Biological Pollution

Damage from exotic pests is increasing all over the world. A major reason is the increased international trade in agricultural products, although there are also other factors such as the growth in tourism. In the past, oceans, rivers and mountain ranges were a barrier to the spread of species. Now, these ancient barriers are being breached.

Of course, not all species which are accidentally introduced are harmful. However, the majority seem to be. A study of 260 exotic insects in Japan found that 74% of these were pest species. Only 4% were beneficial, while the remainder had no particular impact, whether good or bad. In comparison, among Japan's native insect fauna, only 8% of species are regarded as pests. In recent years, there have been a large number of exotic pests colonizing greenhouses, such as whitefly and thrips. Many were imported accidentally in vegetables, fruits and cut flowers.

Impact of Invasive Species

Invasive species cause huge economic losses. They are also a threat to the world's biodiversity. Many of the invaders are distinguished by their ability to compete with native species. They are adaptable and have a high reproductive rate. Some are new predators, against which native species have few defenses. The wild birds of Guam have been almost completely wiped out by the brown tree snake, accidentally imported from Australia. Wildlife officials in Hawaii, home to many beautiful birds, are on constant alert to prevent the arrival of the brown tree snake in military flights from Guam. Similarly, the native birds of New Zealand are under threat from imported predators such as ferrets and opossums.

There is a danger that the natural world may blend into one great monoculture of the most aggressive species. Many scientists think that "biological pollution" may cause more extinctions than pollution from industry and agriculture!

Invasion of Foreign Weeds in Japan

In the last few years, many new species of exotic weeds have appeared in Japan. They are spreading in a different way than in the past. Traditionally, invasive weeds have appeared first near the point of entry (e.g. a sea port or airport). They spread out slowly from there over the years. In contrast, these new weeds appeared suddenly in forage fields all over Japan.

An examination of imported fodder-crop seeds showed that there was no contamination by weed seeds. The culprit seems to be imported feed grain. Weed seeds mixed in with the grain are eaten by cattle and pass through the digestive tract. When farmers spread the manure as fertilizer, they are unknowingly spreading the weed seeds, which then germinate and grow.

Composting the manure is a good way of preventing the weed seeds from germinating. Usually the temperature of compost rises during fermentation. A composting temperature of more than 57^oC will kill nearly all weed seeds.

Index of Images

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  Figure 2 Asian Irrigation Canal Covered in Water Hyacinth

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  Figure 3 The Giant African Snail Has Become a Major Crop Pest in Asia

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  Figure 4 Slurry Spread on Paddy Field in Japan As Fertilizer. It Contains Weed Seeds, Imported in Feed Grain