بررسی تاثیر مدیریت چرای دام بر ترکیب و شاخص‎های مکانی تنوع گونه‌ای (مطالعه موردی: مراتع شهرستان بروجن، چهارمحال و بختیاری)

مدیریت چرای دام یکی از نمودهای اصلی مدیریت جامع حوزه‌های آبخیز بوده به‌طوری‌که بدون آگاهی از اثرات چرای دام بر تنوع زیستی، مدیریت پایدار حوزه‌های آبخیز میسر نمی‌باشد. به همین دلیل آگاهی از تغییرات تنوع و ترکیب گیاهی تحت آشفتگی‏های طبیعی از قبیل چرای دام برای مدیریت بهینه اکوسیستم از اهمیت بالایی برخوردار است. تحقیق حاضر با هدف بررسی تاثیر شدت‎های مختلف چرای دام (کم، متوسط و زیاد) بر مولفه‏های مکانی تنوع زیستی (آلفا، بتا و گاما)، اجزای تنوع بتا شامل روگشت و الگوی آشیانه‏ای و ترکیب گیاهی در مراتع بروجن در استان چهارمحال و بختیاری انجام گردید. نمونه‎برداری در قالب طرح تصادفی - سیستماتیک و با استفاده از 60 پلات 2×2 متری در سه منطقه معرف در سه شدت چرای دام (سبک، متوسط و سنگین) انجام گردید. تجزیه و تحلیل داده‎ها با استفاده از بسته آماری vegan و betapart در نرم‎افزار r انجام شد. نتایج نشان داد که افزایش شدت چرای دام موجب کاهش معنی‎دار مولفه‎های تنوع آلفا، بتا و گاما می‎شود (p-value < 0.01) به‌طوری‌که بیشترین مقدار مولفه‏های تنوع آلفا، بتا و گاما، در شدت چرای کم به‌ترتیب 11.77، 7.22 و 19 و در چرای شدید به‌ترتیب برابر با 3.44، 3.55 و 7 بود. همچنین نتایج این تحقیق نشان داد افزایش شدت چرای دام موجب افزایش معنی‏دار روگشت گونه‏ای خواهد شد که نشان‌دهنده حذف و جایگزینی گونه‏های گیاهی است در حالی‌که افزایش چرای دام تاثیر معنی‏داری بر مولفه الگوی آشیانه‏ای تنوع بتا نداشت. نتایج آزمون nmds نشان داد که چرای دام در شدت‏های مختلف باعث ایجاد اختلاف معنی‎داری در ترکیب جوامع مورد بررسی شده است. نتایج این تحقیق نشان دادند به‌کارگیری شدت چرای سبک می‌تواند به‌عنوان ابزار مدیریتی مناسبی در راستای حفظ و بهبود تنوع زیستی مورد استفاده قرار گیرد.

investigating the effect of animal grazing management on composition and spatial diversity indices (case study: broujen rangelands, charmahal and bakhtiari)

livestock grazing management is one of the main manifestations of integrated watershed management. lack of awareness of the effects of livestock grazing on biodiversity makes sustainable watershed management impossible. therefore, awareness of the variation in plant diversity and composition under natural disturbances such as livestock grazing is important for optimal ecosystem management. this study aims to investigate the effects of different grazing intensity (low, moderate, and high) on spatial diversity components (α-diversity, β-diversity and γ-diversity), turnover and nestedness components of β-diversity and species composition in semi-steppe broujen rangelands in chaharmahal and bakhtiari province. vegetation sampling was conducted based on a random-systematic sampling design using 60 plots of 2*2m in three grazing intensities including low, moderate and high. statistical analysis was performed in r software using “vegan” and “betapart” packages. the results showed that an increase in grazing intensity could significantly (p-value < 0.01) decrease α-, β- and γ-diversity indices, which, in low grazing intensity, were 11.77, 7.22, and 19, respectively, while in high grazing intensity they were 3.44, 3.55, and 7, respectively. in addition, an increase in grazing intensity could significantly (p-value < 0.05) increase the turnover component of β-diversity due to species replacement, while there was no significant effect for nestedness components of β-diversity. the nmds analysis results showed that different animal grazing could drive a significant deviation in plant community composition. the results of this study suggest that the light grazing intensity could be used as a suitable management tool in order to maintain and improve plant biodiversity.extended abstract1. introductionvegetation is one of the main foundations in watersheds and its proper and principled management has an important role in watershed management. considering the effects of livestock grazing on vegetation and choosing the appropriate livestock grazing intensity are among the main manifestations in integrated watersheds management. therefore, without knowledge of livestock grazing effects on biodiversity, sustainable management of watersheds is not possible. so, it is so important to be aware of changes in plant diversity and composition under natural disturbances such as livestock grazing for optimal ecosystem management.2. materials and methodsthis study aimed to investigate the effect of different livestock grazing intensities (low, medium and high) on spatial diversity indices (alpha, beta, gamma), beta diversity components including turnover and nestedness, and plant composition in broujen rangelands in chaharmahal and bakhtiari province (mid-june 2017). sampling was performed based on a random-systematic sampling design using 60 plots of 2 × 2 m in three representative areas in three livestock grazing intensities at the peak season of plant growth. livestock grazing intensity was determined based on distance from watering points (omidipour et al., 2021). in this method, the areas near the watering points (distance less than 200 meters) as severe grazing (high grazing intensity) area, the area with a distance of 200 to 800 meters from the watering points as the medium grazing intensity and the area with a distance of more than 800 meters from the watering points was selected as a low-intensity area (zhao et al., 2007). calculation of diversity indices was done in vegan statistical packages (oksanen et al., 2018) and partitioning of beta diversity into their components (turnover vs. nestedness) was performed using betapart statistical package in r software (baselga and orme, 2012). the effect of livestock grazing on the alpha, beta and gamma diversity indices as well as turnover and nestedness component of beta diversity were investigated based on one-way analysis of variance (anova) and their mean comparison using duncan’s test. to evaluate the effect of livestock grazing on the plant structure and composition of the studied communities, the non-metric multidimensional scaling test (nmds) was used in the vegan statistical package in r software (oksanen et al., 2018).3. resultsthe results indicated that all diversity indices (alpha, beta and gamma) were significantly affected by grazing intensity so increasing the livestock grazing intensity causes a significant reduction (p <0.01) of alpha, beta and gamma diversity indices. the highest values ​​of alpha, beta and gamma diversity were 11.77, 7.22 and 19 in low grazing intensity and 3.44, 3.55 and 7 in the region with high grazing intensity, respectively. partitioning of beta diversity into their component showed that the turnover has a greater role than the nestedness component in deriving beta diversity. the results also showed that increasing the livestock grazing intensity will significantly increase the turnover, which indicates the removal and replacement of plant species while increasing livestock grazing did not have a significant effect on the nestedness component of beta diversity. finally, the results of the nmds test showed that different livestock grazing intensities caused a significant difference in the studied communities’ composition so the three grazing areas in the direction of the first axis were separable from each other.4. discussion and conclusionthe results of this study showed that livestock grazing is one of the main factors affecting plant diversity and composition. it is one of the most important factors in rangeland dynamics that has a negative effect on diversity by increasing the livestock grazing intensity. light and medium grazing create empty spaces (gaps) where other annual plant species can establish. livestock grazing can increase the number of species in the area by reducing inter-and intraspecific competition between plants at low intensities. also, the higher amount of turnover in high grazing intensity indicates livestock grazing effect on removal and replacement of most plants under this grazing intensity. therefore, using light grazing intensity can be used as a suitable management tool to preserve and improve biodiversity.