Dispersal and competition have both been suggested to drive variation in adaptability to a new environment, either positively or negatively. A simultaneous experimental test of both mechanisms is however lacking. Here, we experimentally investigate how population dynamics and local adaptation to a new host plant in a model species, the two-spotted spider mite (Tetranychus urticae), are affected by dispersal from a stock population (no-adapted) and competition with an already adapted spider mite species (Tetranychus evansi). For the population dynamics, we find that competition generally reduces population size and increases the risk of population extinction. However, these negative effects are counteracted by dispersal. For local adaptation, the roles of competition and dispersal are reversed. Without competition, dispersal exerts a negative effect on adaptation (measured as fecundity) to a novel host and females receiving the highest number of immigrants performed similarly to the stock population females. By contrast, with competition, adding more immigrants did not result in a lower fecundity. Females from populations with competition receiving the highest number of immigrants had a significantly higher fecundity than females from populations without competition (same dispersal treatment) and than the stock population females. We suggest that by exerting a stronger selection on the adapting populations, competition can counteract the migration load effect of dispersal. Interestingly, adaptation to the new host does not significantly reduce performance on the ancestral host, regardless of dispersal rate or competition. Our results highlight that assessments of how species can adapt to changing conditions need to jointly consider connectivity and the community context.
Population sizes after 12 generations evolving on tomato plants under different treatments (dispersal/competition). Population sizes was measured as the number of adult females on the experimental complete tomato plants (before removing epigenetic effects).
Population sizes of T. urticae after 9 generations evolving on tomato plants under different treatments of competition and dispersal. Population sizes are measured as number of adult females.
fecundity (number of eggs at day 6) and longevity (number of days females were alive) for females during the fitness tests after 20 generation evolving on tomato. Females were placed on detached bean and tomato leaves for 1 generation. Before the fitness test females were place on common garden (detached bean leaves) for 2 generations to remove juvenile and maternal effects. Dispersal treatments have 4 levels: 1 = 2mites/week, 2 = 3mites/week, 3 = 5mites/week, 4=10mites/week. Fecundity data for females that died of non-natural causes (drowned or disappeared) was excluded from the analysis (NA).
Fecundity (day 6) and longevity data for mites after 8 generations evolving on tomato plants. Data from the fitness test after removing juvenile and maternal effects. Females were placed individually on detached tomato and bean leaves for one generation. Dispersal treatments have 4 levels: 1 = 2mites/week, 2 = 3mites/week, 3 = 5mites/week, 4=10mites/week. Fecundity data for females that died of non-natural causes (drowned or disappeared) was excluded from the analysis (NA).
Fecundity (day 6) and longevity data on detached tomato and bean leaves (fitness test) after removal of juvenile and maternal effects. We used T. urticae females from the stock population (ST - a population reared on bean plants) and from a population that has been adapted to tomato (same strain as the stock population - London strain) for more than 100 generations (STa-reared on tomato plants, but originally coming from bean plants). Fecundity data for females that died of non-natural causes (drowned or disappeared) was excluded from the analysis (NA).