INTRODUCTION
Background: This research addresses the issue of alien invasive and naturalized plant species in natural and semi-natural areas in Israel. Previous research on alien plant taxa in Israel focused on agriculture weeds (Dafni & Heller, 1980,1990), whereas botanical notes and surveys (Danin, 2000, 2004) only mentioned the presence of some alien taxa without further descriptions of their areas of occurrence, the size of their populations and their various stages of invasion. Therefore, there was a need for a detailed study on alien plant taxa that occur, and in numerous cases invade, natural and semi-natural areas in Israel. Moreover, no study has yet attempted to determine which plant taxa and what particular populations should be targeted and prioritized for control in natural areas throughout the country.

One can distinguish three main characteristics regarding Israel's natural
areas: First, although there are numerous protected sites, their surface areas are generally extremely restricted due to the scale of the country. Secondly, nature reserves and national parks in Israel are subject to severe pressure from various human activities connected to very high population density. The type of threat posed by invasive plants to natural areas in Israel is characteristic to developed countries because the main sources of the proliferation of alien plants are linked to ornamental plants used in gardens, tree species previously used for forestry or soil stabilization, and agriculture. A third characteristic of Israel's natural areas which concerns alien plant invasion is the scarcity of wetlands: The development of intensive agriculture over the past 60 years, especially in the lowlands, had led to the drying of numerous wetlands. As a consequence, this type of habitat, which combines the first two characteristics mentioned above, became quite rare and the remaining wetlands are today the type of natural habitat which is probably the most prone to alien plant invasions.

Aims: This research has four objectives: (1) to carry out a survey of the invasive plant species that occur in natural and semi-natural areas in Israel, including an estimation of their population size; (2) to define the invasion stage of each alien plant species according to the most recent terminology; (3) to prioritize the alien plant populations and taxa that should be targeted for control; (4) to build a GIS compatible database of alien plant species in natural and semi-natural areas in Israel; such a database was built for the Science and Conservation Division of the Nature and National Parks Authority.