Quantitative mismatch between empirical temperature-size rule slopes and predictions based on oxygen limitation

Sigurd Einum,Øystein Nordeide Kielland,Claus Bech

doi:10.1038/s41598-021-03051-y

Sigurd Einum, Øystein Nordeide Kielland + Show 1 more

Open Access

https://doi.org/10.1038/s41598-021-03051-y

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Abstract

In ectotherms, adult body size commonly declines with increasing environmental temperature, a pattern known as the temperature-size rule. One influential hypothesis explaining this observation is that the challenge of obtaining sufficient oxygen to support metabolism becomes greater with increasing body size, and more so at high temperatures. Yet, previous models based on this hypothesis do not account for phenotypic plasticity in the physiology of organisms that counteracts oxygen limitation at high temperature. Here, we model the predicted strength of the temperature-size response using estimates of how both the oxygen supply and demand is affected by temperature when allowing for phenotypic plasticity in the aquatic ectotherm Daphnia magna. Our predictions remain highly inconsistent with empirical temperature-size responses, with the prior being close to one order of magnitude stronger than the latter. These results fail to provide quantitative support for the hypothesis that oxygen limitation drives temperature-size clines in aquatic ectotherms. Future studies into the role of oxygen limitation should address how the strength of the temperature-size response may be shaped by evolution under fluctuating temperature regimes. Finally, our results caution against applying deterministic models based on the oxygen limitation hypothesis when predicting future changes in ectotherm size distributions under climate change.

Highlights

In ectotherms, adult body size commonly declines with increasing environmental temperature, a pattern known as the temperature-size rule
Adult or maximum body size commonly declines as a plastic response to increasing environmental temperature experienced during their life, a pattern known as the temperature-size rule[1,2]
Despite the decline in oxygen content at saturation with increasing temperature, fmax increases with increasing temperature (Table 1). When applying these estimates to our model (Eq (3)), body mass was predicted to decline with increasing temperature for both temperature intervals, for both values of the surface area-body mass scaling exponent (c), and independent of whether the model allowed for phenotypic plasticity or not (Fig. 1)

Summary

Introduction

Adult body size commonly declines with increasing environmental temperature, a pattern known as the temperature-size rule. We model the predicted strength of the temperature-size response using estimates of how both the oxygen supply and demand is affected by temperature when allowing for phenotypic plasticity in the aquatic ectotherm Daphnia magna. Our predictions remain highly inconsistent with empirical temperature-size responses, with the prior being close to one order of magnitude stronger than the latter These results fail to provide quantitative support for the hypothesis that oxygen limitation drives temperature-size clines in aquatic ectotherms. Kielland et al.[21] provided an empirical estimate of how supply increases with increasing temperature when allowing for phenotypic plasticity for the zooplankton Daphnia magna, and demonstrated that this change in supply was insufficient to compensate for the increased demand This provided qualitative support for the oxygen limitation hypothesis. We apply the data from that study in a model that provides quantitative predictions on how the maximum body size should respond to temperature if the temperature-size rule is driven by oxygen limitation

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