Pastoralism in the highest peaks: Role of the traditional grazing systems in maintaining biodiversity and ecosystem function in the alpine Himalaya

as Orchidaceae and Asteraceae (especially Anaphalis lactea and Lentopodium leontopodioides) in grazed plots than ungrazed plots. Second, grazing results in the reduction in abundance of dominant species by selectively removing palatable species [67]. This explains higher PD for graminoid species (that are palat- able) in ungrazed areas. The results show that graminoid species represent 78.19% of the total PD in ungrazed quadrats as compared to 44.34% in grazed quadrats. Graminoid species of Cyperaceae and Poaceae families were more palatable than Juncaceae which was grazed after most palatable species were eaten. Additionally, higher PD of less palatable families such as Asteraceace, Primulaceae and Rosaceae (dominated by Potentilla spp) was found in grazed plots. Third, grazing can increase spatial heterogeneity thus increasing habitat diversity. Free ranging livestock may modify soil properties directly by recycling nutrients and concentrating it from a larger area into small patches through their dung and urine deposition [64,80–82]. Grazing may also affect soil properties indirectly through their forage preference and grazing intensity. This may selectively affect the survival of some plants that affect soil properties through their rhizosphere [83,84]. Grazing mediated dispersal, foraging preferences and spatial heterogeneity would increase niche breadth and overlap and may promote patches of rarer species thus increasing diversity. A strong positive correlation between productivity and rainfall has been well documented

across the globe [64,85–87]. In the study site monthly productivity in grazed plots were closely associated with rainfall. The monsoon season in the region starts in May, peaking in July and gradually ending in September and October. Peak ANPP in lower and middle elevation quad- rats was reached in July and gradually tapered off with the receding monsoon winds (Fig 2K). The monsoons gradually travels northwards, towards the higher elevation explaining why the higher elevation quadrats showed a slight delay in peak ANPP (in August) (Fig 2K). Interest- ingly the migration pattern of the local communities is closely associated to temporal trends in productivity with the Lachenpas moving up to the lower and middle zones in June and July and the Dokpas moving down to the lower elevations (study design’s higher zones) around mid-July. Higher ANPP in grazed plots suggests grazing mediated species diversity is an important driver of ANPP. Relationship between species diversity and ANPP A strong positive correlation between diversity and productivity (Fig 2L) suggests higher bio- diversity is an important factor in driving ecosystem functions. The results demonstrating that biomass production increased with diversity conforms to numerous studies from other parts of the world [88–91]. There are three explanations proposed based on experimental studies to understand the role of diversity on productivity. Firstly, complementarity effects occur when more diverse communities facilitate greater niche partitioning allowing for a more complete

utilization of resources resulting in increased biomass production [88,90,92]. Secondly, facili- tative effects of some species promote the establishment and survival of other co-occurring species influencing biomass production [89,93]. Thirdly, sampling or selection effects suggest- ing that more diverse plots have a higher probability of containing species with high biomass [89,91,94,95]. The three effects are not mutually exclusive, rather a combined influence of complementarity, facilitative and selection effect of biodiversity influence biomass in a community. Studies on the impact of grazing on ecosystems have shown mixed results. Negative effects of grazing have been attributed to high grazing intensity owing to overgrazing and high stock- ing rates or to low grazing intensity including the abandonment of grazing [2,21,22,64,65,96]. The general consensus is that a light to medium grazing intensity is most beneficial [2,64,66]. PLOS ONE Role of the traditional grazing systems in maintaining biodiversity and ecosystem function in alpine Himalaya PLOS ONE | https://doi.org/10.1371/journal.pone.0245221 January 7, 2021 11 / 19