The diet composition and trophic niche of main herbivores in the Inner Mongolia Desert steppe

In recent years, degradation of the natural grassland areas of Inner Mongolia has been particularly severe. Many factors have contributed to grassland degradation, including frequent grazing, trampling and damage by herbivores. These factors have not only changed the structure of the plant communities and plant species composition; they have also affected community succession. To examine the effects of grassland degradation on the dietary composition and trophic niches of the main herbivores of the Inner Mongolian desert steppe (Ovis aries, Spermophilus dauricus, Oedaleus asiaticus), we measured changes in the dietary composition and trophic niches occupied by these herbivores under different intensity grazing regimes in June-July 2003 using an n-alkane technique. The results showed that the richness of Stipa klemenzii-dominated communities decreased significantly with increased grazing intensity, whereas the proportion of Salsola collina in the community increased. Salsola collina was the main food source for Ovis aries, Spermophilus dauricus and Oedaleus asiaticus. Six to eight plant species were ingested by Ovis aries but Salsola collina made up the largest proportion of its diet. The other main species ingested by Ovis aries were Allium bidentatum,Cleistogenes songorica and Allium polyrhizum. Five to seven plant species were ingested by Spermophilus dauricus in Stipa klemenzii-dominated communities, but Allium bidentatum and Salsola collina were consumed to a greater extent than other species. Spermophilus dauricus fed less selectively on Salsola collina with increased levels of grazing intensity and more selectively on Stipa krylovii. Overall, Spermophilus dauricus fed most selectively on Allium bidentatum, which had the highest selectivity index score. Five plant species were ingested by Oedaleus asiaticus, but Cleistogenes songorica and Salsola collina constituted the largest proportion of its diet. The selectivity indices of Oedaleus asiaticus for Salsola collina and Allium polyrhizum were lower with increased grazing intensity, while the selectivity indices for Cleistogenes songorica and Stipa krylovii increased. The selectivity index of Oedaleus asiaticus for Allium bidentatum was highest at moderate grazing intensities. Analysis of variance tests on grazing intensity, herbivore identity and plant species factors revealed significant differences between each factor that we examined (P < 0. 05) and significant (P < 0. 01) interactions between any two factors or among the three factors. The proportion of plant species ingested by herbivores in ungrazed plots was significantly (P < 0. 05) lower than that in grazed plots, but there was no significant difference between moderate grazing and heavy grazing intensity plots. The proportion of plant species ingested by Ovis aries was significantly (P < 0. 05) lower than that ingested by Spermophilus dauricus and Oedaleus asiaticus, but multiple comparisons of the diets of different herbivores showed that there was no significant difference between Spermophilus dauricus and Oedaleus asiaticus. Trophic niche breadth of the three herbivores became increasingly narrow with increasing grazing intensity. There was a great deal of overlap among the three herbivores with the greatest trophic overlap occurring between Ovis aries and Spermophilus dauricus. There was also considerable overlap in the diets of Ovis aries and Oedaleus asiaticus, Spermophilus dauricus and Oedaleus asiaticus. In summary, heavy grazing on the Inner Mongolian desert steppe changed a Stipa klemenzii-dominated community into a Salsola collina-dominated community. This succession led to the degradation of the grassland, and to fierce competition for resources among the three main herbivorous species. Therefore, controlling pests and mice population densities to prevent outbreaks is important for grazing livestock and food safety.