Rapid evolution promotes defense-strategy divergence in a widespread plant invader

Rapid evolution of defense strategies in non-native plants contributes greatly to their invasion success. Many studies have evidenced the reduced resistance in non-native plants following specialist-herbivore release, but how these plants adjust their defense portfolio against generalist herbivores in the introduced range remains poorly understood. Using the widespread plant invader Spartina alterniflora as a model species, we systematically compared three defense strategies against generalist herbivores—constitutive resistance, induced resistance, and tolerance—among its three genetic groups (native populations, high- and low-latitude introduced populations) in a common garden. We found that compared to native populations, introduced populations as a whole exhibited enhanced constitutive resistance and reduced induced resistance, but no significant difference in tolerance. Within each genetic group, native populations tended to allocate resources evenly among the three defense strategies, whereas high- and low-latitude introduced populations invested more in induced and constitutive resistance, respectively. These findings suggest that Spartina has undergone adaptive evolution in defense strategies within just 40 years since its introduction to China, adjusting resource allocation among different strategies in response to varying herbivory pressures. Our study underscores the importance of integrating generalist herbivores and multiple defense strategies to understand the evolutionary dynamics of plant invasions.