^{Image by Ngā Manu}

Mammalian Predators

Evidence suggests predation and/or competition with non-native mammals, especially rats and pigs, may be key drivers responsible for native frog declines. The kiore (Rattus exulans) likely caused an initial decline in native frog populations and the extinction of three species before European arrival in New Zealand. Further introductions of non-native mammals have continued the decline in native frog populations. Now, Hamilton’s frog only survives naturally on mammal-free island refugees. It is somewhat difficult to evaluate the exact nature of the effect of non-native mammals on native frogs. Ship rats (Rattus rattus) and feral pigs (Sus domesticus) have been documented predating upon Hochstetter’s and Archey’s frogs in North Island forests. It’s also possible mice may prey on juvenile frogs but further research is needed.

Habitat Change

Historical declines may also be partially attributed to the destruction of forest habitats that followed the arrival of Polynesian and European settlers. As amphibians, Leiopelma frogs have very specific habitat requirements preferring pristine native ecosystems. In particular, Hochstetter’s frog continues to be threatened by localised destruction of stream habitat that occurs as a result of mining, forestry and farming practices. Non-native mammals such as pigs and goats can cause severe disturbance and degradation of habitat which may also affect native frogs.

^{Image by ©Kimball Chen}

^{Image by ©CISR}

Disease

Chytrid fungus (Batrachochytrium dendrobatidis) has been implicated in the worldwide decline of numerous amphibian species. Frogs infected with this fungus suffer chytridiomycosis, a disease that affects their skin and is often fatal. Chytrid zoospores can survive in damp conditions. They may be transported between frog populations in muddy clothing, footwear or by the pet trade. However, it has never been detected in Hochstetter’s frog. It is assumed infection caused one population of Archey’s frog in the Coromandel Peninsula to decline in numbers by over 80%. Subsequent experimental infection of both Archey’s and Hamilton’s frogs showed these frogs produce antimicrobial peptides which inhibit chytrid growth; frogs did not show clinical symptoms. Because Leiopelma frogs seem to have a low susceptibility to chytrid fungus, it might not be as important of a conservation concern as it is for frogs globally.

The Native Frog Recovery Group is an advisory panel of native frog conservation and research experts. It is a mix of researchers and Department of Conservation staff. The group provides expert advice to the Department of Conservation about the recovery of our threatened native frog species. It meets monthly to discuss frog research and conservation priorities to progress towards achieving the goals of the Native Frog Recovery Plan 2013.

Captive populations

There are four captive populations of Leiopelma frogs in New Zealand. Captive populations play important roles as ambassadors for their species. Auckland Zoo has successfully captive bred Archey’s frog. Breeding programs can supplement struggling populations in the wild.

Translocations

Conservation translocations are a conservation management tool used to deliberately move animals to an area in which they have become locally extinct. All translocations require careful planning to guarantee the success of the translocated population. Conservationists must ensure factors that originally caused the local extinction have been removed or controlled. Habitat quality and the number of founding individuals are other important considerations. Ideally, translocations also require long term monitoring to assess the outcome and to determine the factors that have influenced success or failure.  

Translocations occur for a number of reasons. Fragmented populations are very vulnerable to extinction from a stochastic (random) event including: fire, disease or invasion by a non-native mammalian predators. Translocations have been used multiple times to mitigate such threats in both Archey’s and Hamilton’s frog. These began in 1984 with the translocation of 100 Hamilton’s frogs on Maud Island to more environmentally suitable habitat 500 m away from their original location at Boat Bay.  

Translocation can also be used to fulfil objectives of larger restoration projects. In 2006, 60 Maud Island frogs were translocated to Zealandia Ecosanctuary. Zealandia is a 252ha restoration project 2 km from Wellington City and is surrounded by an 8.6 km fence which excludes all introduced species of mammals except mice. This translocation is significant because it is the first translocation of Hamilton’s frog back to mainland New Zealand. 

^{Image by ©Oscar Thomas}

Disease management

The Department of Conservation and frog researchers work hard to reduce the threat of infection by chytrid fungus. Strict biosecurity rules aim to prevent the spread of chytrid fungus and frog populations are regularly monitored for infection. All people visiting areas with frog populations adhere to strict hygiene protocols including not taking packs, bags or gaiters into frog areas, and treating clothing, footwear and research equipment with a biocide known to kill the fungus.

Control of predators

The Department of Conservation runs a predator control programme to protect the Archey’s frog population at Whareorino incorporating both aerial and ground based rat control. Archey’s frog populations within the treatment area are increasing; adult frogs show increased survival and breeding success. Other populations benefit from landscape scale predator control aimed at other species, often birds. Hochstetter’s populations in the predator-free ecosanctuary at Maungatautari are recovering rapidly.

^{Image by ©Oscar Thomas}

^{Image by Department of Conservation}

Population monitoring

Across New Zealand ‘capture-recapture’ studies allow conservationists and scientists to understand trends in native frog populations and their response to conservation interventions including translocation and pest control. Dr Ben Bell and his colleagues from the University of Victoria started long-term population monitoring for Hamilton’s frog on Maud Island in 1976 and Archey’s frog in the Coromandel in 1982. These ongoing studies have revealed important insights into population trends and the life cycles of our native frogs. Native frogs can take up to 7 years to reach breeding maturity and live up to 45 years!