Differences in herbivore damage and performance among Arctium minus (burdock) genotypes sampled from a geographic gradient: a common garden experiment.
Performance of plant species does not necessarily decline as they approach their geographic range limits. One reason for this may be a loss of natural enemies in marginal populations. Such patterns have been found in native species, but also may occur for exotics if they have not already escaped their herbivores in invaded regions. For instance, the Eurasian biennial Arctium minus (common burdock) is attacked by a variety of native and introduced insects in its new North American range. Previously, research has shown that damage by these herbivores strongly decreases towards the northern range limit of this species. This gradient might reflect a genetic cline in resistance to herbivores, or geographic variation in herbivore abundance. To distinguish between these possibilities, herbivore damage to leaves and seeds of A. minus was measured in a common garden experiment with genotypes sampled from 11 populations along a 550 km transect extending from southern Ontario towards burdock's northern range limit. As well, a freezing tolerance experiment was performed with the important lepidopteran seed predator Metzneria lappella, and palatability experiments were performed with two generalists, the snail Cepaea nemoralis and the moth Trichoplusia ni. Although there were some differences in damage among populations, results indicated that latitudinal differences in herbivore damage are not explained by genotypic differences among populations, but instead are likely to result from the absence of herbivores from colder sites. Escape of A. minus from its usual herbivores may increase performance of marginal populations, and contribute to future spread.