Status of Conservation and Decline of Amphibians

Australia, New Zealand, and Pacific Islands

Harold Heatwole, Jodi J.L. Rowley, Harold Heatwole, Jodi J.L. Rowley, Jodi Rowley

• Publisher: CSIRO PUBLISHING
• Year: 2018
• Print ISBN: 9781486308385
• eBook ISBN: 9781486308408
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Amphibians are among the most threatened groups of animals on earth. In part due to their highly permeable skin, amphibians are highly sensitive to environmental changes and pollution and provide an early-warning system of deteriorating environmental conditions. The more we learn about the impact of environmental changes on amphibians, the better we as humans will be able to arrest their demise, and our own.

Status of Conservation and Decline of Amphibians brings together the current knowledge on the status of the unique frogs of Australia, New Zealand, and the Pacific. Although geographically proximate, each region presents unique challenges and opportunities in amphibian research and conservation. This book contributes to an understanding of the current conservation status of the amphibians of each region, aims to stimulate research into halting amphibian declines, and provides a better foundation for making conservation decisions. It is an invaluable reference for environmental and governmental agencies, researchers, policy-makers involved with biodiversity conservation, and the interested public.

1 Introduction

Harold Heatwole and Jodi J. L. Rowley

HISTORY OF THE ENVIRONMENTS AND BIOTA

Australia

Australia’s tectonic plate separated from the Antarctic remnant of Gondwanaland sometime before 55 million years ago and drifted northwards to abut on the Asian plate toward the end of the Oligocene about 25 million years ago (Hall 2002, 2009). During the more than 30 million years for that traverse to occur, the continent was increasingly isolated from floral and faunal exchange with other continents, except possibility for exceedingly rare overwater dispersal of waifs. Amphibians, being highly sensitive to salt water, are poor subjects for such long-distance, overwater dispersal and consequently the present batrachofauna of Australia either originated in Gondwanaland in ancient times and adaptively radiated during the long voyage northward, or arrived much more recently after the Australian and Asian plates collided (Tyler and Lee 2006). Ranidae clearly belongs to the latter category because Papurana damaeli, the sole representative in Australia of this otherwise widespread family, only occurs on Cape York Peninsula and in eastern Arnhem Land. The Australian microhylids also may have originated from an Asian source (see Chapter 2), but the remaining taxa of Australian frogs probably descended from ‘hitchhikers’ aboard ‘Ark Australia’ during its northward journey from Gondwanaland.

During this long passage, the Australian climate and vegetation varied dramatically, with an overall trend toward increasing aridity and an adaptation of some frogs to dry conditions (see Chapter 2). This trend was not a straight-line progression, however, because there were reversals superimposed on overall drying (Fujioka and Chappell 2010). It is likely that there was a pulsation of expanding aridity outward from the centre, leaving coastal pockets of moist habitat and divergence of the isolated populations contained in them, followed by a retreat of aridity, with humid regions reconnecting and the ranges of the newly formed species expanding geographically (Chapter 1 in Heatwole and Taylor 1987). The repeated cycles of speciation of isolated populations followed by expansions of range, led to radiation into a wide variety of habitats and to a few species with unusual adaptations to aridity (see Chapter 2).

The topography of Australia is one of generally low relief, with the Great Dividing Range skewed toward the eastern coast; the climatic pattern is of a monsoonal tropical north, a moist eastern and south-western temperate periphery, and an arid (desertified) core. These topographic and climatic features are reflected in the zonation of vegetation underlying the topical organisation of chapters in this book.

New Zealand

About 83 million years ago, a fragment of continental crust about the size of modern India rotated away from the Australian section of Gondwanaland to form a land mass known as Zealandia (Goldberg et al. 2008). Subsequently, the rift separating Zealandia from Australia widened to form the Tasman Sea and, over about 60 million years, Zealandia thinned and sank until only a much smaller remnant, the present New Zealand, remained above water. A boundary collision of the plate in the late Oligocene gave rise to topographic upheaval to form the Southern Alps.

New Zealand’s biota consists of a mixture of ancient Gondwanan elements and more recent arrivals (Goldberg et al. 2008). In line with their physiological characteristics, frogs do not disperse over salt water easily and, not surprisingly, New Zealand’s frogs are of Gondwanan origin. In fact, they are among the most primitive of frogs, having diverged from more modern taxa in about the mid-Triassic (see Chapter 13). All are endemic to New Zealand and, in keeping with the mesic climate there, are denizens of forests.

The oceanic islands of the Pacific

Oceanic islands are those that arise de novo from the sea without any present or previous direct connection to a continent or isolated continental fragment (such as New Zealand). Chapter 14 and Neall and Trewick (2008) summarise the tectonic history of the Pacific Basin and the islands lying within it, and that will not be detailed here. The salient point is that most oceanic islands of the Pacific are devoid of amphibians, probably because of their great distances from a continental source of terrestrial fauna and the difficulty with which amphibians disperse across salty water. Only three island-groups near the periphery of the Pacific, and hence nearer sources of continental faunas, have native frogs – the Solomon Islands, Palau, and Fiji – although some others have species introduced by humans (see Chapter 14).

THE GLOBAL CONTEXT OF THE DEMISE OF AMPHIBIANS

Throughout geologic history there have been periodic mass extinctions in which the biota of the Earth was reduced cataclysmically (McElwain and Punyasena 2007). The causes of these events have been varied: one perhaps being due to changing patterns of oceanic currents and climatic disruptions resulting from the separation of the supercontinent Pangaea into two, and eventually all seven, of the present continents. The most famous mass extinction, however, was the one marking the transition from the Mesozoic Era to the Cenozoic about 66 million years ago, attributed to the collision of a comet with the Earth, and highlighted by the precipitous disappearance of dinosaurs as well as many other taxa (Alvarez et al. 1980). The eventual fate of all species, of course, is to disappear, either through dying out completely (extinction), or by loss of the ancestral form via its transition into new, and different, taxa (evolution, speciation). Previous mass extinctions were followed by a regenerative cycle as the surviving taxa adaptively radiated into ecological niches left vacant by the demise of victims of the extinction event. Recovery requires millions of years (McElwain and Punyasena 2007).

Currently, almost one-third of all amphibian species are threatened with extinction, making them the most threatened group of terrestrial vertebrates (IUCN 2017). Thirty-three amphibian species are officially listed as recently extinct, but this likely is a severe underestimate. The causes of amphibian declines and extinctions are multiple (Heatwole 2013) and form a complex network that is nearly intractable to solution by virtue of the labyrinth of a large number of interactive links (Plate 1.1). Amphibian decline is global, but is generated by different combinations of factors in different regions, although some causes are common and widespread geographically.

One of the biggest obstacles to halting the decline of amphibian species is a lack of knowledge. At the most basic level, we still do not know how many species of amphibians there are; in the past decade, an average of 157 new species of amphibians have been described annually (AmphibiaWeb 2017). This trend shows no sign of slowing. For the species of which we are aware, we often lack information necessary to carry out informed conservation. Indeed, 24% of all currently assessed amphibian species (84% total known amphibian species to date; AmphibiaWeb 2017) are so poorly understood that their conservation status cannot be determined (IUCN 2017).

The mass extinction now in progress differs from previous ones by virtue of its largely anthropogenic origins – we are the root of most of the causes. This aspect, however, does have a positive side: there is an intelligent, sentient species involved in the event. If we can cause declines that lead to extinction, perhaps we also can devise ways to prevent, or at least ameliorate, them. Consequently, this book has two goals: (1) to contribute to an understanding of this complex phenomenon; and (2) to stimulate research into finding solutions to prevent the occurrence of the worst-case scenario. A lot of research toward these ends already has taken place and has been reviewed (see Preface); the material presented here is by way of application specifically to Australia, New Zealand, and the Pacific Islands.

With such a complex set of causes as illustrated in Plate 1.1, the tracing and quantifying of the links in the network of interactions is a formidable task. We must be vigilant to avoid facile oversimplifications. Too often, once a single correlation has been established, it is accepted as denoting a cause and effect relationship, with that conclusion becoming widely accepted and without delving into the possible intervention of other factors. The oversimplified version may then be enshrined as truth for all occasions in the lexicon of scientific mythology, and the complete suite of reasons bypassed by future investigators. Enigmas are important because they help identify such situations and allow greater focus on reconciling apparent contradictions. For example, Lane and Burgin (2008) reported that at lower elevations in the Greater Sydney region the diversity of amphibians was lower in more urbanised and polluted sites than in natural, less polluted ones, as one would expect, but that at higher elevations, the reverse occurred. In view of the complexity of possible interactions illustrated in Plate 1.1, paradoxes like this are important and should be followed up for verification and further scrutiny, rather than merely dismissed as aberrant. Resolution of such mysteries may lead to a far better understanding than could be achieved merely by unduly emphasising the more predicable outcome.

A common misconception that also tends to disguise the dynamics of natural systems is an optimistic view of their regenerative capacity. It is true that biological systems often exhibit fluctuations around some mean level, and that departures from that level bring into play forces tending to return the system toward the mean, whether the departure is above the mean or below it. The mechanisms directing such returns toward a stable equilibrium collectively are known as negative feedback, and the entire process of oscillation around a set value is called homeostasis. Regulation of body temperature in mammals is an example of such a system. When the body begins to get too hot, certain automatic responses, such as panting, sweating, or seeking shade, occur that tend to cool the body. When the body begins to cool below the equilibrium level, other responses, such as shivering, come into play and raise the body back toward the ‘normal’ level. Such mechanisms are frequent in biological systems and operate at various levels, ranging from the physiology of an individual (as in the above example) to the dynamics of biotic communities and ecosystems. This generality has inspired unwarranted optimism that homeostasis will operate consistently and that, when disturbed, ‘nature’ will automatically restore itself to its original condition, merely if left alone. That is not always the case and the more severe the disturbance, the more likely it is that some non-equilibrium state will prevail or that a new, less desirable, equilibrium will be reached (Rohde et al. 2013). In the worst-case scenario, an irretrievable situation, such as extinction, may alter the equilibrium. For example, amphibian population declines in Central America in the 1990s has resulted in large-scale ecosystem-level effects in stream habitats that persist today (Whiles et al. 2006, 2013). There are limits to homeostasis.

AMPHIBIANS IN AUSTRALIA, NEW ZEALAND, AND THE PACIFIC ISLANDS

Australia has a relatively well-known frog fauna but much remains unknown, even with respect to the true number of species. There are currently 240 native species of frogs known from Australia (see Chapter 2 for a summary of the diversity, distributions, and conservation status), but in the past decade alone, 21 new species, representing 9% of Australia’s known frog fauna, have been discovered. Most of these species were hidden within known ‘species’ that were found to be complexes of multiple, morphologically similar species (e.g. Mahony et al. 2006; Anstis et al. 2016; McDonald et al. 2016), but others have been discovered as the result of surveys in remote or previously unsurveyed areas (e.g. Hoskin and Aland 2011). The discovery of previously undescribed species of frogs in Australia is ongoing, particularly in tropical northern regions. The frog fauna of the Pacific Islands is considerably less well known (see Chapter 14), with just over 30 known species and new species found on almost every expedition (e.g. the Solomons). New Zealand’s native frog fauna currently consists of four species belonging to a single relatively ancient lineage (see Chapter 13).

Although geographically proximate, each region presents unique challenges and opportunities in amphibian research and conser...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Dedication
• Contents of Previous Parts of Volume 11
• Contributors to Part 6
• Preface
• Chapter 1 (68). Introduction
• Chapter 2 (69). A Brief Demographic Overview of Australia’s Native Amphibians
• Chapter 3 (70). Status of Decline and Conservation of Frogs in the Wet Tropics of Australia
• Chapter 4 (71). Frogs of the Monsoon Tropical Savannah Regions of Northern Australia
• Chapter 5 (72). An Update on Frog Declines from the Forests of Subtropical Eastern Australia
• Chapter 6 (73). Frog Declines and Associated Management Response in South-eastern Mainland Australia and Tasmania
• Chapter 7 (74). The Status of Decline and Conservation of Frogs in Temperate Coastal South-eastern Australia
• Chapter 8 (75). Conservation of Frogs in South-western Australia
• Chapter 9 (76). The Status of Decline and Conservation of Frogs in the Arid and Semi-arid Zones of Australia
• Chapter 10 (77). The Impact of an Invasive Amphibian: The Cane Toad Rhinella marina
• Chapter 11 (78). The Role of Ex-Situ Amphibian Conservation in Australia
• Chapter 12 (79). Conservation of Frogs in Australia: State and Federal Laws
• Plates
• Chapter 13 (80). Status of Decline and Conservation of Frogs in New Zealand
• Chapter 14 (81). Amphibians of the Pacific: Natural History and Conservation
• Index