Orthogonal fitness benefits of nitrogen and ants for nitrogen‐limited plants in the presence of herbivores

Abstract

Abstract. Predictable effects of resource availability on plant growth-defense strategies provide a unifying theme in theories of direct anti-herbivore defense, but it is less clear how resource availability modulates plant indirect defense. Ant-plant-hemipteran interactions produce mutualistic trophic cascades when hemipteran-tending ants reduce total herbivory, and these interactions are a key component of plant indirect defense in most terrestrial ecosystems. Here we conducted an experiment to test how ant-plant-hemipteran interactions depend on nitrogen (N) availability by manipulating the presence of ants and aphids under different N fertilization treatments. Ants increased plant flowering success by decreasing the densities of herbivores, and the effects of ants on folivores were positively related to the density of aphids. Unexpectedly, N fertilization produced no changes in plant N concentrations. Plants grown in higher N grew and flowered more, but aphid honeydew chemistry stayed the same, and neither the density of aphids nor the rate of ant attraction per aphid changed with N addition. The positive effects of ants and N addition on plant fitness were thus independent of one another. We conclude that N was the plant's limiting nutrient and propose that addition of the limiting nutrient is unlikely to alter the strength of mutualistic trophic cascades.

Short‐term, low‐level nitrogen deposition dampens a trophic cascade between bears and plants

“…This result suggests that the protective benefits of ants for herbivores were greater with N enrichment, potentially due to increased predation pressure from other arthropod predators (Cushman & Whitham, ; Grinath et al, ). Unlike a recent study in a similar system assessing high‐level N enrichment (Pringle, Ableson, Kerber, Vannette, & Tao, ), the present study found that indirect effects of ants on plants were not independent of N additions.…”

Excessive nutrient input induces an ecological cost for aphids by modifying their attractiveness towards mutualist ants

“…We found that B. cardui ' honeydew from 2NK‐rich plants had 2.4 times more uric acid than those from NPK‐rich treatment, suggesting they are dealing with an excess of N in 2NPK‐rich plants. Moreover, the changes that we observed in honeydew composition are consistent with some previous studies that showed that the nutritional status of plants affects the sugar and the amino‐acid composition of aphid honeydew (e.g., Fischer & Shingleton, 2001; Pringle et al, 2014; but see Pringle et al, 2017). Therefore, it is expected that changes in honeydew are an important mechanism that allows aphids to deal with the nutritional imbalance of their food, regulating the excretion of elements in excess.…”