Effects of nutrient levels on defense against specialist insects in an invasive alligator weed.
Evolution of Increased Competitive Ability (EICA) has suggested that exotic plants may reduce resource allocation to anti-herbivore defenses and increase allocation to growth due to reduced specialist herbivores in introduced ranges. However, the major limitation for the hypothesis may be that, as important defensive strategies, induced resistance and tolerance were seldom tested and little information is known regarding the evolution of defensive strategies as influenced by resource availability. Nine introduced and nine native populations of Alternanthera philoxeroides were compared for fitness and two morphological traits that influence herbivory, specifically specific stem length (SSL, plants with low SSL have hollow stems favoured by a stem-boring specialist insect Agasicles hygrophila) and root/shoot ratio (RSR) under low and high nutrient availability and with or without A. hygrophila. Introduced populations showed greater total biomass (+14.7%) and lower SSL (-27.5%), demonstrating increased growth and decreased levels of constitutive defense. Tolerance to herbivory was similar between introduced and native populations. SSL plasticity to herbivory was greater in introduced populations under high nutrient levels, and was positively correlated with total biomass. These results suggest that although both functional trait values and plasticity contribute to the adaptation of alligator weeds to a wide range of nutrient availabilities and specialist insect herbivory, introduced populations have evolved higher production of biomass, lower levels of constitutive structural defense, and greater plasticity of structural defense in high nutrient conditions, indicating that the present control practice for A. philoxeroides where introducing A. hygrophila will face great challenges in the future.