional Park, which together with its surrounding residential areas is afforded protection to minimise the effects of development on the region. The accessibility and conservation character of this area makes it an ideal area for the study of L. hochstetteri populations. As a first step to characterise the diet and trophic level of L. hochstetteri within streams in the Waitakere Ranges, Auckland, stable carbon and nitrogen isotope analyses were undertaken on a variety of sympatric terrestrial and aquatic plant and animal species, including adult frogs. These results showed that: 1) aquatic and terrestrial food webs were linked by terrestrial inputs into the stream; 2) invertebrate and vertebrate predators separated well into distinct trophic groups; and 3) L. hochstetteri occupied an intermediate trophic position among predators, with a diet, at least as an adult, comprising terrestrial invertebrates. Shortfin eels and banded kokopu were identified as potential predators of L. hochstetteri, but data for rats were inconclusive. The inconclusiveness of these trophic studies, with regard to the effects of ship rats on L. hochstetteri populations, lead me to evaluate the influence of a seven-year ship rat management operation on frog abundance. To achieve a reliable evaluation, the habitat characteristics that had significant influence on frog abundance were identified. Then, it was confirmed that the study areas represented similar habitats in terms of those variables, and finally the effect of the pest-management activities was evaluated. Presence/absence of pest-management operations did not have a significant effect on frog abundance. These results, together with the results of the diet and trophic level analyses, suggested that ship rats do not represent a significant threat for this frog species, at least in the Waitakere Ranges. The results of distribution and abundance investigations indicated that in the Waitakere Ranges frogs are currently widely distributed, relatively abundant and that recruitment has occurred at least in the last ten years. Additionally, in order to identify associations between habitat characteristics and frog distribution and abundance, reliable and specifically designed monitoring methodologies were developed. Although this frog is known to occur in wet areas adjacent to shaded streams in forested catchments, quantitative ecological data previously did not exist to enable characterisation of its habitat. Here, novel data were reported on the current distribution and habitat requirements of this species in the Waitakere Ranges. Statistical modelling demonstrates frogs most likely occur in small, erosive streams with coarse substrates and cold waters, surrounded by mature or undisturbed riparian vegetation, where higher abundances of frogs may be found in steep areas with stable patches of cobbles and boulders lying against larger stream bed elements within the stream channel. Anthropogenic activities, such as clearing or logging, and upstream disturbances that potentially increase silt input into streams were identified as threats to these frog species. Finally, the habitat-use information gathered during this investigation was utilised to develop a spatial decision support system (SDSS) as a tool to assess the quality and quantity of habitat available to L. hochstetteri populations associated with the Auckland Region. These results have important implications for the conservation of New Zealand native frog species and riparian stream habitat.