Data for: Temperature effects on growth rates of Daphnia from different populations

When comparing somatic growth thermal performance curves (TPCs), higher
somatic growth across experimental temperatures is often observed for
populations originating from colder environments. Such countergradient
variation has been suggested to represent adaptation to seasonality, or
shorter favorable seasons in colder climates. Alternatively, populations
from cold climates may outgrow those from warmer climates at low
temperature, and vice versa at high temperature, representing adaptation
to temperature. Using modelling, we show that distinguishing between these
two types of adaptation based on TPCs requires knowledge about (i) the
relationship between somatic growth rate and population growth rate, which
in turn depends on the scale of somatic growth (absolute or proportional),
and (ii) the relationship between somatic growth rate and mortality rate
in the wild. We illustrate this by quantifying somatic growth rate TPCs
for three populations of Daphnia magna where population growth scales
linearly with proportional somatic growth. For absolute somatic growth,
the northern population outperformed the two more southern populations
across temperatures, and more so at higher temperatures, consistent with
adaptation to seasonality. In contrast, for the proportional somatic
growth TPCs, and hence population growth rate, TPCs tended to converge
towards the highest temperatures. Thus, if the northern population pays an
ecological mortality cost of rapid growth in the wild, this may create
crossing population growth TPCs consistent with adaptation to temperature.
Future studies within this field should be more explicit in how they
extrapolate from somatic growth in the lab to fitness in the wild.