Phenotypic selection gradients in a tripartite plant interaction in southern South America

The study of how phenotypic selection by plant mutualists and antagonists varies within ecosystems is crucial for evaluating potential plant adaptations to changing environments, such as urban areas. Here, we describe the interaction intensity of plants with their mutualistic and antagonistic partners, as well as the selective landscapes in one natural and one urban site. We measured direct phenotypic selection on plant traits relevant to three interaction partners: floral tube length, resistance to herbivores and arbuscular mycorrhizal fungi (AMF) colonization. Significant direct phenotypic selection on the three interaction traits was detected in the natural site. We observed disruptive selection on floral tube length, with the two fitness optima closely matching the proboscis peak lengths of the main pollinator species. In addition, we observed stabilizing selection by arbuscular mycorrhizal fungi, favouring plants with around 11% of arbuscular colonization. This result supports the idea that plants may autoregulate mean intraradical arbuscular colonization to maximize fitness relative to carbon allocation to arbuscular mycorrhizal fungi. Finally, we detected directional selection benefiting plants with high resistance to herbivores. Our results highlight the importance of adopting a multispecies, context-dependent approach to better understand the complex ecological interactions that drive plant evolution.