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.