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A new book about unwanted plants is causing consternation among nature lovers, gardeners and policy makers.

"Invasive Plants in Israel," co-published by the Israel Nature and Parks Authority, describes 50 types of invasive plants, some of which pose a real threat to the natural landscape of Israel. They push out local species, take over sandy areas, clog stream beds and reduce water quality.

Over the last few years, ecologist Jean-Marc Dufour-Dror, an adviser to the Environmental Protection Ministry, has been documenting the spread of foreign plant species introduced into the local ecosystem. The book marks another step in his efforts to spread awareness of the growing problem. The threat continues to grow, the books shows, as the discovery a year ago by the authority of a new foreign species in Harod Stream area.

In Israel today there are a total of about 170 species of foreign plants in both unpopulated and urban areas. Dozens of them are considered invasive, which means that the plant has established itself in its new habitat and spreads offshoots through seeds which allow it to cover large areas. In Israel's case, most of these are plants which have been intentionally introduced in parks and gardens, and which have spread. What disturbs environmental protectionists is what Dufour-Dror calls "environment-changing invasive plants" which "are capable of changing conditions, the characteristics of ecological systems, over a large area."

These plants are often species which faced competition in their home habitats which limited their growth, while they have no natural enemies in the places they have invaded.

"The most notorious invasive plant in Israel, the blue leaf wattle, is common only in southwest Australia," Dufour-Dror says. "There it usually lives for 12-15 years, in contrast to its adopted habitats, where it can live 30-40 years. In Australia its growth comes under pressure from a fungus that kills the plant."

The blue leaf wattle has caused serious damage in sandy areas along the local shoreline, completely changing the conditions there and dealing a blow to plant and animal life characteristic of such areas. Another plant, camphorweed, also pushes local plants out of dune areas. Yet another species, the ice plant, affects the chemistry of the soil and prevents local plants from sprouting. The water hyacinth, which has spread to water sources in the Sharon plain, reduces water temperature and the concentration of oxygen.

According to the picture painted by Dufour-Dror, treatment of such species once they have put down roots is complicated. Uprooting does not solve the problem because it leaves the root system in place, and pesticides are likely to be dangerous to the environment and harm non-invasive species as well.

Biological intervention with natural enemies of such plants is problematic since it is hard to estimate whether the fungus or bug won't also damage local plants in the future. At the same time, Dufour-Dror emphasizes that many foreign species have not yet established themselves in Israel and that their spread may be prevented by conventional means at our disposal.