Urbanization weakens mutualisms without affecting antagonisms of a hawkmoth-pollinated plant species

The influence of urbanization on ecosystems extends beyond the impact on individual species, affecting interactions among multiple species, and thereby ecosystem functioning. In this study, we asked whether interaction intensity between plants and their mutualists and antagonists varies along an urbanization gradient. To address this question, we evaluated changes in the interactions of Nicotiana longiflora Cav. (Solanaceae) with arbuscular mycorrhizal fungi (AMF) and nocturnal hawkmoth pollinators as mutualists, and with insect herbivores as antagonists across an urbanization gradient. We assessed female plant fitness, pollination intensity by nocturnal hawkmoths, AMF root colonization, and floral and leaf herbivory across 29 populations and their relationship with urbanization metrics, including type of area cover, degree of light pollution, and impervious surface cover. We found that as impervious surface cover and degree of light pollution increase, hawkmoth visitation, and conspecific pollen load on stigmas significantly decreased. Herbivory did not significantly vary with urbanization metrics. Results are consistent with the known influence of artificial illumination on nocturnal pollinator activity. Total root colonization by AMF increased with impervious surface cover and highly compact urban areas probably due to enhanced net photosynthetic response and greater carbon allocation to roots. In addition, Vesicular colonization increased with natural grasslands. Overall, our findings indicate that urbanization has the potential to influence multiple plant interactions in different ways, ultimately leading to a net negative impact on plant benefits. This could have long-term consequences for plant fitness by indirectly altering how below- and above-ground partners perform their mutualistic or antagonistic roles.