Abstract
Studying species diversity and analyzing of the relationships between plants, environmental factors and disturbance play an important role in studying the dynamics and management of ecosystems. We studied the species diversity patterns in relation to some physiographic factors and grazing along an altitudinal gradient of 1000 meters and at different altitudes with 100-m intervals. The Margalef, Sheldon, and Shannon-Wiener indexes were used to assess richness, evenness and diversity, respectively. Partial Canonical Correspondence analysis was used to investigate the relationship between vegetation, physiographic factors (with an emphasis on altitude) and grazing. The study of the relationship between altitude and grazing showed a significant correlation between these two factors (P <0.01, R2 = 0.55). Results showed that increased species diversity and richness was observed at lower altitudes up to 2200 meters above sea level; however, at the altitude above 2200 m, the species diversity and richness decreased. This result is consistent with the mid-domain effect (MDE) hypothesis. Overgrazing at low altitudes has led to a shift in vegetation toward annual and ruderals species. The areas with intermediate grazing had the highest richness and diversity and the areas with high and low grazing intensity had the lowest richness and diversity. This result is consistent with the intermediate disturbance hypothesis. Species evenness also increased with increasing grazing intensity, although these changes were not significant (P> 0.05). Therefore, the changes in species diversity along ecological gradients cannot be evaluated without considering the grazing intensity.
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