The relative importance of intraspecific and interspecific effects to temperature– size relationships in diatom communities

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Climate warming has been linked to an apparent general decrease in body sizes of ectotherms, both across and within taxa, especially in aquatic systems. We tested the hypothesis that diatoms are smaller at warmer temperatures using a system of geothermally heated streams in Iceland. To do this we examined both community and species levels of organization simultaneously, and developed a statistical method that calculates the proportion of size differences between communities that are due to intraspecific and interspecific effects. We found that there was no consistent relationship between size and temperature at either the population or community level. Therefore, these data provide important counterexamples to both James’ and Bergmann's temperature–size rules, respectively, undermining the widely held assumption that warming favours the small. In addition, we show that interspecific effects were a bigger contributor to whole-community size differences, and are probably more ecologically important than more commonly studied intraspecific effects. These findings highlight the need for multispecies approaches in future studies of climate warming and body size.