S America Ecosystems Under Siege By Alien Invaders

by Yensi Rivero

(IPS) CARACAS -- Youngsters with slingshots have a new mission in the Andean highlands state of Merida in western Venezuela: killing bullfrogs before they can cause serious damage to the local ecosystem.

For their efforts in catching the wary amphibians, they reap a reward: the Environment Ministry offers the equivalent of 60 cents for each female bullfrog, 30 cents for males, and 20 cents per kg of tadpoles.

The bullfrog (Rana catesbeiana) is an invasive exotic species originally native to eastern North America. It has spread widely around the Americas. No one knows exactly how it was introduced in Venezuela.

The unique deep-throated croak has been heard at night for at least the past four years in the chilly highlands of Merida. Biology professors and students from the University of Los Andes eventually detected the source of the strange new noise: the bullfrog, which can measure up to 23 cms and weigh as much as one kg.

The diet of the voracious large frogs ranges from insects to other bullfrogs, and they are prolific breeders. In the Andean region, there are already hundreds of thousands of these invasive amphibians, which are threatening native species of birds, snakes, turtles and even rodents.

The bullfrog "has not yet caused enormous damage, but it could do so," Amelia Diaz, an expert in animal ecology at the University of Los Andes who has done research for the Venezuelan Bullfrog Foundation, told IPS.

The illegal introduction and dispersion of exotic species caused by careless handling is a serious problem in Latin America and the Caribbean.

One of the most high-profile cases is that of the tilapia, which has long been an important food fish in Africa and is now farmed around the world, from Asia to South America. Like the bullfrog, the tilapia is on the World Conservation Union (IUCN) list of "100 of the World's Worst Invasive Alien Species".

Latin American countries like Argentina, Brazil and Mexico, where the tilapia is farmed, have established controls on where the fish can be bred. The species has had a heavy environmental impact, because excess feed and feces pollute water and degrade local habitats.

In addition, native fish species are endangered, because "The tilapia's fast reproduction rate gives it an advantage over other species," Eddy Solorzano, at the Venezuelan Environment Ministry's Office on Biological Diversity, pointed out to IPS.

The Inter-American Development Bank (IDB) projects that tilapia production in the Americas will reach 500,000 tons a year by 2010, and one million tons by 2020.

Another exotic fish species that has had an impact in Latin America is the rainbow trout (Oncorhynchus mykiss), native to the Pacific states of the United States.

The rainbow trout has been released into streams and rivers in Colombia, where studies show that it has caused serious damage to native species of fish, and even waterbirds, like the yellow-billed pintail (Anas georgica).

Farther to the south, in Argentina, the Canadian beaver (Castor canadensis) has had a major impact on local ecosystems by cutting down trees to build dams, which provokes floods in forests, killing trees and creating new ponds.

Perhaps one of the most famous cases of damages caused by an introduced species involves the "Africanized honeybee".

In 1956, researchers in Brazil imported queen bees from Africa in an attempt to develop a more productive honeybee that would be better-suited to conditions in Brazil than the European honeybee.

But in 1957, 26 African queen bees, along with swarms of European worker bees, escaped from an experimental apiary near the city of Sao Paulo.

The "Africanized honeybees" that resulted from the cross-breeding process are more aggressive than European honeybees, and have killed more than 1,000 people since they began to gradually spread northward through South America, Central America, and eastern Mexico.

By 1990, the so-called "killer bees" had reached southern Texas, and in 1995 they began to be found in California.

Africanized bees have also killed numerous domestic livestock and wild animals.

Although the bullfrog does not pose that kind of threat, biologists and authorities in Venezuela believe "it is very important to keep it from spreading, because the Andean ecosystem is fragile," said Solorzano.

The United Nations Environment Program (UNEP) has warned that the pressure from introduced species like bullfrogs on native species and their habitats poses a threat to the region's biodiversity.

In Venezuela, the illegal introduction of species is punishable under laws on the environment by up to three months in prison and fines ranging from $1,500- 5,000 (equivalent to between 10 months and three years of the minimum monthly salary).

A single bullfrog can lay from 50,000 to 70,000 eggs. In a lake and the surrounding valley between the Merida towns of Jaji and La Carbonera, the Bullfrog Foundation estimated that there were 600,000 bullfrogs.

That number has been reduced to one-third, or one-sixth according to the most optimistic estimates, by teams that use nets to capture the frogs, and which destroy the vegetation that serves as their breeding-ground.

In countries like Argentina, Brazil, Cuba and Mexico, such efforts might look like a waste, because bullfrogs are commercially bred in those countries to sell frog legs to restaurants.

In the United States, live bullfrogs can fetch $5.50 a kg, and frozen frog legs around $10 a kg.

But no experienced breeders have set up shop yet in Merida, and biologists and boys with slingshots continue tracking down the distinctive, deep-pitched call in the aim to curb the invasion of the big amphibian from the North.