Systematic data for phenological analyses and population trends
To analyze phenological change in adult flight periods of Lepidoptera,
we use c. 20 years of systematically collected data from butterfly
transects and moth traps. For butterflies, we combined data from two
butterfly surveys conducted in agricultural landscapes in Finland during
1999-2017 (Fig. 2): 1) volunteer transects with varying length and
sampling frequency, and 2) professional transects with standardized
length and sampling frequency. For moths, we use data that have been
collected in the Finnish national moth monitoring scheme (Nocturna)
during 1993-2017 (Leinonen et al. 2016, 2017).
The raw data and how they were refined to fit the purposes of
phenological analyses and population trend estimation in this study are
described more in depth in Text S1. For example, to ensure enough
phenological resolution, we removed all sampling events in the moth data
with a sampling interval longer than ten days. All the observation dates
for butterflies and moths were converted into Julian dates (day of
year), to represent the timing of adult flight. To ensure sufficient
data for each analyzed species per year, we included only species with
observations from at least ten years during the 19- (butterflies) and
25- (moths) year study periods and with at least 30 observations from
each of the included years. This procedure left us with a total of 289
species. As a consequence of this conservative inclusion of data, our
set of study species does not contain rare species. Lastly, to exclude
the effect of potentially differing phenological timing in recently
colonized areas, we also excluded records of species from sites located
further north of the estimated NRB for T1 (see section
below and Text S2). The final data for phenological analyses and
population trend estimations consisted of 725 106 and 355 970 abundance
observations of the 289 species, respectively.