The Trouble With Frogs

Forest and Bird, Issue 311, 1 February 2004, Page 6

The Trouble With Frogs

is a physician and

biologist who has bred reptiles and amphibians for many years. He has written several scientific papers on the subject. Contact him at TurtleDoc@xtra.co.nz

—MARK FELDMAN

efore the arrival of the forests of New Zealand sheltered millions of native frogs. This moist, cool environment was ideal for the frogs of the Leiopelma genus because most are able to reproduce without needing standing bodies of water for their tadpoles. These frogs lay a relatively few, large eggs that can incubate out of water. The entire tadpole stage of development occurs within the egg and a tiny froglet hatches out at the end of the process. This reproductive adaptation made it possible for the Leiopelma frogs to live throughout the undisturbed forests, but disaster struck when

the Maori arrived bringing the Pacific rat or kiore. Within a short time, three of the native New Zealand frogs were extinct and the three remaining species survived only in isolated pockets in remote areas. Today those three surviving species linger on but their numbers are declining and one, Archey’s frog, is on the verge of extinction in the Coromandel. The demise of the native frogs and the arrival of people opened up a niche that has been slowly filling with frogs brought from Australia in the 1800s. Of the seven species introduced as a food source for ducks, three still survive in New Zealand. These frogs all require still water for their tadpoles to

mature in. The burning of the forests and the creation of thousands of small bodies of unshaded water by farmers and land developers provided just the right environment for these species to spread quickly throughout the country. Of those three, only the whistling tree frog is still doing well in Australia. The other two species, the green and golden bell frog and the southern bell frog, are in serious trouble in their native environment. These two seem to be falling prey to the same forces that are causing the extinction of many species of frogs worldwide. No-one’s quite sure why it’s happening or why only some species are affected. Some of the suggested causes are draining wetlands, road building, introduction of the mosquito fish Gambusia affinis, atmospheric pollution, herbicides draining into water, an unknown viral disease and a Chytrid fungal infection. This fungus has been implicated over and over again but it is unlikely that it is a primary cause. The demise of the frogs is probably due to some other factor that makes it possible for the Chytrid fungus to infect their delicate skin. There is concern that these

Australian frogs may be starting to die here too. The Chytrid fungus has been identified in New Zealand but we’re still not sure of the extent of the danger. One danger that has been clearly identified in the North Island is the gambusia or mosquito fish. This tiny American fish, only three to four centimetres long, has been widely introduced around the world because it eats mosquito larvae. They are voracious feeders and able to live in waters too warm or shallow for other fish. Unfortunately they eat a lot more than just mosquito larvae. Gambusia also consume frog eggs, young tadpoles and even tadpoles their own size, which they kill by slowly nipping away at their tails. Although declared a noxious fish in New Zealand, gambusia are still being spread because of their ability to keep down mosquito populations. Unfortunately, they are killing off frogs our in the process. Very few frog eggs or small tadpoles can survive in ponds where gambusia flourish.