The most remarkable migrants: Systematic analysis of the Western European insect flyway at a Pyrenean mountain pass

In 1950, David and Elizabeth Lack chanced upon a huge migration of insects and birds flying through the Pyrenean Pass of Bujaruelo, later describing the spectacle as combining both grandeur with novelty. The intervening years have seen many changes to land use and climate, posing the question as to the current status of this migratory phenomenon, while a lack of quantitative data has prevented insights into the ecological impact of this mass insect migration and into the factors affecting it. To address this, we revisited the site in autumn over a 4-year period and systematically monitored diurnal insect species and numbers. We document an annual mean of 17.1 million day-flying insects from 5 orders moving south, with 'mass migration' events associated with warmer temperatures, the presence of a headwind, sunlight, low windspeed, and low rainfall. Diptera dominated the migratory assemblage and annual numbers varied by more than fourfold with larger annual migration flows associated with higher autumn temperatures in Northwest Europe. Finally, using observed environmental thresholds for migration, we estimate an annual 'bioflow' of at least 14.6 billion day-flying insects migrating south over the whole Pyrenean Mountain range, highlighting the importance of this route for seasonal insect migrants.

1950年，戴维·拉克（David Lack）与伊丽莎白·拉克（Elizabeth Lack）偶然在布哈鲁埃洛比利牛斯山口（Pyrenean Pass of Bujaruelo）观测到大规模昆虫与鸟类迁徙现象，后续将该壮观景象描述为兼具宏伟性与新奇性的奇观。数十年间，当地土地利用与气候均发生诸多变化，这引发了学界对该迁徙现象当前现状的疑问；同时，由于缺乏定量数据，学界始终难以深入解析此次大规模昆虫迁徙的生态影响及其驱动因素。为填补这一研究空白，我们在为期4年的秋季监测周期内重返该研究站点，系统性地对日间飞行昆虫的物种组成与种群数量开展监测。本次研究记录到，年均有1710万只隶属于5个目的日间飞行昆虫向南迁徙，其中“大规模迁徙”事件与气温偏高、存在逆风、日照充足、风速较低及降雨稀少显著相关。双翅目（Diptera）在迁徙昆虫类群中占据主导地位，且年迁徙总数量的波动幅度超过4倍；欧洲西北部秋季气温越高，当年的昆虫迁徙规模便越大。最后，基于观测得到的迁徙环境阈值，我们估算出整个比利牛斯山脉范围内，每年向南迁徙的日间飞行昆虫总生物流量（bioflow）至少达146亿只，凸显了该迁徙路线对于季节性昆虫迁徙的重要意义。