Investigating the effect of the sample size and plot size on the numerical indices of rangeland biodiversity

This research was carried out in an enclosure rangeland with four habitats of Artemisia sieberi, Zygophyllum eurypterum, Stipa barba and Amygdalus scoparia in the Shaskouh protected area, South Khorasan province. All the plant species of the rangeland were identified and counted in 200 plots, due to the different vegetation form and distribution of each of the dominant species, in each of the four habitats, 20, 40, 60 and 80 plots (respectively) were established with two plot size: 2 × 2 meters and 4 × 4 meters (60 plots each). Numerical indices and diversity ordering curves were used to compare four habitats. Alpha and gamma species richness were determined by numerical indices and single sample rarefaction and pooled rarefaction curves were used to compare species richness and determine data adequacy. The results showed that the total species richness (gamma) varies between 51 and 54 species. The habitat with 80 plots has the highest richness and diversity (S=43 and H=3.27) and 20 plots has the lowest richness and diversity (S=6 and H=1.41). The results of SHE analysis showed that with the increase in the number of plots, species evenness decreased. The pooled refraction curve showed that by increasing the number of plot to 110, all plant species will be observed. With the increase in plot size all values of species richness and diversity indices increased significantly (p≤0.01). Species evenness indices were not affected by plot size. The results of the present research showed that the numerical indices of species richness and diversity are dependent on plot size, and this is one of the disadvantages of evaluating biodiversity in rangeland using numerical indices.