Abstract
The evolution of increased competitive ability (EICA) hypothesis predicts that escape from intense herbivore damage may enable invasive plants to evolve higher competitive ability in the invasive range. Below-ground root herbivory can have a strong impact on plant performance, and invasive plants often compete with multiple species simultaneously, but experimental approaches in which EICA predictions are tested with root herbivores and in a community setting are rare. Here, we used Brassica nigra plants from eight invasive- and seven native-range populations to test whether the invasive-range plants have evolved increased competitive ability when competing with Achillea millefolium and with a community (both with and without A. millefolium). Further, we tested whether competitive interactions depend on root herbivory on B. nigra by the specialist Delia radicum. Without the community, competition with A. millefolium reduced biomass of invasive- but not of native-range B. nigra. With the community, invasive-range B. nigra suffered less than native-range B. nigra. Although the overall effect of root herbivory was not significant, it reduced the negative effect of the presence of the community. The community produced significantly less biomass when competing with B. nigra, irrespective of the range of origin, and independent of the presence of A. millefolium. Taken together, these results offer no clear support for the EICA hypothesis. While native-range B. nigra plants appear to be better in dealing with a single competitor, the invasive-range plants appear to be better in dealing with a more realistic multi-species community. Possibly, this ability of tolerating multiple competitors simultaneously has contributed to the invasion success of B. nigra in North America.
Highlights
The evolution of increased competitive ability (EICA) hypothesis predicts that escape from intense herbivory and subsequent genetically based re-allocation of resources to increased growth and reproductive output may help invasive plant species to colonize novel habitats in the invasive range [1]
In the absence of the community, biomass of invasive-range B. nigra plants was reduced by the presence of A. millefolium (-13.9%; Fig 1C), while this was not the case for native-range B. nigra plants (+ 0.7%; Fig 1C)
Invasive-range B. nigra plants suffered less from the presence of the community (-24.6%) than native-range B. nigra plants (-40.6%), regardless of the presence/absence of A. millefolium. This was reflected by a significant Range x Achillea x Community (RxAxC) three-way interaction (Table 1; Fig 1C)
Summary
The evolution of increased competitive ability (EICA) hypothesis predicts that escape from intense herbivory and subsequent genetically based re-allocation of resources to increased growth and reproductive output may help invasive plant species to colonize novel habitats in the invasive range [1]. The EICA hypothesis predicts that plants from the invasive range that evolved lower anti-herbivore defenses should evolve increased competitive ability. When grown under common conditions in the absence of herbivores, invasive-range plants should exhibit significantly higher growth and reproductive output than conspecific plants from the native range [1]. In most of the experiments testing predictions of the EICA hypothesis, invasive-range and conspecific native-range plants were grown under common conditions in the absence of competitors [3]. Tests for evolution of increased competitive ability in invasive plants require manipulative experiments that compare the performance of invasive-range and conspecific native-range plants under competition in a community setting
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