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Over The Horizon

Frog Fadeout Fears

Scientists monitoring the rapid decline in frog species worldwide warn that whatever is killing the frogs may eventually affect humans.

They are alarmed by frog population crashes occurring all over the world at the same time, with no apparent common cause. Frogs are disappearing rapidly from both remote pristine habitats and developed areas.

Many factors have been suggested as possible causes. Disease, increased UV radiation, environmental pollutants, hormone-mimicking pesticide residues and climate change are among the possibilities. However, because there has been little data collected, scientists still don't know.

"Amphibians are thought to be especially vulnerable to environmental changes," says a senior lecturer in zoology at the University of Canterbury, Dr Bruce Waldham.

"Their sudden disappearance may signal impending danger to our ecosystem and to ourselves. Especially puzzling is the lack of common factors linking population declines in different species."

During the past 12 months, Department of Conservation reports have noted declines in native and introduced frogs in New Zealand. Concerned landowners have been contacting DoC asking "where are my frogs?"

A research project is about to start gathering data on native and introduced frog populations. The Lottery Grants Board's Science Research Committee has provided $5,000 for equipment to monitor UV radiation and gather weather information from frog habitats around New Zealand.

Waldham, along with Lincoln University ecology technician Alastair Freeman, and Dr Phillip Bishop, a post-doctoral researcher at Canterbury, make up the research team. Their data will provide information for drawing up conservation plans -- if it's not too late for the frogs, Waldham adds.

"If we find out what the causes are, this may give us a clue about what we can do differently. But because they have declined so rapidly all over the world, we can't be optimistic.

"Frogs may be like the canary in the cage down the mine -- a warning system. They may be more sensitive. They readily take up chemicals through their skin and they are small, so it probably takes pretty low quantities of chemicals to affect them, compared to larger animals such as humans."

One theory is that environmental factors contained in the atmosphere, such as pesticide residues, are carried around the world and affect frogs, even in remote habitats.

Some pesticides contain components similar in form to the frogs' natural hormones, "so it could be that this is stuffing up their reproductive systems."

The Australian golden bell frog is still present in New Zealand although it's close to extinction in its native home, due in part to the 2000 Olympics. The construction site for the stadium and village for 10,000 athletes is centred on clay pits which are home to one of the last known sites of golden bell frogs in Australia.

The popular whistling frog may also be dwindling in numbers across the Tasman. It's thought high UV radiation levels are killing the frog eggs.

"But we have UV radiation here in New Zealand, so it doesn't explain why they are doing better here than in Australia."

Waldham says both these introduced species probably have relatively short life spans, and breed annually, compared to native New Zealand frogs which live for 20 years or more and may not breed every year.

"If there was an environmental factor, such as a pesticide that mimics their hormones, we would see the effect on the introduced frog species first, because they reproduce more often and lay many more eggs."

The scientific programme will monitor frogs at various sites around the country over successive years. The same populations will be surveyed repeatedly to identify any population changes. Along with weather and UV radiation recording, water at or near breeding sites will be tested, and genetic characteristics will be analysed to assess the susceptibility of frogs to disease.

"It may be that the same factors causing the decline of introduced frogs will affect native frogs. The introduced frogs seem to be disappearing faster."

Waldham says there's a long tradition of frogs being generally overlooked in scientific circles.

"The decline in populations could have been a gradual process, but because not many people have studied them, we don't know."

"On the other hand, some frogs don't mate every year and are difficult to study. And many have cyclical population rises and falls over periods as long as ten years.

"Although scientists like to be cautious, with such a large number of species declining all over the world we are now motivated to go out and get accurate counts of frog population sizes and secondly, to correlate from research the different factors that could be adversely affecting frog numbers."