Crop heterogeneity is positively associated with beneficial insect diversity in subtropical farmlands

Increasing crop configurational heterogeneity – smaller crop fields with more field margins – has been repeatedly found to support farmland biodiversity. But research on compositional crop heterogeneity – the number and evenness of crop types – has usually shown only weak effects. However, much of this research has been conducted in large-scale temperate agroecosystems. We examined smallholder subtropical agroecosystems in southern China to assess the effects of crop heterogeneity on beneficial insect biodiversity. In addition to pollinators (bees, apoid and vespid wasps, butterflies), we studied dung beetles and dragonflies/damselflies, which are not usually considered in cropland heterogeneity studies, but are abundant in these multi-functional agroecosystems. We sampled these taxa in 468 transects placed inside 52 farms across three seasons (summer, spring, winter), collecting data on 27,245 insects belonging to 160 species. We found a strong positive effect of crop compositional heterogeneity (measured by Shannon-Wiener index) on dung beetle and dragonfly/damselfly diversity. Bees/wasps and butterflies, conversely, were positively affected by crop configurational heterogeneity (measured by cumulative field margin length). Field margin type, categorized by the structure of the dominant crop types, was consistently an important explanatory variable, with weedy margins having high insect diversity. The presence of a vegetable crop on one side of the field margin, compared to non-vegetable monocultures on both sides, increased diversity in 3/4 taxon-season comparisons made for rice, and 6/9 comparisons made for sugarcane or corn. Synthesis and applications. We demonstrate that crop compositional heterogeneity can support insects that respond to differences among crop types, including taxa that play a key role in nutrient cycling (dung beetles) and natural pest control (dragonflies/damselflies). Incorporating structurally diverse crops into monoculture Asian agroecosystems can reduce the adverse effects these intensive systems have on beneficial insects, and increase crucial ecosystem services.

提升作物配置异质性——即采用更多田边带的小型农田格局——已被多项研究反复证实可助力农田生物多样性维持。但针对作物组成异质性——即作物类型的数量与均匀度——的相关研究，其结果通常仅呈现微弱效应。然而，此类研究大多开展于大规模温带农业生态系统中。我们针对中国南方小农经营的亚热带农业生态系统展开调研，以评估作物异质性对益虫生物多样性的影响。除传粉昆虫（蜜蜂、蜜蜂总科蜂类与胡蜂类、蝴蝶）外，我们还研究了农田异质性研究中通常未被纳入考量的蜣螂与蜻蛉（蜻蜓/豆娘）——这类昆虫在这类多功能农业生态系统中种群数量颇为丰富。我们在52个农场内布设的468条样带上开展了三个季节（夏季、春季、冬季）的采样工作，共收集到隶属于160个物种的27245只昆虫样本。研究结果显示，作物组成异质性（以香农-威纳指数（Shannon-Wiener index）衡量）对蜣螂与蜻蛉的多样性具有显著正向影响。与之相对，蜜蜂/胡蜂类与蝴蝶的多样性则受到作物配置异质性（以累计田边带长度衡量）的正向驱动。以优势作物类型结构划分的田边带类型始终是重要的解释变量，其中杂草丛生的田边带可支撑更高的昆虫多样性。相较于两侧均为非蔬菜单作种植模式的田边带，田边带一侧种植蔬菜的处理，在水稻的3/4类群-季节对比中提升了昆虫多样性，在甘蔗或玉米的6/9类群-季节对比中亦呈现出相同效果。综合与应用：本研究证实，作物组成异质性可助力那些对作物类型差异产生响应的昆虫类群，其中包括在养分循环中发挥关键作用的蜣螂，以及在自然虫害防控中起到重要支撑作用的蜻蛉。在亚洲单作农业生态系统中纳入结构多样的作物，可降低这类集约化农业系统对益虫的负面影响，并提升关键生态系统服务功能。