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

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. CHAPTER 1. General introduction 1.1 BIODIVERSITY CRISIS AND CONSERVATION Biodiversity connotes the richness and variety of life on Earth. During the United Nations Conference on Environment and Development in Rio de Janeiro in 1992, this variety of life was distinguished at three different levels: genetic variation, the number of species and the variety of ecosystems (Selvik 2004). The concept of biodiversity is used widely, and its relatively rapid establishment within science and popular science is an indication that it is considered an important issue. Indeed, biodiversity is the very basis of human survival and economic well-being, and encompasses all life forms, ecosystems and ecological processes (Singh 2002). Species extinctions are, of course, perfectly natural. All species begin in some restricted setting and then spread; most subsequently undergo differentiation, and eventually all species come to an end (Levin and Donald 2002). The diversity of species at any point in time is simply the result of these ongoing processes, which can wax and wane in intensity (Singh 2002). As long as the rate of speciation equals or exceeds the rate of extinction, biodiversity will remain constant or increase. In past geological periods the loss of existing species was eventually balanced or exceeded by the evolution of new species. However, the current losses are exceptional— 100 to 1000 times those of past rates (Primack 2004; Brooks et al. 2006). This recent episode of extinction is due almost exclusively to human activities (Leakey & Lewin 1996; Lövei 2001). The major threats to biological diversity are habitat destruction, habitat fragmentation, habitat degradation (including pollution), global climate change, overexploitation of species, invasion of exotic species, increased spread of disease, and synergisms among these factors (Melchias 2001; Primack 2004). These seven threats to biodiversity are all caused by an expanding human population’s ever-increasing use of the world’s natural resources (Singh 2002; Primack 2004; Brooks et al. 2006; Chivian & Bernstein 2008). Assuming no radical transformation in human behaviour, we can expect important changes in biodiversity and ecosystem services in the next 40 years (Jenkins 2003). Although, the impact of species deletions on ecosystem function and stability is still a subject of debate among ecologists (Singh 2002: Jenkins 2003), this does not mean, of course, that we can continue to manipulate or abuse the planet ad infinitum. In fact the processes driving extinction are eroding the environment services on which humanity depends (Millennium Ecosystem Assessment 2005). The later suggests that it is wise to take a precautionary approach and make serious attempts towards the conservation of biodiversity. The leading response to the biodiversity crisis since the late 19 century has been the creation of protected areas (Adams et al. 2004). However, other basic strategies for biodiversity conservation include ex situ strategies, such as the establishment of botanical and zoological gardens, reduction of anthropogenic pressure on natural populations by cultivating them elsewhere, and rehabilitation of threatened species through a variety of strategies (Singh 2002). Popular interest in protecting the world’s biodiversity has intensified during the last few decades (Caughley & Gunn 1996). The desire of the scientific community to face this challenge has prompted the development of conservation biology as an integrated scientific discipline. Conservation biology represents a synthesis of many basic sciences, such as community ecology, biogeography, environmental law; that provide principles and new approaches for the applied fields of resource management, such as agriculture, management of natural areas and fisheries management (Wilson 1992; Primack 2004). In New Zealand there is an ongoing biodiversity decline (Craig et al. 2000). New Zealand was one of the last places on earth to be settled by humans and since the settlement around 1000 years ago, humans and their accompanying pests have caused the extinction of several animal species (Ministry for the Environment 2002). For example, the flightless avifauna of at least 38 species has been reduced in a few centuries to nine, most of which are currently endangered (Jenkins 2003). Three of seven species of frogs have become extinct (Towns & Daugherty 1994; Bell 1994) and the remaining species have suffered drastic reductions in their distribution and population size (Bell et al. 2004a). Rain forest have been reduced from an original 78% of land area to approximately 23% (Ministry for the Environment 1997), and land use change analysis shows a continued loss of nearly 175 km of indigenous habitat betwee