Body size ’shrinkage’ due to warming reflects reduced survival and scales up to intensify the effects of food abundance on population growth rate

Background/Question/Methods

Reduce body size as a response to warming is considered a major ecological response to climate change, along with the shifts in species distribution and seasonal changes in life cycle events. One potential explanation for this ‘shrinking effect’ is the temperature-size rule, which states that high temperature causes an increasing in the developmental rate, which is not matched by a higher growth rate. As a result, individuals experience faster growth, but attain lower adult body size at high temperature. Although many studies have found support for the temperature-size rule, food availability and its own temperature dependence are often major confounding factors of individual growth. Moreover, the population-level consequences of the mismatch between development and growth and its effect on life span and fecundity are still unclear. Here we used a full factorial design and reared over 400 Daphnia magna individuals at two temperatures (15 and 25^oC) and two food levels (high and low). We measured body size and number of offspring daily over the life span of all individuals. We then used a size-structured matrix projection model to investigate the combined effects of temperature and food abundance on population growth rate.

Results/Conclusions

Our results are consistent with the temperature-size rule: at both high and low food abundance, individuals at higher temperature grew faster, achieved size of maturity earlier and had lower adult size than individuals kept at 15^oC. High temperature also caused individuals to have lower clutch size, shorter life span and although they reproduced more frequently, they had lower life-time reproductive output. As a consequence of shorter life span, high temperature caused smaller size at death at low, but not at high food abundance. The projection model showed that at 25^oC, population growth rate attained its highest and lowest values at high and low food abundance, respectively. These results suggest that low survival could explain the 'shrinkage' due to warming observed in many ectotherms and that food abundance is an important confounding factor that can alter these results. In addition, temperature could intensify the amplitude of population variation caused by natural fluctuations in resource and, therefore, increase the risk of extinction under climate change scenarios.