Data from: Pivotal effect of early-winter temperatures and snowfall on population growth of alpine Parnassius smintheus butterflies

Geographic range shifts in species’ distributions, due to climate change,
imply altered dynamics at both their northern and southern range limits,
or at upper and lower elevational limits. There is therefore a need to
identify specific weather or climate variable(s), and life stages or
cohorts on which they act, and how these affect population growth.
Identifying such variables permits prediction of population increase or
decline under a changing climate, and shifts in a species’ geographic
range. For relatively well studied groups, such as butterflies, geographic
range shifts are well documented, but weather variables and mechanisms
causing those shifts are not well known. The Holarctic butterfly genus
Parnassius (Papilionidae) inhabits northern and alpine environments
subject to variable and extreme weather. As such, Parnassius species are
vulnerable not only to long-term changes in average conditions but
especially to short-term extreme weather events. We use population growth
estimates for the alpine butterfly, Parnassius smintheus, from 21
populations in the Rocky Mountains of Canada, over a 20-year interval,
combined with techniques of machine learning (randomForests) and
parametric modeling to identify the important weather variables
determining population growth. We do this to determine the seasons and
life-stages of P. smintheus most affected by climate change. Extreme
minimum and maximum temperatures in November, in combination with November
snowfall, affect annual population growth most, more so than do mean
temperatures in November, and more so than weather at any other time of
year. Populations decline both in years with low extreme minimum
temperatures in November, and especially in years with high extreme
maximum temperatures in November, indicating that overwintering eggs are
particularly vulnerable to early-winter weather. Snowfall ameliorates the
negative effects of extreme temperatures, particularly for extreme warm
events. Results provide insight into biological mechanisms by which
over-wintering eggs might be affected by early winter weather. Short-term
extreme weather in November, acting on a single pivotal life-stage (egg)
is a far better predictor of population change of alpine Parnassius
smintheus butterflies than is the general index of climate, the Pacific
Decadal Oscillation (PDO).