Local escape of an invasive plant, common ragweed (Ambrosia artemisiifolia L.), from above‐ground and below‐ground enemies in its native area

Abstract

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.

Does relatedness of natives used for soil conditioning influence plant-soil feedback of exotics?

“…This result indicates that soil pathogens' effect on studied species dominated over the effect of soil microbial mutualists. Our results agree with the findings of Agrawal et al (2005) and van Grunsven et al (2007), whereas MacKay and Kotanen (2008) or Callaway et al (2004) showed that symbionts were the dominant component of the soil microbiota. Unfortunately, we cannot say which species or even guilds of soil microbiota were responsible for the observed pattern.…”

Reduced availability of rhizobia limits the performance but not invasiveness of introduced Acacia

“…Other studies that have examined the influence of soil biota on species performance have found that interactions with natural enemies in the soil are generally more pronounced in species' native ranges (e.g. Klironomos 2002;Reinhart et al 2003;Reinhart & Callaway 2006), particularly in association with conspecifics (MacKay & Kotanen 2008). However, we found that positive interactions dominated seedling performance, since seedling growth in the treatment soils was considerably higher than in the sterile control soils.…”

The History of Ragweed in the World

“…These genes have a role in producing secondary metabolites and stress tolerance, as well as in degradation of xenobiotics (Hodgins et al, 2013 . After having been settled in Europe, ragweed spread quickly, since herbivorous and granivorous species harrmful for A. artemisiifolia in its homeland were missing from Europe (Mackay and Kotanen, 2008).…”