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）偶然发现一场规模浩大的昆虫与鸟类迁徙群途经布哈鲁埃洛比利牛斯山口（Pyrenean Pass of Bujaruelo），随后将此番盛景描述为兼具恢宏格局与新奇特质的奇观。后续数十年间，土地利用方式与气候环境均发生诸多变化，由此引发了关于该迁徙现象当前现状的探究议题；同时，定量观测数据的缺失使得学界难以深入了解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，我们于秋季重返该研究位点，开展了为期4年的系统性监测工作，对日间飞行昆虫的物种组成与种群数量进行了标准化记录。我们记录到，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"事件与温暖气温、逆风条件、日照充足、低风速及低降雨量显著相关。双翅目（Diptera）在迁徙昆虫类群中占据绝对优势，年迁徙规模的波动幅度超过四倍；欧洲西北部秋季气温越高，当年的昆虫迁徙总量往往越高。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（Pyrenean Mountain range）范围内，每年向南迁徙的日间飞行昆虫总"生物流"至少达146亿只，凸显了这条迁徙路线对季节性昆虫迁徙者的重要性。