Mutualism is not restricted to tree-killing bark beetles and fungi: the ecological stoichiometry of secondary bark beetles, fungi, and a scavenger

1. All bark beetles are in symbiosis with fungi. Although obligate mutualisms with fungi are common with tree-killing bark beetles (primaries), fungi associated with non-tree-killing bark beetles (secondaries) are usually dismissed as commensals. 2. Using an ecological stoichiometric approach, we show secondaries are also involved in nutrition-based mutualisms, some of which appear obligate, and that differences in symbiont provisioning efficiency have a potent effect on beetle carbon (C): nitrogen (N): phosphorus (P) ratios. 3. Some secondary beetles have high P contents and require efficient P provisioning via fungi, while others have low P contents that may allow them to exploit less efficient fungi or a broader range of species with variable efficiencies. A co-occurring scavenger that feeds on nutrient-poor bark beetle frass (excrement/boring residues) exhibited the lowest phosphorus content yet recorded for an invertebrate. 4. Our results generally support the growth-rate hypothesis that posits differences in C:P and N:P ratios in consumers are due to differential allocation of P to P-rich RNA to support growth. However, while the beetle species that accumulated the most biomass was considerably enriched in P and that with the least biomass was P-poor, one beetle species that was P-rich was also small possibly due to limitation by an element other than P. 5. Our results indicate that fungi are important to a broader range of bark beetles than previously recognised. Additional research is needed to describe how these various symbioses influence forest ecosystems via differential effects of fungi on host beetle fitness.