Differences in performance and herbivory along a latitudinal gradient for common burdock (Arctium minus)

Background/Question/Methods

Populations of plants near the limits of their geographic distribution may experience different biotic pressures than central populations. For instance, if herbivores are scarce in marginal populations, performance of their host plants may benefit relative to core populations. However, if herbivore tolerance decreases in marginal populations, performance of plants in core populations may be higher. The objective of this project was to study the plant-insect interactions of an invasive species, common burdock (Arctium minus), across its northern range while considering changes in reproductive success, morphological traits, physical defenses and herbivory. Morphological (height, diameter etc.), reproductive (number of seeds, reproductive mass), herbivore (leaf and capitular damage) and physical defense (number of trichomes, penetration force for leaves) data were collected for 535 individuals of burdock from 107 different populations (12 locations) along an 850km latitudinal transect, representing a gradient from more central (southern) to marginal (northern) populations. A soil-feedback experiment was also conducted in which seeds of burdock were planted in media inoculated with field soil from different locations; after 3 months, above- and below-ground biomass, as well as length of the longest leaf were measured.

Results/Conclusions

Although there were no significant differences in height with latitude, analysis showed that northern plants had increased diameter, produced more seeds, and invested more energy in reproductive mass. In addition, damage by 6 out of the 7 measured guilds of herbivores decreased on approaching the range limit.  As a result, the northernmost populations had 16 times less leaf damage than the southernmost populations. Pre-dispersal seed predation was also 9 times higher in southern populations. In addition, northern plants invested less energy in physical defense, had fewer trichomes per unit area, and a lower force required for penetration. In the soil-feedback experiment, marginal soil initially produced smaller plants (decreased above- and below ground biomass), but these surpassed more southern populations in size in a second iteration. Collectively, these results suggest that marginal populations experience relaxed pressure from natural enemies, and may allocate energy from defense to increased growth and reproduction.