Browsing by Author "Callaway, Ragan M."

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    Escape from natural enemies depends on the enemies, the invader, and competition
    Lucero, Jacob E.; Arab, Nafiseh Mahdavi; Meyer, Sebastian T.; Pal, Robert W.; Fletcher, Rebecca A.; Nagy, David U.; Callaway, Ragan M.; Weisser, Wolfgang W. (2020-09-11)
    The enemy release hypothesis (ERH) attributes the success of some exotic plant species to reduced top-down effects of natural enemies in the non-native range relative to the native range. Many studies have tested this idea, but very few have considered the simultaneous effects of multiple kinds of enemies on more than one invasive species in both the native and non-native ranges. Here, we examined the effects of two important groups of natural enemies-insect herbivores and soil biota-on the performance ofTanacetum vulgare(native to Europe but invasive in the USA) andSolidago canadensis(native to the USA but invasive in Europe) in their native and non-native ranges, and in the presence and absence of competition. In the field, we replicated full-factorial experiments that crossed insecticide,T. vulgare-S. canadensiscompetition, and biogeographic range (Europe vs. USA) treatments. In greenhouses, we replicated full-factorial experiments that crossed soil sterilization, plant-soil feedback, and biogeographic range treatments. We evaluated the effects of experimental treatments onT. vulgareandS. canadensisbiomass. The effects of natural enemies were idiosyncratic. In the non-native range and relative to populations in the native range,T. vulgareescaped the negative effects of insect herbivores but not soil biota, depending upon the presence ofS. canadensis; andS. canadensisescaped the negative effects of soil biota but not insect herbivores, regardless of competition. Thus, biogeographic escape from natural enemies depended upon the enemies, the invader, and competition. Synthesis:By explicitly testing the ERH in terms of more than one kind of enemy, more than one invader, and more than one continent, this study enhances our nuanced perspective of how natural enemies can influence the performance of invasive species in their native and non-native ranges.
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    Negative plant-soil feedbacks increase with plant abundance, and are unchanged by competition
    Maron, John L.; Smith, Alyssa Laney; Ortega, Yvette K.; Pearson, Dean E.; Callaway, Ragan M. (2016-08)
    Plant-soil feedbacks and interspecific competition are ubiquitous interactions that strongly influence the performance of plants. Yet few studies have examined whether the strength of these interactions corresponds with the abundance of plant species in the field, or whether feedbacks and competition interact in ways that either ameliorate or exacerbate their effects in isolation. We sampled soil from two intermountain grassland communities where we also measured the relative abundance of plant species. In greenhouse experiments, we quantified the direction and magnitude of plant-soil feedbacks for 10 target species that spanned a range of abundances in the field. In soil from both sites, plant-soil feedbacks were mostly negative, with more abundant species suffering greater negative feedbacks than rare species. In contrast, the average response to competition for each species was unrelated with its abundance in the field. We also determined how competitive response varied among our target species when plants competed in live vs. sterile soil. Interspecific competition reduced plant size, but the strength of this negative effect was unchanged by plant-soil feedbacks. Finally, when plants competed interspecifically, we asked how conspecific-trained, heterospecific-trained, and sterile soil influenced the competitive responses of our target species and how this varied depending on whether target species were abundant or rare in the field. Here, we found that both abundant and rare species were not as harmed by competition when they grew in heterospecific-trained soil compared to when they grew in conspecific-cultured soil. Abundant species were also not as harmed by competition when growing in sterile vs. conspecific-trained soil, but this was not the case for rare species. Our results suggest that abundant plants accrue species-specific soil pathogens to a greater extent than rare species. Thus, negative feedbacks may be critical for preventing abundant species from becoming even more abundant than rare species.