Effects of grazing on insect communities in the midsection of the northern slope of Tianshan Mountain

The adjustment of grazing intensity is one of the important means of coordinating grassland protection and utilization, and its changes have an important driving effect on the community structure of grassland biodiversity. Insects are one of the important components of grassland ecosystems, and the material cycle and stability of ecosystems are of great significance. This study investigates the structure and diversity of insect communities under different utilization conditions such as grazing prohibition (CK), light grazing, and heavy grazing in the middle section of the northern slope of the Tianshan Mountains, using insect net scanning and Mahalanobis net collection methods to explore the response of insect community diversity to grazing intensity in mountain meadow grasslands. A total of 18 146 insects were collected in this study, belonging to 8 orders and 45 families. The dominant families were Chloropidae, Larvaevoridae, and Muscidae, accounting for 18.77%, 12.20%, and 16.37% of the total number of individuals, respectively. From the perspective of the functional group level of insect communities in mountainous meadows and grasslands, the order of family level numbers is herbivorous insects (44.93%), predatory insects (28.66%), parasitic insects (15.85%), omnivorous insects (7.26%), and saprophytic insects (3.29%). Compared with CK, the total number of insects under mild grazing conditions increased significantly, while the number of herbivorous and predatory insects also increased significantly (P < 0.05). Under heavy grazing conditions, the total number of insects decreased while the number of herbivorous and predatory insects significantly decreased (P < 0.05). However, the number of omnivorous insects under heavy grazing conditions was higher than that under light grazing conditions, but there was no significant difference compared to CK. As grazing intensity increases, the Shannon Wiener index of herbivorous, predatory, parasitic, and omnivorous insects first increases and then decreases, and the Shannon Wiener index is significantly higher under light grazing conditions than CK and heavy grazing, while the Shannon Wiener index of corrosive insects is the highest under heavy grazing conditions. The Simpson index of herbivorous, predatory, and parasitic insects showed a trend of first increasing and then decreasing under all three grazing conditions, with the highest value under mild grazing conditions. There was no significant difference in the Peilou index between herbivorous, predatory, and omnivorous insects (P < 0.05), while the Peilou index of saprophytic insects showed a trend of first decreasing and then increasing, with CK > HG > LG (P < 0.05). The results of RDA redundancy analysis indicate that insect community diversity is significantly correlated with plant density, plant Margalef, Shannon wiener, and Simpson index. In summary, it can be seen that under grazing prohibition and grazing conditions, the number and diversity of insect community structures tend to decrease, while light grazing promotes an increase in the diversity of insect communities and their functional groups.