Title: The ecological stoichiometry of mutualism and antagonism in a bark beetle-fungi symbiosis
Abstract: Insect herbivores face some of the greatest deficiencies in N and P in their food. We applied ecological stoichiometry theory (EST) to investigate how a bark beetle deals with such elemental mismatches via symbiotic facilitation. Western pine beetle feeds in outer bark, an extremely nutrient deficient tissue. Its ability to survive there is supported by two obligate mutualist fungi. A third fungus acts as a powerful antagonist with larvae often dying in their first instar. We investigated how the three fungi influence nutrient transfer (specifically N and P) in the context of EST and the growth rate hypothesis (GRH) that predicts that growth rates reflect P availability (for production of rRNA). We found the mutualists transfer N and P from sapwood through phloem and finally into the outer bark to where the larvae and new adults feed. The antagonist, instead, moves N and P from sapwood only to phloem, leading to starvation of larvae. However, even with mutualists, P remains low in bark accounting for the insect’s slow growth rate and extremely low threshold elemental ratio (TER, the point where limitation shifts from P to C), the lowest TER yet recorded for a consumer. Although EST has seldom been applied to mutualistic systems, it provides a powerful means to assess elemental flow and symbiont efficiency independent of difficult, artificial, and for obligate mutualisms, often impossible, feeding studies.