Choice of voles among genotypes of birch seedlings: its relationship with seedling quality and preference of insects.

Selective feeding by herbivores on establishing seedlings has been suggested to affect genotype frequencies in several plant populations. The existence of genotypes susceptible to herbivores calls for an explanation in such populations. In the present study we assessed the choice of multiple herbivores, field voles (Microtus agrestis) and insects, among genotypes of silver birch (Betula pendula) representing variation occurring in a naturally regenerated stand. We examined how food choice of voles and insects is related to each other, competitive ability among the seedling genotypes and variation in soil fertility. We set up a field experiment and randomly assigned seedling populations, composed of mixed genotypes, to fully crossed insect exposure and fertilization treatments. After the first growing season we exposed a half of the seedling populations to vole herbivory. Voles selected clearly among the genotypes: they preferred the fastest growing seedlings as well as those with a low density of resin droplets on their stems. The preference of voles and insects among the genotypes was tightly correlated. We conclude that the effects of herbivory compensate those of intraspecific competition in this system and thus favor coexistence of genotypes differing in their susceptibility to herbivores.