Science: Little Green Fern

When the great Kariba Dam on the Zambesi River was finished in 1959, every prospect about it was pleasing. Besides generating 1,500,000 kilowatts of power, the dam would create a lake 175 miles long where protein-starved black Rhodesians could catch fish and white Rhodesians could swim and sail. But Kariba Lake had hardly begun to fill with water when a vicious enemy showed its deceptively pretty face. A delicate, floating water fern named Salvinia auriculata appeared in patches that spread with astonishing speed. By last week dense mats, some of them strong enough to support a man, covered 15% of the lake, and local scientists were in a dither. Professor Boris I. Balinsky of Johannesburg’s University of the Witwatersrand warned that Salvinia might turn the lake into a swamp and eventually dry it up.

Salvinia auriculata is a native of tropical America, and no one knows for sure how it got to Africa. One theory is that a 19th century missionary imported it to ornament a pond. The fern’s hairy, half-inch-long leaves grow in pairs on a slender stem. Each broken-off bit of stem can start a new colony. Great islands of weed drift around Kariba Lake, entangling boats and clogging harbors. Fishery experts had been counting on Kariba to support an important fishing industry, as other African lakes do, but under Salvinia’s thick floating mats the water contains too little food or oxygen to sustain fish.

To fight the little green fern, Rhodesia has already spent nearly $3,000,000 and has little to show for it. No chemical has yet been found that will kill the weed and leave fish unharmed. No native animal eats the weed. One possibility is to import manatees, the tropical American sea cows that are used in British Guiana to eat ditches clear of vegetation (TIME. Dec. 19). Another possibility is the coypu, or nutria, a South American aquatic rodent that has a voracious appetite for water plants. It reproduces almost as fast as Salvinia, and the scientists fear that it might devastate Africa as European rabbits did Australia.

Some experts believe that the fern’s explosive growth is caused by nutrients in the lake water; when the nutrients have been exhausted, they argue, Salvinia will not grow so fast. Even more optimistic is a group that is trying to make Salvinia a valuable local crop; if ways can be found to harvest it cheaply, it might prove to be acceptable cattle feed, and protein extracted from its leaves might be good food for humans. One Rhodesian industrialist claims that dried, compressed Salvinia might make fine fiberboard. But none of these schemes are working yet, and the little green fern remains the victor so far.