Tree Species Diversity, Canopy Height, and Soil Nutrient Availability Shape Leaf Insect Herbivory in Zhoushan...

Plant-insect interactions regulate ecosystem processes and productivity. Most studies have focused on the effect of plant diversity on insect herbivory in grasslands, croplands, and natural forests. However, the direction of this effect remains controversial. Unlike these ecosystems, island ecosystems possess distinct geographical features and provide natural experimental settings for studying trophic interactions. Nevertheless, the effects of plant diversity, island characteristics, and their interactions on insect herbivory in such environments have scarcely been explored. To test the relationships between plant diversity and total herbivory, as well as the herbivory of specific damage types, and to find the key determinants of insect herbivory in island ecosystems, 24 sample plots were established on 24 representative islands in 2019 in Zhoushan Archipelago, China. To ensure the reliability and repeatability of the results while minimizing inter-annual variation in insect herbivory, 36 sample plots were set up on 30 different islands in 2021; all previous plots of 2019 were included in those of 2021. This study investigated insect herbivory in both years and measured plant species richness, climate factors, tree height, and soil nutrient availability in 2021. Polynomial (quadratic) regression was employed to examine the relationship between insect herbivory and plant diversity, and multiple linear regression (MLR), the stepwise regression model, and variation partitioning analysis (VPA) were used to identify significant patterns of insect herbivory. Our findings showed that tree species diversity was positively correlated with total herbivory and leaf mining in both 2019 and 2021, but only positively associated with sucking in 2019 and chewing in 2021. The extent of plant diversity effects varied among different damage types. In 2019, tree species diversity, tree height, soil nutrition, and their interactions collectively explained 60% of the variation in sucking. Tree species diversity, soil nutrition, and their interaction accounted for 32% of the variation in mining. But there were no significant relationships between climate factors and insect herbivory in this study. Synthesis. The results of this study partially support our initial hypothesis. Specifically, plant species diversity increased insect herbivory on islands, indicating that herbivorous insects on islands are almost generalists. However, the extent of effects among different damage types was inconsistent, which may stem from inter-annual competition among herbivorous insect species. The discontinuous effects on sucking and chewing between 2019 and 2021 may also be ascribed to this. Additionally, in 2019, sucking was jointly regulated by plant species diversity, tree height, soil nutrition, and their interactions, with community structure and total nitrogen emerging as particularly important factors. For mining in the same year, plant diversity, soil nutrition, and their interaction were significantly correlated, indicating that soil environments with high nitrogen but low carbon and phosphorus favor mining species. However, climate factors showed no significant relationships with insect herbivory in both years, suggesting that microclimates may mediate external climate effects. These findings provide new insights into leaf insect herbivory patterns in island ecosystems.