Temperature is a major factor affecting population abundance and individual performance. Net reproductive rate (R0) and intrinsic rate of increase (r) differ in their response to different temperature regimes, and much of the difference is mediated by generation time (Tg). Here, we evaluate the effects of thermal mean and variability on R0, r and Tg, at four population densities in Drosophila melanogaster. The results show that R0, r and Tg present differential responses to thermal variation. Although temperature effects on R0 and Tg are non-linear, r response was negligible. R0 and Tg comprise a generational time scale, while r is at a chronological time scale. Thus, we argue that individuals growing under different thermal environments perform similarly on a chronological scale, but differently on a generational scale.

Thermal performance curves have provided a common framework to study the impact of temperature in biological systems. However, few generalities have emerged to date. Here, we combine an experimental approach with theoretical analyses to demonstrate that performance curves are expected to vary predictably with the levels of biological organization. We measured rates of enzymatic reactions, organismal performance and population viability in Drosophila acclimated to different thermal conditions and show that performance curves become narrower with thermal optima shifting towards lower temperatures at higher levels or organization. We then explain these results on theoretical grounds, showing that this pattern reflects the cumulative impact of asymmetric thermal effects that piles up with complexity. These results and the proposed framework are important to understand how organisms, populations and ecological communities might respond to changing thermal conditions.

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