The effect of livestock grazing intensity on the frequency distribution pattern and species abundance of rangelands in Sarbisheh, South Khorasan

Livestock grazing has a direct or indirect effect on the density, diversity, and distribution of vegetation cover in Rangelands. Excessive livestock grazing is one of the destructive pressures on rangelands, which causes a decrease in diversity, and loss of sensitive plant elements. In this regard, maintaining species diversity and abundance is one of the goals of managing natural ecosystems. The concept of species diversity is a combination of two interconnected components of richness and uniformity. Species richness means the number of species per unit area and uniformity means the distribution of individuals and plant species per unit area. Many species diversity indices have been invented so that species diversity can be studied using them. Considering the importance and role of both components of species diversity, ecologists want to evaluate and study indicators that examine both components and determine the contribution of each. In this regard, several methods have been proposed for the evaluation and studies of plant diversity. Two major groups of these methods include numerical indices (richness, uniformity, and diversity) and parametric indices (for example, frequency distribution models, category-frequency diagrams, dominance-diversity, and diversity grading curves). In this research, the condition of vegetation cover, frequency, abundance, and diversity of species in three rangelands with light, medium, and heavy grazing intensities in the rangelands of Sarbisheh City is investigated.
 
This research was carried out in the plain rangelands of west of Sarbisheh City located in South Khorasan Province. First, after the field visit and observing the effects of destruction in the soil and vegetation caused by the intensity of livestock grazing and the presence of invasive plants in the studied rangelands, In the whole field of research, The effects of light, medium and heavy grazing (each in three separate plant types) were observed. Sampling in each plant type was done randomly and systematically. The percentage of plant cover and density was evaluated by visual and counting methods, species abundance based on the Ranquier method, species diversity using numerical indices, and species abundance using statistical and ecological models. Kruskal-Wallis non-parametric test was used to compare vegetation characteristics and species diversity in three intensities of livestock grazing. In order to select the best model for fitting the Four species data with statistical models, akaike's information criterion (AIC) was used.
 
The result of the Kruskal-Wallis non-parametric test showed that the intensity of livestock grazing had no significant effect on any of the studied characteristics. Nevertheless, the rangeland under light grazing had a higher coverage percentage and plant density (42.27 % and 28,000 plants per ha) than the rangeland under medium and heavy grazing. According to the Shannon-Wiener index, very low diversity was assessed in two rangeland with light and medium grazing intensity (H=1.78 and H=1.69, respectively), and low diversity was assessed in the rangeland with heavy grazing intensity (H=2.02). Rangeland with moderate grazing intensity is relatively lower in species diversity (H=1.69 and 1-D=0.66) and higher in richness (15 species) and species dominance (D=0.34). Based on the AIC criterion, the results showed that rangelands with light, medium, and heavy grazing intensity were best fitted with the logarithmic series, lognormal series, and logarithmic series models, respectively. In communities where the sampling volume is large, she recommended three logarithmic, Shannon, and Simpson indices, which have low to moderate sensitivity to plot size and are widely used.
 
The results of the present research showed that species abundance patterns change due to the intensity of livestock grazing. By comparing the results of species abundance models in the current research with other research, it can be said that in areas with high species richness, with increasing grazing pressure (from low to high), the distribution of species abundance changes from the normal log series (representing a stable society) to a geometric or log-normal series (representing an unstable society) But in the present research, because the rangeland has a poor condition in terms of species richness and diversity, species abundance distribution models are not a suitable indicator for distinguishing the three intensities of livestock grazing.