Insectivore diet and abundance determine the contribution of bird species to services and disservices in an agricultural ecosystem

Insectivorous birds provide an essential ecosystem service in agriculture by feeding on arthropods considered pests but can also consume arthropods considered to be natural enemies of such pests. Therefore, depending on the dietary composition of the birds, intraguild predation could outweigh pest control services. This study addressed the agronomic quality, defined as the trade-off between ecosystem services (pest control) and disservices (intraguild predation), of 26 insectivorous bird species in culturally and economically important cider apple orchards in northern Spain. We used DNA-metabarcoding techniques to analyze bird’ diets. First, we examined whether the trophic position of bird species can be inferred from functional traits related to foraging and movement behavior and from the degree of insectivory in their diets. Then, we tested whether bird abundance and trophic position influenced agronomic quality, based on the proportion of arthropod crop pests and natural enemies in their diet. Finally, we combined bird abundance, insectivory and agronomic quality to infer the potential contribution of each bird species to pest control. Bird trophic position was positively related to the degree of insectivory, with this effect being modulated by traits related to body size. The trophic position of birds was inversely related to their agronomic quality. Nevertheless, the agronomic quality only slightly affected the differential contribution of bird species to the whole assemblage effect. Overall, we found the potential of insectivorous bird species to control pests can be estimated based on their trophic position. Yet, in agroecosystems with uneven avian species abundance, the potential contribution of bird species to pest-control services may be driven by their quantitative contribution rather than by the bird’s per capita, qualitative effects. Finally, our results suggest that rare insectivorous birds may have a redundant role in pest control, due to the overwhelming functional dominance of common species.

食虫鸟类在农业生产中提供至关重要的生态系统服务：它们取食被视为农业害虫的节肢动物，但同时也会取食这些害虫的天敌节肢动物。因此，根据鸟类的饮食组成，集团内捕食（intraguild predation）可能会抵消害虫防控服务的效益。本研究针对西班牙北部兼具文化与经济重要性的酿酒苹果园中26种食虫鸟类，分析了其农艺价值——即生态系统服务（害虫防控）与反服务（集团内捕食）之间的权衡。我们采用DNA宏条形码（DNA-metabarcoding）技术分析鸟类的饮食结构。首先，我们探究能否通过与觅食和移动行为相关的功能性状，以及饮食中食虫性的程度，来推断鸟类的营养位阶；其次，我们基于鸟类饮食中作物害虫与天敌的占比，检验鸟类丰度与营养位阶是否会影响其农艺价值；最后，我们结合鸟类丰度、食虫性程度与农艺价值，推断每种鸟类对害虫防控的潜在贡献。研究结果显示，鸟类的营养位阶与食虫性程度呈正相关，且这一关系受体型相关性状的调控；鸟类的营养位阶与其农艺价值呈负相关。不过，农艺价值仅对鸟类群落整体效应的差异化贡献存在轻微影响。总体而言，我们发现可通过营养位阶估算食虫鸟类的害虫防控潜力。然而，在鸟类物种丰度分布不均的农业生态系统中，鸟类物种对害虫防控服务的潜在贡献可能更多取决于其数量贡献，而非单只鸟类的定性效应。最后，本研究结果表明，由于常见物种在功能上占据绝对主导地位，稀有的食虫鸟类在害虫防控中可能仅发挥冗余作用。