Ecological stoichiometry of mountain pine beetle, its mutualist fungi, and the disease white pine blister rust in whitebark pine

Ecological stoichiometry can help clarify how symbionts and other co-occurring organisms mediate nutrient deficiencies for hosts. We used ecological stoichiometry (comparisons of elemental compositions in food vs consumer) to investigate whether obligate mutualist fungi (Grosmannia clavigera, Ophiostoma montium) of the tree-killing bark beetle Dendroctonus ponderosae (mountain pine beetle) and the invasive tree pathogenic fungus, Cronartium ribicola (causal agent of the disease white pine blister rust) influenced availability of carbon, nitrogen, and phosphorus to the beetle in Pinus albicaulis (whitebark pine), as well as how these elements varied among three populations of the tree. Nitrogen was 33% higher when phloem was colonized by the mutualist fungi, but phosphorus did not differ. In contrast to the mutualist fungi, infection of trees by the blister rust pathogen was not correlated with changes in carbon, nitrogen, or phosphorus. The contents of these elements were relatively homogeneous in living whitebark pine across populations. Our results indicate that blister rust disease does not shift phloem nutrients in ways that make trees more suitable for bark beetle colonization. However, nitrogen enhancement of beetle diets by fungal mutualists could help support the high colonization densities needed for the mass attacks required to kill trees.