Twenty-two years ago, research presented at the World Congress of Herpetology Conference sent shock waves through the amphibian community. Drastic declines and extinctions were predicted globally, and this has rung true for many species, including the Leiopelmatid frogs of Aotearoa, New Zealand.
Eighty million years ago Zealandia broke away from the supercontinent Gondwana. It then took a further 20 million years of tectonic movements and volcanic activity to form the landmass we now regard as Aotearoa. Having been the last major archipelago of islands to be colonised by humans, this allowed for an extended period of evolution of plant and animal species in the absence of terrestrial mammalian predators. Imagine giant insects, ancient reptiles, and flightless birds thriving in a pre-human world.
Often referred to as “living fossils”, the Leiopelmatid frogs are among this “archaic” fauna that originated from Gondwana. Having changed little in over 150 million years, Leiopelmatid frogs are among the most unique amphibians in the world and survived when most other creatures disappeared. Extant species of Leiopelma include three terrestrial and morphologically similar taxa: Archey’s Frog (Leiopelma Archeyi), Hamilton’s Frog (L. hamiltoni) and Maud Island Frog (L. pakeka), and one semi[1]aquatic species, Hochstetter’s Frog (L. hochstetteri). Leiopelma species are small, with snout-vent-length (SVL) ranging from up to 40 mm for the smallest species, Archey’s Frog, to 51 mm for Hamilton’s Frog. They are carnivorous, eating a range of invertebrates such as beetles and mites and all four Leiopelma species are generally nocturnal. But what actually makes Leiopelma so unique? Or as Professor Phil Bishop, a New Zealand frog expert, has said, “so weird?”
While many of us have watched tadpoles swimming around in backyard ponds, eagerly awaiting the morning chorus, Leiopelmatid frogs sat silently in the hills above. Endemic Aotearoa frogs lack external eardrums and vocal sacks that are characteristic of modern frogs, with Leiopelma species not known to vocalize in a social context. And forget about watching tadpoles, Leiopelma instead hatch as tailed froglets, the size of your pinkie nail. So why are our frogs, our taonga (treasured), in trouble? Biota in Aotearoa was extensively altered after human colonisation. The arrival of Polynesians between 1280-1300 AD led to the burning of forests, introduction of kiore (Rattus exulans), and the exploitation of large birds, such as moa. European settlement followed around 350 years later and accelerated the trend of native species decline by habitat loss/ disturbance, the introduction of exotic plants and animals, and disease. During this period, three Leiopelma species became extinct.
The highest proportion of threatened native species in the world can be found in Aotearoa. One would not find a single New Zealander who would dispute the devastating contribution introduced mammalians have had to our fauna and flora. Prior to the arrival of humans, the only land mammals in New Zealand were small bats. Animal groups therefore evolved to become functional equivalents of mammals or, like the frogs, evolved traits such as longevity, low reproduction rates and freeze tactics. Our frogs also have cryptic colouration and lack vocalisation that could have helped them evade native predators such as the extinct Laughing Owl (Sceloglaux albifacies). However, these features provide limited protection against introduced mammalian species with highly developed olfactory receptors such as rats (Rattus spp.) and pigs (Sus scrofa).
To protect frogs against mammalian predation, Maud Island frogs and Hamilton’s frogs are restricted to predator-free o f-shore islands. Maud Island Frogs are located on islands in the Marlborough Sounds. Stephens Island, where a population of Hamilton’s Frog inhibits a small area on the 150 hectare island, is in the Cook Strait. On the other hand, Archey’s and Hochstetter’s Frogs co-occur with mammalian predators on the mid-upper North Island and these mainland frog populations seem to be in a losing battle against mammalian predators. The remains of Archey’s Frogs in Whareorino have displayed bite characteristics of rats and anecdotal accounts suggest pig predation. The Department of Conservation (DOC) are monitoring areas with varying rat control to determine the effect rodents have on frog populations and how we might combat these effects.
Working as an ecologist, I am regularly told by the public “…back in my day the place was crawling with them”, in reference to any native species. More often than not, they now point out the introduced Australian herpetofauna, praying that these croaking frogs and shiny skinks are our beautiful natives. Their disappointing looks and frowned muttering of the word “Australian” might sum up the competitive streak between the two countries more than their awareness of the mysterious Aotearoa herpetofauna.
The Archey’s Frog is the world’s most Evolutionary Distinct and Globally Endangered (EDGE) amphibian species. That is a big title to hold for a tiny New Zealand frog. But Archey’s Frogs are not alone in their concerning threat status. Maud Island Frog and Hamilton’s Frog are classified as ‘Threatened – Nationally Vulnerable’ and Hochstetter’s frog as ‘At Risk- Declining’ by the New Zealand Threat Classification System. Mammalian predation is not only to blame, but so is disease and habitat modification, as according to the New Zealand Native Frog Recovery Plan 2013-2018. With so few individuals left, the majority of New Zealanders will never see these weird and wonderful creatures. How can we convince the public to fight for something they may never see?
“The main problem is we are destroying their homes”, comments Professor Phil Bishop. Habitat disturbance and destruction continues to be a major player in species decline, with forested cover now remaining at 26% in Aotearoa. Just over 50% of the land area has now been modified for human activities, such as urbanisation, agricultural production and exotic forestry plantations. The distribution of these areas reflects the economic value of the land, with protected areas skewed towards hilly, mountainous landscapes that are less desirable for agricultural and forestry practises but may not be ideal for all native species to seek refuge. Habitat modification impacts microclimates and ecosystem processes that are essential for amphibian survival and reproduction. Leiopelma are particularly vulnerable because they occupy discrete home ranges and depend on particular microclimates to live. They are also a relatively immobile species and therefore cannot easily relocate when their home ranges are adversely affected (one frog has been recorded by Professor Ben Bell as remaining in the same survey area for 37 years!). Habitat loss has also contributed to the fragmentation of populations of Hochstetter’s frog on the mainland, with isolation shown to cause smaller, cytogenetically distinct populations that are at risk of potential inbreeding.
Leiopelma populations have also been impacted by disease. Chytridiomycosis (disease caused by Chytrid fungus) is suspected to have contributed to a population crash in Archey’s Frogs on the Coromandel Peninsula between 1996- 2001. Interestingly, Chytrid fungus was eliminated by Archey’s Frogs in clinical trials. However, we don’t know the percentage of the wild population able to eliminate Chytrid, as is the effect the disease could have on other Leiopelma species. As Professor Ben Bell and others have put it, “…we understand so little of the pathology and epidemiology of any disease that might be affecting Archey’s Frogs…” (Bell et al. 2004). Management, therefore, is difficult to implement.
These population declines for Leiopelma can have huge implications for the overall survival of the species due to the current isolation and size of populations. Additionally, the slow maturity (3-4 years) of the species and their reliance on environmental cues, such as high humidity and rainfall, and low clutch sizes (2- 19 eggs), can result in lower lifetime productivity, which is exacerbated by in high predation or stressful environments. Aotearoa frogs are protected however under the Wildlife Act, and may not be captured, collected or disturbed without permission from DOC. DOC, Auckland Zoo and researchers annually monitor native frogs to determine survivorship of populations. Current research on frogs includes reactions to introduced predators, frog movement and behaviour, monitoring methodology and habitat use.
With the growing human population in New Zealand, climate change, and continued mammalian predation, ongoing research and conservation is required to ensure management of existing and future Leiopelma populations. This is a difficult task. The implementation of Predator free 2050 (eradication of rats, possums and mustelids by 2050) does give hope for the future and helps to engage the public in the protection of our threatened wildlife. Predator-free islands have also been a saving grace. As cliché as it might sound, working together towards common goals will increase the effectiveness of conservation in Aotearoa.
A huge hole has been left in the amphibian world with the passing of Professor Phil Bishop – an outstanding researcher and frog enthusiast who dedicated his life to amphibian conservation in Aotearoa. All hope is not lost for our frogs, however. In the wise words of Phil Bishop “…if we give Archey and his friends a helping hand, they will be around for the next millennium”. And with combined management and research efforts, I believe we will not allow our toanga to croak.
References
Bell B.D., Carver S., Mitchell N.J., & Pledger S. (2004). The recent decline of a New Zealand endemic: how and why did populations of Archey’s frog Leiopelma archeyi crash over 1996-2001? Biological Conservation, 120: 189-199
