Nutrient availability affects mutualistic interactions: the case of plant-aphid-tending ants.

LESCANO M. N; QUINTERO, C.; FARJI-BRENER, A. G.; BALSEIRO, E.

Lincoln

Congreso; Conference on Biological Stoichiometry; 2023

University of Nebraska

In terrestrial systems, it has been historically assumed that soil N additions will linearly increase herbivore performance. However, excess of soil fertilization, by resulting in high-N plants, could be detrimental to herbivores. Besides the physiological effects, soil fertilization can lead cascading effects that alter the interactions between organisms and trigger multiple ecological consequences. Through experiments under three increasing fertilization levels (unfertilized, NPK-rich, and 2NPK-rich soils), we studied how the enhancement of soil nutrient availability, by modifying the C:N ratio of thistles, affects the performance and homeostatic response of the aphids, and the consequent attraction of their mutualistic ants. We found that fertilized soils increased the biomass and reduced the C:N ratio of thistles, and also increased aphid abundance. Despite the huge reduction in the C:N ratio of thistles with increased soil nutrients, aphids were able to maintain their stoichiometric homeostasis through changes in honeydew production and composition; in both fertilization treatment, the quantity of honeydew secreted was at least 50% lower than in the unfertilized treatment, while the honeydew N concentration was 2.5 to 6.4 times higher compared with the unfertilized treatment. In addition, in the highest fertilization treatment, the aphids increased the content of uric acid excreted in their honeydew. This could explain that aphid-tending ants have a marked preference for honeydew from intermediate rich-substrates (i.e., honeydew with a lower C:N ratio than that of aphids from unfertilized plants, and lower level of uric acid than that of 2NPK-rich plants), and thus, why the aphid-infested thistles had the highest number of aphid-tending ants when they grew on intermediate rich-substrates. When compared with intermediate fertilization, the highest level of fertilization used here was neither deleterious nor beneficial in terms of aphid performance, but it could imply an ecological cost due to the decrease in the attraction of protective mutualists. This study provides key insights into the complex role of bottom-up cascading effects triggered by increases in soil nutrient availability and highlights the importance of evaluating not only the physiological and population cost and benefits of it but also the ecological ones; especially when it alters mutualistic interactions.