Altitudinal variation in ant–aphid mutualism in nitrogen transfer of oak (Quercus liaotungensis)

Abstract

Ant–plant relationship is a model for the study of the ecology and evolution of interspecific interactions. In direct ant–plant mutualism (i.e., plants providing food or nesting places for ants, and ants protecting the plants in return) ants provide nutrients to plants. However, whether a similar mechanism exists in indirect ant–plant mutualism (i.e., an ant–aphid–plant system) remains unknown. In this work, we used the 15N stable isotope method to study altitudinal variations in the roles of ants in the nutrient transfer of oak (Quercus liaotungensis). Our work shows that ants deliver nitrogen in indirect ant–plant interactions, and that the effect of nutrient transfer differed significantly with altitude. Ants’ trophic level at high altitudes was significantly lower than that at low altitudes, indicating that the degree of ant–aphid mutualism was greater at high altitudes, which may be beneficial in nitrogen transfer. Our work suggests that ant–aphid mutualism might be context dependent, such that it affects nutrient transfer in the food web, and that this context dependency is an important factor that influences altitudinal variation in nutrient transfer.