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As thermal regimes change worldwide, projections of future population and
species persistence often require estimates of how population growth rates
depend on temperature. These projections rarely account for how temporal
variation in temperature can systematically modify growth rates relative
to projections based on constant temperatures. Here,we tested the
hypothesis that time-averaged population growth rates in fluctuating
thermal environments differ from growth rates in constant conditions as a
consequence of Jensen’s inequality, and that the thermal performance
curves (TPCs) describing population growth in fluctuating environments can
be predicted quantitatively based on TPCs generated in constant lab
conditions. With experimental populations of the green alga Tetraselmis
tetrahele, we show that nonlinear averaging techniques accurately
predicted increased as well as decreased population growth rates
influctuating thermal regimes relative to constant thermal regimes. We
extrapolate from these results to project critical temperatures for
population growth and persistence of 89 phytoplankton species in naturally
variable thermal environments. These results advance our ability to
predict population dynamics in the context of global change.
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