A broader flight season for Norway's Odonata across a century and a half

As global climate continues to change, so too will phenology of a wide range of insects. Changes in flight season usually are characterised as shifts to earlier dates or means, with attention less often paid to flight season breadth or whether seasons are now skewed. We amassed flight season data for the insect order Odonata, the dragonflies and damselflies, for Norway over the past century-and-a-half to examine the form of flight season change. By means of Bayesian analyses that incorporated uncertainty relative to annual variability in survey effort, we estimated shifts in flight season mean, breadth, and skew. We focussed on flight season breadth, positing that it will track documented growing season expansion. A specific mechanism explored was shifts in voltinism, the number of generations per year, which tends to increase with warming. We found strong evidence for an increase in flight season breadth but much less for a shift in mean, with any shift of the latter tending toward a later mean. Skew has become rightward for suborder Zygoptera, the damselflies, but not for Anisoptera, the dragonflies, or for the Odonata as a whole. We found weak support for voltinism as a predictor of broader flight season; instead, voltinism acted interactively with use of human-modified habitats, including decrease in shading (e.g., from timber extraction). Other potential mechanisms that link warming with broadening of flight season include protracted emergence and cohort splitting, both of which have been documented in the Odonata. It is likely that warming-induced broadening of flight seasons of these widespread insect predators will have wide-ranging consequences for freshwater ecosystems.

随着全球气候持续变化，诸多昆虫类群的物候亦将随之改变。当前学界多将昆虫飞行季（flight season）的变化特征概括为发生日期或发生期均值的提前，却较少关注飞行季跨度（flight season breadth），亦或是当前飞行季分布是否出现偏态（skew）。我们汇编了过去一个半世纪以来挪威蜻蜓目（Odonata）昆虫（涵盖蜻蜓与豆娘）的飞行季数据，以探究飞行季变化的模式。本研究借助纳入了年度调查投入波动不确定性的贝叶斯分析（Bayesian analyses），估算了飞行季均值、跨度与偏度的变化情况。我们重点聚焦飞行季跨度，假设其将与已被证实的生长季（growing season）扩张趋势相契合。本研究探讨的特定机制为化性（voltinism，即每年发生的世代数）的变化——随气候变暖，化性通常会升高。研究结果显示，有充分证据表明飞行季跨度有所增加，但关于飞行季均值偏移的证据则薄弱得多，且此类均值偏移多呈现延后趋势。均翅亚目（Zygoptera，豆娘）的飞行季分布偏度已呈右偏，但差翅亚目（Anisoptera，蜻蜓）以及整个蜻蜓目类群均未出现该变化。我们发现化性作为飞行季跨度扩大的预测因子的支持证据较弱；相反，化性与人类改造生境的利用存在交互作用，这类生境包括荫蔽程度降低（例如因木材采伐导致的荫蔽减少）。其他将气候变暖与飞行季跨度扩大关联起来的潜在机制包括延长的羽化期以及同生群分裂（cohort splitting），这两种现象在蜻蜓目中均有记载。由气候变暖引发的这类广布昆虫捕食者飞行季跨度扩大现象，很可能会对淡水生态系统（freshwater ecosystems）造成广泛的生态影响。