An unFROGettable journey

The last of the captive population of pepeketua/Hamilton’s frogs have been released from their enclosure following the opening of a new boardwalk built to protect them from human foot traffic. This boardwalk was made possible through generous funding from John Nankervis's estate. Lead Ranger Ellen Irwin shares some of the mahi/work that went into planning for this release.

Most folks who visit Zealandia, particularly day visitors, may not get to see what is (in my humble opinion) one of Zealandia’s coolest residents, pepeketua/Hamilton’s frog. Formerly known as Maud Island frogs, they are one of three indigenous frog species in Aotearoa and are our rarest and largest native amphibian. Once widespread throughout the lower North and upper South Island, their numbers declined due to habitat loss and invasive predators. At one stage, only two small populations in the Marlborough Sounds remained.

I have always been a big fan of frogs. They are such incredible creatures: with their permeable skin, they are often used as environmental indicators of habitat quality and pollution—but they can also survive in a wide variety of environments, with some adapted to live through harsh winters, becoming partially frozen in the process!

However, the frogs of Aotearoa are in their own league of weird and wonderful. For example, our native pepeketua don’t have vocal sacks, and therefore don’t croak regularly (they can still make tiny chirps when alarmed). So if you hear froggy sounds, it’s likely to be an introduced species from Australia.

Hamilton’s frogs live amongst rock piles and leaf litter so have almost no webbing on their feet. They also don’t have a tadpole stage, preferring instead to lay eggs in the ground or trees. The males guard these eggs for several months until froglets hatch. They then carry them around on their backs until the froglets are fully developed—the ultimate dad piggyback.

How did these frogs make their way to Zealandia? Pepeketua from Te Pākeka/Maud Island and a captive population at Canterbury University were translocated here in 2005. They were released into three fully enclosed pens (the display pen on Te Māhanga Track and two larger off-track pens), as it wasn’t known at the time how the frogs would fare with kiwi and low levels of mice. In 2012, 100 more frogs were released outside one of the off-track pens within a ‘kiwi-proof fence’ to assess how they would do in the presence of mice.

In 2020, in kōrero/discussions with the Department of Conservation Frog Recovery group, we started looking into what we would need to have an entirely free-roaming population of pepeketua. There had been minimal long-term monitoring of the population inside the kiwi exclosure fence and almost none outside. A frog had been spotted along Te Māhanga Track, so we knew some frogs were living outside of the pens. We then began to gather information and plan how we might do a survey.

There are a few things that make Hamilton’s frogs tricky to monitor. They spend a lot of time nestled under leaf litter and rocks and don’t emerge every night. Emergence also varies depending on factors like rainfall, humidity, and temperature. The number of frogs detected also depends hugely on the observers. These frogs are experts at camouflaging, and it can be very easy to miss them.

We also had nothing to compare our surveys to. Most frog monitoring had taken place on islands, but as Zealandia has the only mainland population and the only population co-existing with mice and kiwi, we didn’t know what number of frogs might indicate a healthy population, but we were hoping to find some evidence that the frogs were surviving and reproducing in the presence of these predators.

Eventually, with a lot of help from many frog experts, we surveyed the frogs over several nights in May 2022. As expected, the number of frogs found each night varied, but we were hopping with excitement to find what seemed to be a ‘good’ number of frogs in all the areas! We even found gravid (full of eggs) females and tiny froglets in the wider sanctuary, outside the protective fences and enclosures, indicating that some individuals were surviving and breeding which was awesome news.

Over the next few years, the kōrero continued with both the Frog Recovery group and mana whenua, and eventually, it was decided that frogs could be released. However, given the population was in an area near a popular track, members of the Frog Recovery group recommended that we build a boardwalk spanning the ~150m of Te Māhanga Track where the majority of the pepeketua population was. While the frogs can climb trees*, they aren’t fantastic climbers. The boardwalk was built so frogs couldn’t climb it to keep them safe from people at night (e.g., visitors or researchers). We were also advised to move the captive frogs to a new nearby site suitable for frogs, within a kiwi-proof fence. While frogs could climb out, the fence would protect some of the small population.

And that brings us to today. As we completed the finishing touches on the project, I reflected on the long journey to get here. This has been a project nearly two decades in the making, involving dozens of people, studies, surveys, and discussions. When the frogs were first brought here, it wasn’t known how they would do, both on a mainland site and in the presence of other creatures that might eat them. Much of conservation is highly experimental, as we can’t know exactly how nature is going to react—but sometimes we have to take calculated risks to learn and to improve future outcomes for a species.

In many ways, this project exemplifies the best parts of conservation—working in partnership with a bunch of different folks, from mana whenua to academics to other conservation practitioners, not to mention the contractors who built the boardwalk with such care for the frogs and took incredible measures to minimise the environmental impact of their work. It was also a great exercise in adaptive management, as we constantly modified our approach as new information came to light.

Pepeketua are also the last species we had in captivity at Zealandia, so releasing them marks a special moment for the sanctuary—a new chapter where we can back ourselves to be a place where creatures like Hamilton’s frog can thrive and be seen by folks without the need for them to be in a pen.

I am also really excited about the future of this population: we are the only place on the mainland where Hamilton’s frogs live in the wild and the only place where they co-exist with mice and kiwi. This means, outside of remote offshore islands, this is the only place where folks can see these beautiful creatures in the wild, and we may be able to unpack gnarly questions that have implications for the conservation of this species at a national level. I am stoked to see these decades of hard mahi by so many come to fruition, and I can’t wait to see where it leads us next.

*Interestingly, many frogs in our surveys along Te Māhanga Track were spotted in trees. One of our anuran experts informed us that she observed this behaviour many a time on Te Pākeka/Maud Island and that as dawn breaks the frogs will literally drop from the trees!