Dietary overlap and co-existence of sympatric wild yak, Tibetan wild ass and Tibetan antelope in Arjin Shan National Nature Reserve, Xinjiang Province, China
Authors: Shi Jianbin , Lu Feiying , Li Xiaowen , Zhang Zihui , Su Xukun , Dong Shikui , Xu Huadong , Zhang Xiang 
Published: July 2016

Context
Wild yak (Bos grunniens), Tibetan wild ass (Equus kiang, also known as kiang) and Tibetan antelope (Pantholops hodgsoni, also known as chiru) have been sympatric on the Tibetan Plateau for numerous generations. There is concern over potential dietary competition among them, particularly in winter when their forage resources are scarcer, but little is known about their dietary interactions.

Aim
We aimed to determine diet composition and dietary overlap of the three ungulate species, and to assess the extent of potential dietary competition among them in order to enhance our understanding of mechanisms underlying their co-existence. We predicted that the three species would widen their choice of forages in winter (December) compared with summer (June–July), and that there would be diet segregation among them, particularly in winter when food resources were scarcer.

Methods
We collected fresh faecal samples of the three species and reference forage samples from the field during our transect line surveys in the summers of 2012 and 2013, and winter of 2012 in Arjin Shan National Nature Reserve (ANNR) of Xinjiang Province, China. Slides of faecal samples were prepared for microhistological analysis, and plant fragments in faecal samples were identified under microscope. Relative percentage frequency of diet plant species, diet breadth and dietary overlap were calculated.

Key results
The three species consumed similar forage plants, but varied greatly in their percentage use of the available forages. Wild yak mainly fed on sedges in summer and graminoids in winter, supplemented with forbs. Kiang consumed mainly forbs in summer and browse in winter, and chiru mainly fed on graminoids and forbs in summer. Wild yak had the widest choice of forages, and kiang had the narrowest one in summer, but both wild yak and kiang became less selective in their food choice in winter, resulting in higher diet breadth. The dietary overlaps among them, as represented by Pianka’s index, were generally low, ranging from 0.26 between chiru and kiang to 0.51 between wild yak and kiang. The dietary overlap between wild yak and kiang decreased from summer (0.51) to winter (0.35), suggesting an increased dietary divergence.

Key conclusion
Our study reveals that the three species are ‘opportunistic feeders’, able to cope with coarse forage resources. There is dietary segregation among them as indicated by low dietary overlap, suggesting risk of potential competition for diets. We suggest that competition may have shaped their dietary divergence, contributing to their co-existence in ANNR.

Implication
Although there is dietary segregation among the three species in ANNR, it is not possible to make definitive conclusions about interspecific competition from these results. Further studies are needed to investigate if there is spatial and habitat segregation among them, which may facilitate their dietary divergence and thus contribute further to their co-existence in ANNR. Continued efforts are also needed to monitor their population dynamics and status of forage resources in order to prevent overgrazing.