Effect of genetic variance in plant quality on the population dynamics of a herbivorous insect.

1. Species diversity can affect many ecological processes; much less is known about the importance of population genetic diversity, particularly for the population dynamics of associated species. Genetic diversity within a host species can create habitat diversity; when associated species move among hosts, this variation could affect populations additively (an effect of average habitat) or non-additively (an effect of habitat variance). Mathematical theory suggests that non-additive effects of variance among patches should influence population size, but this theory has not been tested. 2. This prediction was tested in the field by asking whether aphid population dynamics parameters on strawberry plant genotype mixtures were additive or non-additive functions of parameters on individual plant genotypes in monoculture using model fitting. 3. Results show that variance in quality among plant genotypes can have non-additive effects on aphid populations, and that the form of this effect depends on the particular plant genotypes involved. 4. Genetic variation among plants also influenced the spatial distribution of aphids within plant populations, but the number of plant genotypes per population did not affect aphid populations. 5. These results suggest that predicting the behaviour of populations in heterogeneous environments can require knowledge of both average habitat quality and variance in quality.