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Keywords:

  • Ambrosia artemisiifolia;
  • annual ragweed;
  • biological invasions;
  • common ragweed;
  • Enemy Release Hypothesis;
  • fungal pathogens;
  • insect herbivores;
  • Janzen-Connell Hypothesis;
  • natural enemies

Summary

  • 1
    The Enemy Release Hypothesis proposes that exotic species gain an advantage in new regions because their natural enemies are lost during invasion; however, enemy release could also occur as a result of much smaller scale movements within an invasive plant's native range, i.e. if it escapes enemies while colonizing new sites. Few studies have considered this possibility, and none has compared escape from multiple guilds of enemies.
  • 2
    We studied an invasive North American weed, Ambrosia artemisiifolia (common ragweed), to determine whether it escapes above-ground or below-ground enemies at a local scale in its native range. Exotic populations of this plant are known to have escaped above-ground enemies in Europe.
  • 3
    Experimental populations of ragweed isolated from existing populations by just 100 m experienced reduced levels of damage by invertebrate folivores and seed predators; however, there was no clear evidence that this escape led to improved performance.
  • 4
    Plants inoculated with unsterilized soil grew better than plants in sterile soil, likely reflecting a need for mycorrhizal colonization. Inoculum from ragweed populations was as beneficial as inoculum from nearby ragweed-free sites, suggesting only a small initial role for site-specific pathogens.
  • 5
    Performance declined over time when plants were serially inoculated with non-sterile soil, indicating negative feedback with the soil biota. This feedback was stronger for inocula derived from ragweed populations vs. those from nearby ragweed-free sites.
  • 6
    Seeds buried in ragweed populations were less likely to germinate subsequently than seeds buried in nearby ragweed-free sites. This difference was not reduced by treatment with fungicide; consequently, the mechanism is unclear.
  • 7
    Synthesis. These results indicate that ragweed can escape both above-ground and below-ground enemies by dispersing to new sites, resulting in reduced levels of folivory, seed predation, soil feedback and perhaps losses to seed pathogens. Although consequences often may be small, this demonstrates that enemy release can occur for native as well as exotic populations of this species, linking possible mechanisms of local spread with long-distance invasion.