Species diversity can significantly affect plant community stability, invisibility and dynamics. Livestock grazing is a selective process, which may increase frequency of some species, but reduce that of rare and susceptible species. Light grazing may provide opportunities for entrance of new species into a closed plant community, and hence increase its species diversity. This research was aimed to test whether an increase in species diversity due to light grazing provides biological resistance against the entrance of new species, or it may increase diversity of both native and invasive species, via increasing heterogeneity and resource availability. A study was conducted at 8 sites along a livestock grazing gradient, started from highest use in areas close to watering points towards the no-grazing site at an enclosure. Study area was in a semi-arid grassland, in Baharkish, Quchan, Iran. Plant sampling were conducted in spring 2016, at the time of full vegetative growth. Within each site, 10 plots of 1.5 m2 were randomly established, in which number of plant species, and frequency and canopy cover of each species were recorded. Plant traits such as life form, growth form, maximum height, width and length, leaf length and area, and leaf dry and wet weight were measured for all plant species in each quadrat. Livestock grazing intensity was quantified by measuring micro-traces and dungs. Different species diversity indices were measured and compared along the grazing gradient. Highest species diversity was under light grazing intensity, whereas highest frequency of invasive species were under the most intense livestock grazing intensity. These results indicate biological resistance under the light grazing intensity, in which higher species richness prohibited extension of invasive species within the plant community.

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