An organism’s biomolecular composition may constrain its ability to meet the challenges of survival and reproduction. Ecological stoichiometry (ES), the study of element balance in living systems, provides a framework for linking the chemical composition of organisms to functional traits impinging on ecological interactions. A central idea in ES, the growth rate hypothesis, posits a positive causal relationship between whole body phosphorus (P) content and growth rate in organisms due to the dependence of protein synthesis rate on the concentration of P-rich ribosomal RNA. This hypothesis has been tested frequently in aquatic systems, but relatively little is known about the importance of P variation in terrestrial animals. Here we test one component of the growth rate hypothesis by examining the relationship between whole-insect P concentration and RNA concentration both among and within grassland insect species; this work is the first broad scale comparison of whole-organism RNA concentration among terrestrial invertebrates.

Results/Conclusions

We found that whole-insect RNA concentration varied more than 4-fold across species and showed a strong phylogenetic signal. Moreover, variation in RNA concentration accounted for much of the variation in insect P concentration in phylogenetically adjusted comparisons. Within species, we found that RNA concentration consistently varied with P concentration between life stages, as juveniles generally contained higher concentrations of RNA and P than conspecific adults. In addition, RNA concentration often significantly decreased with body mass among conspecifics within a life stage, which coincided with similar negative allometries for P concentration. Our results suggest that the substantial variation in P concentration both among and within grassland insect species is clearly linked to allocation to a biomolecule (RNA) that has documented impacts on key life history traits. A continued focus on these relationships should reveal how material constraints underlie important functional traits in terrestrial insect communities.