Invasive plant species are from the major treats to the stability of mountainous ecosystems. High and selective livestock grazing create spaces for expansion of invasive plants in the expense of native rangeland species. Climate variability and species diversity may also cause contrasting effects on the invasibility of noxious plants. Relationship between plant community diversity and invasibility were studied at different distances from livestock watering points (WP) and at three climatic regions i.e. arid (200 mm), semiarid (380 mm) and dry sub-humid (600 mm), in the mountainous rangelands of northeast Iran. 90 quadrates were established along 18 transects, which had been located within 9 macro-plots in 3 regions. Results indicate, positive relationship between species diversity versus invasibility along the rainfall gradient (abiotic factor) but negative relations along the livestock grazing (biotic factor) gradient. Both species diversity and invasibility increased along the rainfall gradient from the arid to dry sub-humid regions. However in each climatic regions, livestock grazing had negatively affected species diversity but increased community invasibility near WP, where the livestock had induced highest grazing pressure. In conclusion, our results indicate high importance of species diversity for protecting the mountain ecosystems against the expansion of invasive plants. Nevertheless, high livestock grazing can diminish species diversity, by creating open spaces and providing more opportunities for expansion of invasive plants. Hence it is a serious threat to stability of terrestrial vegetation in the mountainous ecosystems.

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