Detecting climate signals cascading through levels of biological organization

Threats to species under climate change can be understood as a time at which the signal of climate change in ecological processes emerges from the noise of ecosystem variability, defined as ‘time of emergence’ (ToE). Here we show that ToE for the great tit ( Parus major ) will occur earlier at the level of population size than trait (laying date) and vital rates (survival, recruitment) under the RCP 8.5 scenario, suggesting an amplified climate change signal at the population level. ToE thus varies across levels of biological organization that filter trends and variability in climate differently. This has implications for the detection of climate impacts on wild species, as a shift in population size may precede changes in traits and vital rates. Further work would need to identify the ecological level that may experience an earlier detection of the climate signal for species with contrasting life histories, climate trends and variability.