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Bright lights, big city, tiny frog: Romer’s tree frog survives Hong Kong

  • Discovered in the 1950s, Romer’s tree frog has so far been declared extinct, rediscovered, immediately declared Critically Endangered, been seriously threatened by an international airport, and become the focus of one of the first ever successful, wholesale population relocation projects conducted for an amphibian.
  • At just 1.5 to 2.5 centimeters (0.6 to 1 inch) in length, this little brown frog lives at just a few locations within the sprawl of Hong Kong Island, as well as on a few outlying islands. It lives in moist forest leaf litter on the forest floor, and depends on temporary fish-free pools of water for breeding.
  • When Hong Kong planned a major new international airport within the shrinking habitat of the Romer’s tree frog, scientists responded quickly, studying the animal’s lifestyle, eating and breeding habits; they then instituted a captive breeding program at the Melbourne Zoo, and launched a restoration program. It worked.
  • While some restoration site populations have since failed, others continue to thrive. And with new protections now in place, scientists hold out some hope that Romer’s tree frog may be a Hong Kong resident for many years to come.
A Romer’s tree frog calls in the leaf litter. Photo courtesy of Kadoorie Farm and Botanic Garden

Romer’s tree frog is, in every respect, an animal that keeps out of the way. Start with its size: averaging just 1.5 to 2.5 centimeters (0.6 to 1 inches) in length, the tiny brown frog isn’t just the smallest frog living inside the territory of Hong Kong, but among the smallest in the world.

Despite its name (and suction-cup toes), it climbs rarely and reluctantly, preferring instead to hide in the leaf-litter of the island forests, aided by a cryptic color pattern of brown skin and faint black “X” markings on its back. There it spends its short life hunting termites and crickets, and being hunted in turn by centipedes and wolf spiders. The tree frog emerges from cover only to court and lay its eggs in shallow, out of the way pools and ditches, where fish can’t reach them.

It is, in short, not the sort of animal that you would expect to make a fuss. But the Romer’s tree frog inhabits the fringes of an urban wilderness, at the mouth of a river that has been part of the biggest spike in human population in history. As such, it keeps hopping into and out of the ecological limelight:

Since its discovery in the 1950s, the species has been declared extinct, rediscovered, immediately declared Critically Endangered, then seriously threatened by development, and eventually became the focus of one of the first ever, wholesale relocation projects conducted for an amphibian.

Along the way, the diminutive frog has become an icon of biodiversity for Hong Kong, one of the world’s most densely populated urban territories. Amongst a global landscape of frog species ravaged by habitat loss and chytrid fungal epidemics, the tree frog is also a rare example of an ongoing, hard-won success story.

Romer’s tree frogs are the smallest frogs in Hong Kong, and among the smallest in the world. Photo by Yorkie Wong

Blinking in and out of existence

The city of Hong Kong stretches across a tangle of islands and bays at the mouth of the Pearl River Delta, where gleaming urban sprawl and undeveloped rural land coexist side-by-side. When the British initially arrived in the 1800s, the islands were mostly occupied by fishermen and charcoal burners, but the city soon began to explode in size. By the 1950s, the population hit more than two million, and continued rising rapidly. Much of the available land on Hong Kong Island was consumed by the growing city.

Michael Lau, one of the Romer’s tree frog’s primary researchers, with a conservation shipment of the rare frogs. Photo courtesy of Chris Banks

The smaller delta islands, however, remained less affected, becoming a sanctuary for species lost to Hong Kong Island. In 1952, John D Romer, a British World War II veteran and naturalist was exploring a remote cave on Lamma Island when he found miniscule brown frogs hopping about among the moss and leaf litter. He had little chance to make a close study of their biology, however, and a year later, the cave roof partially collapsed, sealing the opening. It was assumed then that the frog had likely been endemic to the single cave system. When subsequent searches by Romer and other researchers turned up no frogs elsewhere, the species was declared extinct.

Then, in 1984, the Urban Council of Hong Kong asked a trio of naturalists to write a field guide to the amphibians and reptiles of Hong Kong. One of those naturalists was Michael Lau, then a PhD student from Hong Kong University. Lau and his colleagues wondered if Romer’s tree frog might still survive on Lamma Island, and in their treks through the rainy forests they kept a sharp eye out for it.

“I found some tadpoles in another cave on Lamma that I had not seen before,” Lau recalled. He suspected that they belonged to the elusive tree frog. When he and his colleagues returned to the new cave, they struck pay dirt: adult and tadpole Romer’s tree frogs, alive and well.

“With better knowledge on this species such as its distinctive calls, breeding habitats and the characteristics of the tadpoles,” Lau said, “we were able to find this species on two more islands, Lantau and Chek Lap Kok.” The frogs tiny size and retiring nature had led to them being completely overlooked.

Then came the airport

Despite its rediscovery, Romer’s tree frog was not out of harm’s way. The growth of Hong Kong had left the species confined to three outlying islands, and the animals were not especially common on any of them. Then in the late 1980s, one of those islands, Chek Lap Kok, was chosen as the site of a new international airport. The development required the island’s forests and hillsides to be razed to the ground.

Researchers like Hong Kong University’s David Dudgeon and Lau, who by then had begun working for the Hong Kong branch of the World Wildlife Foundation, viewed the airport construction plan with alarm. “It was obvious then that the main ecological issue for the project was how to deal with the Romer’s tree frog,” Lau said. He pushed for a chance to relocate the suddenly threatened frogs. The Hong Kong government responded by saying that it didn’t have the money to fund such a project.

Amidst a storm of media coverage and debate, the Hong Kong Jockey Club — not your typical tiny frog fanciers — stepped in to fund the Hong Kong University conservation effort. The Romer’s tree frog would have a chance, but only if everyone acted fast. Construction had already begun on the northern tip of the island.

Romer’s tree frogs sometimes display variable skin patterns. Photo courtesy of Chris Banks

At that time, Lau said, there were very few cases of successful amphibian relocations in the world to guide the team’s actions. It is very difficult to move a species wholesale, especially species that prefer specialized habitats. It’s not enough simply to collect all the animals you can get your hands on; they have to be able to survive and breed in their new home.

With just under a year to relocate the frogs and little to no information available about their ecology and behavior, Lau and his colleagues had to figure out what kinds of forest environments the animals preferred, the precise dimensions of the pools and puddles they bred in, and what they ate.

Undaunted, the team launched a study of Romer’s tree frog biology and breeding habits over the course of 1991. They discovered that the frogs preferred relatively undisturbed forest, with deep drifts of leaf litter, plus seasonal pools and puddles of rainwater. Any water, in other words, without fish.

As airport construction on the northern end of the island steadily moved south, Lau collected frogs and assessed breeding locations. Initially he only took females, he said, as these were harder to locate. But, within the last three months of the investigation, he began taking all the frogs and tadpoles he could find.

He kept them in his quarters at Mai Po Nature Reserve, in small tanks that closely imitated their natural environment, complete with trays of shallow water for them to breed in. He didn’t need to wait long for results: one night the males set up an enthusiastic chorus. The following morning, a thrilled Lau found tiny, 1 millimeter black eggs enveloped in jelly attached to the leaves and twigs in the water tray.

Captive breeding

To carry out the relocation successfully, overseas partners were needed in order to build up the species’ numbers. So Hong Kong WWF faxed a letter to the world’s zoos, seeking an institution willing to start up a breeding program. The letter reached Chris Banks, Manager of Conservation Projects at the Melbourne Zoo. Banks, who had long been interested in the role of zoos in conservation, jumped at the opportunity, agreeing to take in about 30 frogs from Chek Lap Kok.

Breeding tanks at the Melbourne Zoo were used to help build up frog populations in captivity. Photo courtesy of Chris Banks

Against all expectations, Banks said, he and his colleagues at the Melbourne Zoo found the tiny frogs to be easy guests. The tiny animals were kept in small acrylic terrariums, with drifts of leaf litter and as near an approximation to their lost home as the zoo could create. Like Lau’s population, the frogs bred easily and rapidly, enough so that keeping them fed was occasionally a bit of a challenge. “Over the first 2 to 3 years,” Banks recalled, “we often had more than 200 little frogs…. That required a lot of very small insect food.”

Banks also took an opportunity to visit the island of Chek Lap Kok with Lau, which he called a surreal experience. Half the island had by then been cleared down to sea level, while the other half was scattered with the remains of emptied villages. Pots, filing cabinets and refrigerators littered the ground, catching rainwater. In an ironic twist, Lau recalled, the flooded junk made a perfect breeding ground for the Romer’s tree frog. Strangely, the frogs of Chek Lap Kok might have experienced one last population boom before the construction snuffed them out altogether.

Once the scientists had a healthy population of captive frogs, the question was where to release them. Chek Lap Kok was of course off limits. There were still small populations of the frogs on the islands of Lamma and Lantau, but Lau worried that releasing the frogs in such places would dilute the genetic diversity of the species.

There were plenty of protected forests on Hong Kong Island itself, but many of these were on slopes, and lacked the seasonal pools the frogs needed to breed. Lau solved this problem by installing shallow pots on the hillsides to collect rainwater — much as the abandoned refrigerators and flooded junk had done back on Chek Lap Kok.

The forests of Hong Kong are the frog’s natural habitat; not exactly the safest place to be a frog (note the fenced off frog breeding pond). Photo by Yorkie Wong
Release sites were designed to catch rainwater, creating temporary breeding pools. Photo courtesy of Kadoorie Farm and Botanic Garden

Return to the wild

The initial releases of frogs proved successful. Over the course of 1993 and 1994, over 1,100 frogs and 1,600 captive-bred tadpoles were released at 8 different sites in Hong Kong, including the Kadoorie Farm and Botanic Garden, Hong Kong Zoo and Tai Po Kau Nature Reserve.

One restoration — Tin Fu Tsai, a forest mostly of invasive plants — failed immediately. Over subsequent years, two other sites failed as well: the Hong Kong Zoo restoration went belly up, Lau speculates, because of the release of exotic mosquitofish into the breeding pools, which spelled doom for the tadpoles. Failure also came at Kadoorie Farm.

According to Gary Ades, the head of the Fauna Conservation Department at Kadoorie Farm, local frogs already in residence proved inhospitable to the little tree frog: “We found this rocky stream species [of frog] was actually using the breeding pots and apparently feeding on the adult [Romer’s tree] frogs when they entered the pools to breed.”

Nor was that the only problem: the transplanted frog population declined after a few years because the installed breeding pots were being washed out by heavy rain. And when there wasn’t too much rain, there wasn’t enough — the moist leaf litter where the frogs lived dried out during a prolonged dry season. Ades told Mongabay that the research team isn’t giving up, however, and is currently getting ready to try again at a new site at Kadoorie Farms.

The Melbourne Zoo likewise lost its entire population, Banks said, after ants invaded their aquaria. But luck was again with the species — the invasion only occurred after the majority of the frogs had been resettled.

Pots were placed in the forest to help the tree frogs breed in fish-free waters, a not altogether successful strategy. Photo courtesy of Kadoorie Farm and Botanic Garden

A better future?

By and large, however, the relocation and restoration effort proved to be a success, with stable populations established, and now residing in five protected sites including those at the Tai Po Kau Nature Preserve.

In Lau’s opinion, the Romer’s tree frog is more secure now than it’s been in years, especially because it’s now finally on the government’s radar. It is still listed as Endangered by the IUCN Red List, as its range is severely divided and its habitat is increasingly degraded. However, it now receives at least a modicum of protection: the species is designated a Protected Wild Animal under Hong Kong ordinances, with hunting, collecting or disturbing of its habitat carrying a hefty penalty of HKD $100,000 (USD $12,894). In addition, one of the frog’s habitats on Lantau Island has received a “Site of Special Scientific Interest” designation, which restricts development around it.

There are still challenges ahead, of course. New infrastructure projects on Lantau Island are worrying, for example. But Lau is confident that an enhanced understanding of the species’ survival requirements will help mitigate the effects of construction there.

Banks cautions that protection on the ground is still spotty, with any proposed building project having the potential to impact frog populations farther away from the city center. There’s also always the risk of climate change drastically altering the rainfall patterns the frogs depend on to provide seasonal pools.

Finally, there’s the simple fact that humans can be hard to live among. According to Wing Tsui, a worker with the Hong Kong Conservation Department, park visitors have a tendency to break or steal the frog breeding pots for reasons nobody’s figured out yet.

Still, for a species as retiring as Romer’s tree frog, the extra attention helps. The little frog has already proven its extraordinary resilience under urban stress. With a little continued care, maybe it can make its way in the big city for a long time to come.

A Romer’s tree frog crouches on club moss, showing its distinctive back markings. Photo courtesy of Kadoorie Farm and Botanic Garden


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