Journal of Biogeography
SUPPORTING INFORMATION
Latitudinal shift in thermal niche breadth results from relaxed selection on heat
tolerance during a climate-mediated range expansion
Lesley T. Lancaster, Rachael Y. Dudaniec, Bengt Hansson and Erik I. Svensson
Appendix S1 Supplementary methods.
Additional information on sampling methods
We collected wild-caught, adult, individual Ischnura elegans from Swedish populations for
thermal tolerance experiments in June–early August 2012 and June–late July 2013. In 2012
we captured individuals from 15 populations in southern Sweden (mean latitude 55.73°, SD
0.22°), which is representative of the northern part of the species’ core ancestral range
(hereafter the core region). In 2013, we captured adult individuals from 11 populations at the
leading edge of the species’ range in central Sweden (hereafter the edge region; mean latitude
59.61°, SD 0.15°).
We aimed to match the core and edge regions for the distance between populations and the
location of each study region in reference to the coastline, in order to minimize non-climatic
habitat differences between regions (Fig. 1 in main text). In both core and edge regions,
individuals were captured using butterfly nets between 09:00 h and 13:00 h, during which
time we visited multiple populations on each day. Individuals were collected from the
shorelines of large ponds and lakes, characterized by low, patchy, often disturbed, vegetation,
and shallow, sandy, reed beds. Thus microclimatic variation and opportunities to
thermoregulate behaviourally were similar in both regions, although unmeasured differences
in biotic interactions and microclimates between regions may underpin some aspects of
thermal-tolerance evolution and range expansion that were beyond the scope of this study
(Sears et al., 2011).
Captured individuals were then transported to Lund University in Skåne (core region) or the
Swedish University of Agricultural Sciences’ Grimsö Field Station in Västmanland (edge
region) for thermal experiments. In transit, the damselflies were kept cool in mesh cages and
provided with moistened cotton wool as a water source.
In total, we sampled damselflies on 61 unique days covering the adult flying season, from 31
May to 3 August in 2012 and from 18 June to 27 July in 2013. Near-daily trials ensured
adequate sample sizes for each population and spread sampling across the entire adult flying
season in each region (Fig. S1).
Core populations were sampled primarily in 2012, and edge populations were sampled only
in 2013. We repeat-sampled nine of the 15 core populations in 2013 and compared those
individuals’ thermal tolerances to tolerances of individuals sampled over the equivalent time
period in 2012, to estimate any possible yearly variation in physiological tolerances that could
compromise our interpretations. In a cox proportional hazard model, the effect of year (2012
versus 2013) on cold-tolerance phenotypes in core populations was 0.016 ± 0.24 SE (z = 0.07,
P = 0.95) and the effect of year (2012 versus 2013) on heat-tolerance phenotypes in core
populations was 0.001 ± 0.25 (z = 0, P = 1). Although it was not feasible to estimate yearly
variation in thermal tolerance within the edge region, we concluded that, at least in the core,
2013 was no different from 2012 in damselfly thermal tolerances, and thus phenotypes were
directly comparable across years. Similarly, any yearly variation in weather patterns during
our sampling interval could have affected our results regarding the effects of recent weather
on thermal tolerance plasticity. Within both core and edge regions, however, daily
temperature patterns during the sampling interval were highly congruent across years:
maximum, mean and minimum temperatures for Eskilstuna (edge region) were 29, 17, 7 °C in
July 2013 versus 27, 17, 6 °C in July 2012, and for Malmö (core region) 29, 18, 8 °C versus 27,
16, 9 °C; data from the Swedish Meteorological and Hydrological Institute,
http://www.smhi.se. Thus although edge populations were only sampled in 2013, recent
weather events experienced by those individuals in 2013 were similar to conditions they
would have experienced if they had been captured in 2012, the same year in which most of
the thermal trials were conducted in core populations.
Additional information on thermal trial methods
During both heat- and cold-ramping trials, damselflies were held in individual enclosures and
provided with ventilation and cotton wool soaked in water, to prevent the confounding effects
of asphyxiation and dehydration. The final temperatures of 2 °C (lower tolerance) and 43 °C
(upper tolerance) were established during pilot studies as those at which approximately 50%
of the individuals were knocked down (i.e. no longer standing). These temperatures are
similar to recorded temperature extremes in southern and central Sweden
(http://www.smhi.se). Experimental temperatures were validated using iButton data loggers
(Maxim Integrated, San Jose, CA, USA) placed in individual containers and distributed
throughout the thermal chamber during trials. Overnight cold-ramping trials typically lasted
16 h (± an SD of 1.5 h), and cold-ramping trial duration had no effect on cold-tolerance
phenotypes (effect = 0.006 ± 0.03, z = 0.22, P = 0.83).
Additional information on weather station data
We used the Eskilstuna weather station for edge populations, located 16–84 km from the
study sites (mean distance = 50 km). For core populations, we used data from the Malmö
weather station, 9–110 km from the study sites (mean distance = 29 km). Weather data were
provided by the Swedish Meteorological and Hydrological Institute, http://www.smhi.se. Data
from these stations collected in 2012 and 2013 were used in our analyses of effects of recent
weather events on adult damselfly thermal tolerances, and to compare regional weather
variation among years (see above).
REFERENCES
Sears, M.W., Raskin, E. & Angilletta, M.J. (2011) The world is not flat: defining relevant thermal
landscapes in the context of climate change. Integrative and Comparative Biology, 51,
666–675.
Daily temperatures on capture
and non-capture dates
(a)
20
15
A
10
160
170
edge populations
core populations
12
7-day temperature minimum (° C)
Daily mean temperatures (° C)
edge populations
core populations
150
Variation in 7-day temperature minima
experienced by captured individuals
(b)
180
190
200
capture date (day of year)
210
10
8
6
B
4
150
160
170
180
190
200
210
capture date (day of year)
Figure S1 Weather variation in core and edge regions during the sampling interval. (a) Daily temperatures during the field season on capture
dates (black symbols) and non-capture dates (grey symbols). Capture dates spanned the peak flight season for Swedish I. elegans, and
experiments were terminated when adult individuals could no longer be captured in sufficient numbers for thermal trials. (b) Variation in
weekly minimum temperatures experienced by captured individuals from core versus edge regions.