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The South East Asian snail disaster

March 31, 2004

A promising enterprise became an economical and ecological disaster. The golden apple snail (Pomacea canaliculata) that was brought to Asia in 1980 to be cultured in ponds for human consumption instead spread through rice fields, irrigation channels and wetlands. It had a voracious appetite for rice seedlings and soon became a dreaded pest in the rice fields. In the Philippines alone, accumulative crop losses since the snail introduction is estimated to 1 billion US dollars. The snail is still spreading in South East Asia and it has recently invaded North Australia, Hawaii and southern USA. Recent research, however, shows that the invasive snail is a serious threat to natural environments as well.

This is because the snail consumes the aquatic plants in natural wetlands. These wetlands harbor great biodiversity, and many of the animals depend on aquatic plants for their survival at some life stage. These wetlands are also an important resource in the "unofficial economy" since the rural poor harvest plants, catch fish and collect drinking water in the wetlands.

- The ongoing invasion is nothing but an ecological disaster says Nils O L Carlsson at Lund University, who has quantified the effects of the invasive snail on natural wetlands in Laos and Thailand.

Entrepreneurs brought this South American snail to Taiwan in 1980. The intention was to grow these large snails (may grow to the size of a medium apple) in ponds to sell them at local food markets and export them to Europe. The project backfired. The Asian markets turned cold to the taste of the snail and all export to Europe was banned when it was found that the snail may be an intermediate host for a parasite that destroy the central nervous system in man.

At this time, however, the explosive invasion had already started. The snail is originally from Southern Brazil and Argentina and there is no feeding, growth or reproduction in the winter. At the higher mean temperatures in South East Asia growth and reproduction is greatly enhanced.

- The female snails lay around 300 eggs weekly and year around and flooded bushes in the wetlands often contain up to 100 000 eggs, says Nils Carlsson and he continues:

- When the snails attacked rice seedlings the invasion received a lot of attention. The farmers panicked and used both registered and non-registered pesticides. The invasion has therefore led to unsustainable use of chemicals and large negative non-target effects on many organisms, including man. Ironically, the snails effect on natural systems has been almost totally neglected, as the measurable economical damage from the snail was thought to be smaller in these systems.

- The wetlands are crucial for the diversity of both in-water, amphibious and land organisms, and rural people harvest plants, catch fish and collect drinking water in these systems. When the invasive snails consume all the plants, these productive systems turn into stagnant ponds with alga blooms.

Nils Carlsson is the first to quantify the effects of the snail in natural wetlands. He found that snail densities in these are as high as in infested rice fields and that the snails consume almost all plants. When the plants are consumed, the snails recycle large amounts of nutrients and these nutrients enhance phytoplankton growth instead. This is therefore a great threat to all organisms that depend on aquatic plants as food, refuge or spawning substrate. The values of the ecosystem services the wetlands provide rural people also drop dramatically after a snail invasion.
This snail invasion is thus not only a threat to rice production but also a serious ecological threat to all invaded wetlands.

- In both rice fields and wetlands you see a lot of people with cut and wounded feet says Nils. There are broken and sharp snail shells everywhere.

Besides higher mean temperatures, a popular explanation to the snails enormous success in South East Asia has been release from natural enemies, but Nils does not agree. Hunting and fishing pressures are very high in rural Asia. In Laos for instance, you can drive through rice fields and wetlands for an entire day without seeing a single bird. Nils has conducted experiments in Laos where he found that at least four native fish species, one native freshwater turtle and one native freshwater crab could consume the snail.
- More studies may confirm that the "immune system" of these environments have been fished and hunted away and that there are strong economical reasons to conserve biodiversity in this part of the world.

Vetenskapsrådet (The Swedish Research Council)