Detailed Material and methods
Study area and data
Terrestrial arthropods were collected using pitfall traps weekly through the activity
season usually from mid-May to early-September, depending on snowmelt and snowfall, between
1996 and 2013 as part of the Zackenberg Basic Monitoring Program [15]. Each pitfall trap consisted
of a yellow plastic cup 10 cm in diameter buried flush to the ground to collect flying and surfacedwelling arthropods. Each plot consisted of eight pitfall traps during 1996–2006 and four traps
thereafter in a gridded design (10m × 20m, 5 × 20m after 2006). The window trap plot consisted of
two two-chambered traps that were placed perpendicular to one another. The window trap plot (plot
1) and the pitfall trap plots in arid heath (plot 5), mesic heath (plots 3 & 5) and fen (plot 2) were
operational during the entire study period 1996–2009, whereas one snow-bed plot (plot 6) and one
additional arid heath plot (plot 7) were only operated during the periods 1996–1999 and 1999–2009,
respectively (see [15] of main text for more details).
Ambient temperature was recorded hourly throughout the study period by an
automated weather station located at 2m above ground in the center of the sampling area [see S1
and for further climate information]. Temperatures at Zackenberg rise dramatically during the
month of May and peak during the months of June and July, declining again during August [S2].
Snowmelt is monitored daily during spring and was assessed as the first day of the year when less
than 50% of the area in the sampling plots was snow covered and was averaged over plots. Fen,
grassland, Salix arctica snow-bed, Cassiope tetragona heath, and Dryas spp. heath are the most
dominant vegetation types, but Vaccinium spp. are also found in the study area [S3].
Butterfly species
In species such as these with holometabolous (development with egg, larval, pupal
and adult stages) life cycles, growth and ultimate size is achieved during the larval feeding stage.
These species, like many others in the Arctic, reproduce and die upon reaching their adult stage
during their final summer; hence, adult size is entirely the result of larval dependent growth.
Boloria chariclea has been documented feeding on species of Dryas and Salix in a roughly 2:1 ratio
and occasionally on Vaccinium [S4], while Colias hecla is thought to feed upon Vaccinium and
Salix in a 1:1 ratio sometimes feeding on Dryas [S5]
Data analysis
The R package AICcmodavg, [S6] was used for model selection with the top models
being based upon Akaike weights. The R package lme4 was used to test the random effects of day
of year when specimens were collected and plot [S7]. In the ‘individuals as replicates’ models we
calculated the conditional and marginal coefficients of determination [S8] using package MuMIn
[S9] when random effects (day of year and plot) were included in the models using individuals as
replicates where each individual, rather than the annual mean, serves as the statistical unit of
observation. This enabled us to test for the effects of within season variation using day-of-year
(rounded to the nearest 7th day) that the specimen was collected. To test that the butterflies were
measured consistently, we randomly measured a subset of 10 B. chariclea 10 times and calculated
repeatability (results presented in electronic supplementary material) using package ICC [S10].
S1. Hansen BU et al. 2008 Present-day climate at Zackenberg. Adv. Ecol. Res. 40, 111–149. (doi:
10.1016/S0065-2504(07)00006-2)
S2. Høye TT, Forchhammer MC 2008 The influence of weather conditions on the activity of higharctic arthropods inferred from long-term observations. BMC Ecology 8, 8 (doi:10.1186/1472-67858-8)
S3. Elberling B et al. 2008 Soil and plant community-characteristics and dynamics at Zackenberg.
Adv. Ecol. Res. 40, 223–248. (doi: 10.1016/S0065-2504(07)00010-4)
S4. Roslin T, Wirta H, Hopkins T, Hardwick B, Várkonyi G. 2013 Indirect interactions in the high
arctic. PLOS ONE 8, e67367. (doi:10.1371/journal.pone.0067367)
S5. Hopkins T. 2012 An extended food web from Greenland: Adding birds, spiders and plants to a
parasitoid-lepidopteran web. MSc dissertation, University of Helsinki, Finland.
S6. Mazerolle MJ. 2014 AICcmodavg: Model selection and multimodel inference based on
(Q)AIC(c). R package version 2.0-1. http://CRAN.R-project.org/package=AICcmodavgR package
version 2.0-1. http://CRAN.R-project.org/package=AICcmodavg
S7. Bates D, Maechler M, Bolker B, Walker S. 2014 lme4: Linear mixed-effects models using
Eigen and S4. R package version 1.1-5. http://CRAN.R-project.org/package=lme4
S8. Nakagawa S, Schielzeth H. 2013 A general and simple model for obtaining R2 from generalized
linear mixed-effects models. Methods Ecol. Evol. 4, 133-142. (doi:10.1111/j.2041210x.2012.00261.x)
S9. Bartoń K. 2014 MuMIn: Multi-Model Inference. R package version 1.12.1.
http://CRAN.R-project.org/package=MuMIn
S10. Wolak ME, Fairbairn DJ, Paulsen YR. 2012 Guidelines for Estimating Repeatability. Methods
Ecol. Evol. 3:129-137 (doi :10.1111/j.2041-210X.2011.00125.x)