Mixed effects of ant–aphid mutualism on plants across different spatial scales

Abstract

Scale affects how we observe, explain, and predict patterns and processes in ecology. The pattern of biotic interactions can be scale-dependent, but it is not clear how biotic interactions perform at different scales. We tested the effects of aphid-tending ants Lasius fuliginosus on the oak tree Quercus liaotungensis by excluding ants at two scales (branches and whole trees). Furthermore, we explored the variation of the effects on different-size branches and trees. The ants had significant anti-herbivory effects, a positive effect on aphids, but a negative effect on galls. Contrary to previous assumptions, effects of ants on plants and herbivores are similar between the branch and plant scales. The size of the branch or tree also had no impact on the ecological effects of ants, indicating the ecological effect of ant–aphid mutualism is consistent within and among individual trees. The homogeneous distribution of aphids may be the key reason for the scale-independent effect of the indirect ant–plant interaction. Our finding highlights the importance of biotic defense for plants across different spatial scales.