Disentangling the ecological complexities of tripartite plant-herbivore-pathogen interactions in Himalayan grasslands along environmental gradients.

Dynamics of tripartite interactions between plants, insect herbivores and fungal pathogens in grasslands remain poorly understood, despite its substantial implications for regulating community structure under global environmental change. Hence, we investigated insect herbivory and fungi-induced leaf damage in temperate Himalayan grasslands along an elevation gradient. We also assessed the extent of this damage to different plant functional groups (grasses, forbs, and legumes), under grazing pressure and nutrient enrichment conditions. We found that at low-elevation sites legumes suffered highest herbivore damage and grasses showed greater damage by fungal pathogens. The damage by insect herbivores sharply declineed with elevation in grasses and legumes, but not in forbs. While fungal pathogen damage was found in all functional groups, it did not show significant variation across elevations in any of the groups. Additionally, damage caused by both herbivores and fungal pathogens increased significantly with the standing crop and this relationship was stronger in sites that are annually mown compared to the grazed sites. Nutrient enrichment had differential impacts on leaf damage across functional groups, with increased insect herbivory observed in grasses and forbs but not in legumes. Conversely, pathogen damage showed a marked increase in grasses and legumes, highlighting the functional group-specific responses to nutrient input. Overall, this study highlights the functional group-specific responses to environmental factors, such as elevation and nutrient enrichment, thus indicating the need for factoring these in the grassland management models. It also indicates that low-intensity or rotational grazing in grasslands could help sustain the critical top-down biotic interactions.