Overrun by the neighbors: landscape context affects strength and sign of local adaptation.

The adaptive deme formation hypothesis states that plant-feeding insects may adapt to individual host plants. To date, no empirical study has examined the strength of such adaptations from a spatially explicit perspective. In this study, we quantify local adaptation of six specialist insect species at multiple sites, predicting that spatial variation in local immigration rates will result in variation in the strength of local adaptation. We use a previously parameterized metapopulation model to estimate the proportion of immigrants in focal populations, clonal trees to measure local adaptation in a reciprocal common garden experiment, and a linear model to test for a link between the strength of immigration and local adaptation across species. As predicted, local adaptation generally varies inversely with the fraction of immigrants in a population. When immigration is high, local populations remain in a maladapted state. Importantly, our results imply that patterns of adaptation may vary predictably at a relatively small spatial scale (among individual host trees within a landscape) and that, hence, measures of local adaptation will make most sense in a spatial context.