Experimental Grazer Exclusion Increases Pollination Reliability and Influences Pollinator-Mediated Plant-Plant Interactions in Tibetan Alpine Meadows

1. Co-flowering plant species often interact through shared pollinators, with effects ranging from positive (facilitation) to negative (competition). It remains unclear how this variation relates to variation in floral density, floral trait distinctiveness, and local environmental conditions. We studied the effect of grazer exclusion, a proposed local management strategy, on pollinator-mediated plant-plant interactions in heavily degraded alpine meadows of the Qinghai-Tibet Plateau. 2. We studied the effect of experimental grazer exclusion on plant reproduction and pollinator-mediated reproductive interactions quantified through pollen transfer networks. We also explored potential mechanisms of pollinator-mediated interspecific pollen transfer and its effect on plant reproductive fitness, including local floral abundance and floral trait distinctiveness among co-flowering species. 3. Grazer exclusion led to greater pollen deposition onto stigmas. The overall quantitative effects of pollinator-mediated interspecific interactions on the receptor species were mainly positive (facilitative) or neutral (with no detectable effect). The frequency of positive relative to negative quantitative effects for pairwise donor-receptor pairs tended to increase after grazer exclusion. Plants with floral traits similar to those of local ‘hub species’ appeared to benefit from pollinator-mediated interactions. 4. Our results suggest an overall positive effect of excluding grazers during the plant growing season on plant reproduction. Facilitative species interactions predominate in harsh environments such as the alpine, and the benefits of pollinator-mediated interactions among plants seemed to exceed the cost of conspecific pollen loss associated with pollinator sharing. This suggest that species invasions into alpine plant communities, an expected consequence of climate change, may not necessarily have negative effects on the reproduction of resident plant species.