Animal and plant populations widely differ in their ability to adapt to a new environment. This variation may not be only driven by selection imposed by the abiotic environment, but also by the community context. Theory suggests that both dispersal and interspecific competition may drive variation in adaptability, either through positive effects (increasing genetic variation or selection) or through negative effects (migration load or competitive exclusion). However, experimental evidence for the individual and combined effects of these processes is scarce.
Here, we experimentally investigate the effects of dispersal and competition on local adaptation in a model species, the two-spotted spider mite (Tetranychus urticae). We exposed 56 populations to a new, toxic, host plant for twenty generations. Populations varied in (1) the number of immigrants they received from an ancestral, non-adapted population and (2) the co-occurrence with an already adapted spider mite species (Tetranychus evansi).
Our study shows how dispersal and competition jointly drive local adaptation. A humpback-shaped relationship between local adaptation and dispersal confirms that dispersal can both positively and negatively affect adaptation. Competition on one hand can hinder adaptation to new habitats by decreasing population size and survival, but on the other hand can help adaptation by exerting stronger selection and counteracting the negative effects of dispersal.