3.2 The influence of climate change on the size of S.
araneus
The weather records from this period revealed several significant trends
indicative of a changing climate (scatterplots of six climate parameters
are shown in Figure S2). The mean annual temperature increased
significantly (F 1,51=28.1,P <0.0001; Figures 4c and S2a) from 6.1 °C to 8.2 °C (as
calculated from the regression line). There was also an increase in the
mean July temperature (F 1,51=10.5,P =0.0021; Figure S2b), from 17.2 °C to 19.7 °C, but not in the
mean January temperature (F 1,51=2.29,P =0.136; Figure S2c). The relationship of the sum of annual
precipitation with time was nonlinear (quadratic term:F 1,50=9.30, P=0.0037), with an initial increase
leading to a maximum in the mid-1970s, followed by a decrease until 2004
(the decrease in 1971-2004 was statistically significant:F 1,32=11.25, P =0.0021, linear regression)
(Figures 4d and S2d). The period of high precipitation was mainly due to
increased rainfall in June and July. Continuous increases in mean annual
and mean July temperatures coupled with a significant decline in
precipitation resulted in an increased nonlinear soil moisture deficit
over the years (quadratic term: F 1,50=7.69,P =0.0078; Figures 4e and S2e). This change in the deficit was not
significant until 1971 (F 1,18=0.20,P =0.660, linear regression), but there was a significant increase
in the soil moisture deficit after 1971
(F 1,32=11.9, P =0.0016). Interestingly, the
beginning of the increase in drought corresponded with the inflection
points of the smoothed relationships of the skull size vs. years and
with the clear decrease in skull size later on (Figure 4). The number of
days with snow cover significantly decreased over the study period
(F 1,51=4.57, P =0.0374) (Figure S2f).
We found no significant relationships between skull length and the
climate parameters in either juvenile or adult shrews in the whole time
series, i.e., between 1953 and 2004. However, when we reduced this
period to between 1971 and 2004 (the time of the significant decrease in
skull length in adults), the relationship between the skull length and
the moisture deficit in the soil became highly significant
(F 1,20=6.91, P =0.0161,r2 =0.257); an increased deficit resulted in a
shorter skull length (Figure 5).
The analysis of the relationships between skull height and climate
between 1953 and 2004 revealed significant temperature effects (Table
2). Skull height decreased significantly with increasing mean daily
temperatures during the expected lifetime in both juvenile and adult
shrews. We also found an impact of winter weather (during the subadult
stage) on adult skull height the following summer: the increase in the
lowest average daily temperature in February (but not in January) was
associated with the decrease in skull height in summer, but this
relationship was marginally significant. Moreover, summer skull heights
were greater when there were more days with snow cover in the previous
January and February. However, the relationship between skull height and
the number of days with snow cover >=5 cm in January and
February did not reach the significance level (Table 2). Following this
pattern in other analyses, there were no significant relationships
between skull width and climate parameters. Including more than one
weather variable in a model never increased the overall significance.