The Impact of Peganum harmala L. Invasion on the Density and Species Diversity of Native Plants

Abstract

Given the increased human activity, its destructive impacts on natural resources are no longer confined to specific regions. Among these impacts is the introduction of non-native and invasive plant species. The adverse effects of these species on natural and agricultural communities, human and livestock health, and ecosystem characteristics are significant, leading to continuous research in this field. In this study, the effects of varying frequencies of the invasive plant Peganum harmala L. on the density, composition, richness, diversity, and species dominance of native plants were examined in the rangelands surrounding the integrated livestock farming project for nomadic communities in the Zirkouh region of South Khorasan Province. Livestock presence and changes in plant composition were assessed using photographs taken in 2002, 2012, and 2022. Following the initial assessment with 0% frequency (enclosure area), plots with 15%, 30%, 45%, 60%, and 75% frequencies of the invasive plant were selected, with three representative zones chosen in each area. In each zone, twenty randomly placed four-square-meter plots were established. In each plot, density, plant cover percentage, litter percentage, bare soil and gravel percentage, and rangeland condition classes were determined. Based on the counted plant bases, indices of richness, species diversity (Shannon-Wiener and Simpson indices), species evenness (Pielou index), and species dominance (Simpson index) were calculated. The Jaccard index was employed to assess the species similarity. Statistical comparisons were made using one-way ANOVA and modified LSD means comparison tests. The results showed that the effect of frequency of P. harmala on the percentage of vegetation cover, litter, and soil and gravel, as well as the density of annual native plants in the area was significant (p≤0.05). The highest Shannon-Wiener diversity index (2.48) was calculated in the area with 45% frequency of the invasive plant. The highest species richness was observed in the areas with 15% and 45% frequency of the invasive plant (with 19 and 18 plant species, respectively). Due to the invasion of P. harmala, the density of annual native plants initially decreased and then increased. However, no significant change was observed in the density of perennial native plants. The results indicate that high livestock pressure favors annual plants.