Effects of urbanization on local adaptation and eco-evolutionary feedbacks in white clover

Urbanization is causing unprecedented ecological and evolutionary change worldwide. However, empirical evidence of local adaptation to urban environments is limited. Here we conduct a reciprocal transplant experiment using white clover (Trifolium repens) originating from urban and rural populations and distributed equally into five urban common gardens and five rural common gardens. Half of the plants in each garden produced the antiherbivore chemical defence hydrogen cyanide (HCN), and the other half lacked the defence, as this trait is known to exhibit genetic clines along urbanization gradients. Based on measurements of multiple vegetative and sexual fitness traits as well as ecological interactions with herbivores, pollinators and mutualistic root bacteria, we detect divergent selection on HCN between urban and rural environments, where HCN improved fitness in rural environments and reduced fitness in urban environments. Urban and rural white clover populations show genetic divergence that drove a tradeoff in life history strategies: urban plants invested more in vegetative growth, whereas rural plants produced more flowers and seeds in both urban and rural gardens. Finally, we demonstrate eco-evolutionary feedbacks, with increased herbivory at rural sites, and increased pollinator visitation to acyanogenic plants at urban sites. This study contributes to understanding how urbanization affects evolution and feeds back to influence broader ecosystem processes.