Counting on the frog chorus

PUBLISHED : Sunday, 07 August, 2011, 12:00am
UPDATED : Sunday, 07 August, 2011, 12:00am


When frogs in a field croak in chorus, they are not answering one another. They are in competition. Generally, only male frogs use their vocal sacs, which puff up like large balloons from their throats, to project their identity - distinct for every species - into the night.

They are in hopes that some yonder female will be attracted to their coaxing noise and hop to their sound. The females listen for the deepest croaks, which signify a bigger, fitter mate.

'But sometimes frogs are tricky,' says Teresa Ma, an ecologist by training who is doing research for her masters degree on frog habitats in Hong Kong.

Scientists say smaller frogs will call from within drainage pipes so their voices will amplify; average-sized frogs perch next to their particularly large brothers in the hope that if a female comes for the larger frog's call, they can try to intercept her or, if multiple females come, well, take advantage of the extra resources.

Every week during the mating season, usually from late February to September, Ma listens to frog calls in Long Valley, a 25-hectare stretch of wetland in the New Territories and home to many of Hong Kong's competing frogs.

The frogs are lucky there: they call in the midst of 400 fields reserved for small-scale farmers in a unique collaboration between the farmers, the Conservancy Association and the Hong Kong Birdwatching Society. The farmers receive cash from the non-government organisations (NGOs) to compensate for money lost while growing diverse crops instead of traditional agricultural practices which usually focus on cash crops.

For the environmental NGOs, this arrangement means preserving some of the last agricultural wetlands left in Hong Kong. For the ageing farmers, it means earning a sustainable livelihood.

For the frogs, it means a habitat with both wet and dry areas which their amphibious selves - which hop over land, sleep in mud, and breed in water - appreciate.

Ma's job is to listen to frog calls in different types of fields - each about 900 square metres - and record their numbers in each, which means comparing rice paddies with lily ponds or often, organic or non-organic plots of land.

She is trying to see whether frogs are more abundant in certain locales.

'We believe that this is the first study in the world comparing how frogs do in organic and non-organic wet agriculture,' says Ma's adviser, Professor Nancy Karraker of Hong Kong University, a specialist in amphibian research in Southeast Asia.

Ma's research has important implications; if, as preliminary results show, frogs do better in organic lands, it could have implications for their survivability and be an indicator of a whole ecosystem's health.

Frogs can only hop or swim short distances; unlike many mammals or birds, they are constrained by the environments in which they live. Mammals and birds also do not directly absorb many environmental contaminants but rather eat or drink them so toxins accumulate in their body fat, which then gets eaten higher up the food chain - all the way to humans.

Not so for frogs.

'Amphibians don't drink water like we do,' says Karraker. 'They take water in through their highly permeable skin and a lot of their breathing is through their skin.'

Since chemicals are taken up directly through their skin in the water, frogs are some of the first to die in polluted areas; they are sometimes compared to canaries in coal mines. If the canary dies, it means that the mine is filled with poisonous gases. If frogs do not survive in a particular area, since they live and breed and even breathe in water, this could be a sign that other species' deaths could follow. Ma is trying to prevent that from happening.

Hundreds of years ago, much of the low-lying areas in Hong Kong were wetlands or often, simultaneously, rice paddies. For an agricultural society, it was common for natural wetlands to be converted into paddies, yet these wet paddies, called 'square wetlands' by scientists, still preserved important wildlife habitat. So frogs and farmers could live in harmony.

'Even though they altered the wetlands, everywhere there are rice paddies, there are frogs by the ton,' says Karraker.

But in the 1960s and 1970s, dry agricultural crops became more valuable and rice from the mainland was cheaper and more widely available. Many of Hong Kong's wetlands were drained and filled. According to a convention on wetlands report on Hong Kong, in 1954, the year of Hong Kong's first-ever comprehensive survey of agriculture, there were 9,450 hectares of paddy.

By 1969, there were only 5,870 hectares; in 1980, 10. Nowadays, Long Valley with its 150 hectares of land is one of the few agricultural wetlands left in Hong Kong.

Even they were under threat a few years ago when the railway line to Lok Ma Chau was proposed. But the presence of a locally important species - a bird called the greater painted snipe - helped protect the area from development and today the train passes underground.

Standing in Long Valley now, one can look out at the lights of Shenzhen and at the same time be utterly transfixed by the sound of rural life, interrupted every now and then by the whoosh of the underground train. This place is one of the last refuges for wildlife before the urban jungle of modern China.

'Wetlands are dirty, sloppy, bug-infested places, but that doesn't fit with the culture here very well,' says Karraker. 'People care about the streams, about the forests, but few people care about the wetlands here.'

It's true: wetlands are not grand like forests and mountains. But they serve important functions - they are some of the most biologically diverse ecosystems on earth, on a par with tropical rainforests. The deep roots of plants in coastal wetlands hold soil in place so strongly they can withstand storm surges of water and prevent flooding. Environmental pollutants can bind to sediments in wetlands and be filtered out.

Yet for Hong Kong, Karraker says: 'None of that stuff goes on here, because almost all wetlands are so altered, they don't serve those functions any more.'

But, being the tropics, they still provide habitats for the 500 species of birds that winter in Hong Kong and are home to about 300 butterfly species (compare this to Britain which has only 56 species of butterflies). The roots, leaves, and bark of many plants in Hong Kong's wetlands are used in Chinese medicine. But the wetlands themselves are disappearing, unnoticed.

Long Valley joins only a handful of other wetlands in Hong Kong, including the Mai Po Nature Reserve and Nam Sang Wai. Recently, developers wanted to put a golf course in the Nam Sang Wai wetlands; environmentalists are still suspicious about the origins of a fire in the area in January.

Now the plants and trees have come back and the only traces of a fire are smudges of black against the bark. Bikers ride through the grassy paths, women in wedding dresses pose for their marriage photos, and wildlife photographers capture the intricacies of toads sitting in the mud.

Finding those subjects is not always easy, though. The photographers have to look and listen closely.

'It's the ornate pygmy frog ... listen,' Ma says, hand cupped over her ear as she performs her frog survey. It sounds like the edge of a spiral notebook being scraped back and forth: a tinny, jagged seductive noise. The calls happen every night during mating season, but to hear them you have to pay attention.

On the first day of her ecology of Hong Kong course at Hong Kong University, Karraker asked her class, 'How many of you have seen a frog?' Out of 125 students, one raised his hand, although frogs and toads are present even on the HKU campus.

At the end of the course, when she asked again, one-third of the students raised their hands.

'They learned to look,' says Karraker.


The number of frog species in Hong Kong, including the lately discovered South China cascade frog. There are three toad species