Herbivory Differentially Affects Plant Fitness in Three Populations of the Perennial Herb Lythrum salicaria along a Latitudinal Gradient

摘要

Herbivory can negatively and selectively affect plant fitness by reducing growth, survival and reproductive output, thereby influencing plant population dynamics and evolution. We documented intensity of herbivory and experimentally examined its effect on survival, growth and reproductive output in three natural populations of the perennial herb Lythrum salicaria along a latitudinal gradient from southern to northernmost Sweden over two years. The intensity of herbivory and the effects of herbivory on plant fitness were strongest in the southern population and intermediate in the central population. The mean proportion of the leaf area removed ranged from 11% in the southern to 3% in the northern population. Herbivore removal increased plant height 1.5-fold in the southern and 1.2-fold in the central population, the proportion plants flowering 4-fold in the southern and 2-fold in the central population, and seed production per flower 1.6-fold in the southern and 1.2-fold in the central population, but did not affect plant fitness in the northern population. Herbivore removal thus affected the relative fecundity of plants in the three populations: In the control, seed output per plant was 8.6 times higher in the northern population compared to the southern population, whereas after herbivore removal it was 2.5 times higher in the southern population compared to the northern. Proportion of leaf area removed increased with plant size, but tolerance to damage did not vary with size. The results demonstrate that native herbivores may strongly affect the demographic structure of L. salicaria populations, and thereby shape geographic patterns of seed production. They further suggest that the strength of herbivore-mediated selection varies among populations and decreases towards the north.