Increased temperature variation poses a greater risk to species than climate warming

Abstract

Increases in the frequency, severity and duration of temperature extremes are anticipated in the near future. Although recent work suggests that changes in temperature variation will have disproportionately greater effects on species than changes to the mean, much of climate change research in ecology has focused on the impacts of mean temperature change. Here, we couple fine-grained climate projections (2050-2059) to thermal performance data from 38 ectothermic invertebrate species and contrast projections with those of a simple model. We show that projections based on mean temperature change alone differ substantially from those incorporating changes to the variation, and to the mean and variation in concert. Although most species show increases in performance at greater mean temperatures, the effect of mean and variance change together yields a range of responses, with temperate species at greatest risk of performance declines. Our work highlights the importance of using fine-grained temporal data to incorporate the full extent of temperature variation when assessing and projecting performance.

Keywords: climate change; ectotherm; performance; temperature.

Figure 1.

(a) A typical thermal performance curve (TPC) for relative performance (fitness or a proximate biological rate; black line) as a function of environmental temperature (equation (2.1)). T_opt marks the temperature at which performance is greatest and T_max marks the critical transition to negative values at high temperatures. Owing to the nonlinearity of this curve, species that experience temporal variation in temperature will have a mean long-term performance 〈w〉 that differs from the value predicted by the mean of their environment (owing to Jensen's inequality). The distribution of instantaneous performance and long-term performance means are shown for nominal ‘cold’ and ‘warm’ temperature distributions (b,e), distributions with increased variance (c,f) and distributions with positive skewness (d,g). In ‘cold’ conditions, increasing the variance leads to an increase in long-term performance, whereas positive skewness has little effect. In ‘warm’ conditions, increasing variances and positive skewness both lead to reductions in long-term performance. The mean temperatures of ‘cold’ and ‘warm’ distributions are equal [17,24] across (b)–(d); variance is equal for (b) and (d).

Figure 2.

Relative long-term performance of a species with thermal performance curve shown in figure 1 as a function of the mean, standard deviation and skewness (a–c) of a normal distribution environmental temperature. Relative performance values below −1 are shaded in dark grey and are not resolved as they decrease quickly beyond this point. Along σ = 0, where no environmental variation is present, contours are given exactly by the TPC. In regions to the left of the white lines, an increase in the mean temperature yields an increase in performance; to the right of the white line, an increase in mean temperature yields a decrease in performance. Exact values of are given by .

Figure 3.

Estimated performance over 10 years of historical local thermal variation (a) for each of the 38 ectothermic invertebrates collected at the locations shown in (e). (b–d) The change in performance as the mean, s.d. and positive skewness of historical local climates are independently increased. (f) The projected change in performance using a decade (2050–2059) of simulated site-specific daily maximum and minimum surface air temperatures (see Methods). (g–i) The change in performance observed when only the mean (a), s.d. (b) or both mean and s.d. (c) of historical climates are altered to match future predictions. In (a), performance is estimated relative to performance at the optimum (T_opt). The mean change alone accounts for 32% of the predicted change in (f), the s.d. change alone for 54%, but interactively they account for 93%.