Abstract
Research on post-establishment evolution in nonnative plant populations has focused almost exclusively on testing the Evolution of Increased Competitive Ability (EICA) hypothesis, which posits that the lack of specialized herbivores in the invaded range drives evolution in nonnative plant populations. Fifteen years of conflicting EICA test results suggest that selection pressures other than specialized herbivory are important in driving post-establishment evolution in invasive species. Alternative hypotheses, such as the Evolution of Reduced Competitive Ability (ERCA) hypothesis, have been proposed but have received little attention or testing. We argue that the lack of consensus across studies that test EICA may be due in part to the lack of consistent definitions and varying experimental design parameters, and that future research in this field would benefit from new methodological considerations. We examined previous work evaluating post-establishment evolution and evaluated the range of study systems and design parameters used in testing the EICA hypothesis. Our goal was to identify where different uses of ecological terms and different study parameters have hindered consensus and to suggest a path forward to move beyond EICA in post-establishment evolution studies. We incorporated these methods into a design framework that will increase data harmony across future studies and will facilitate examinations of any potential selection pressure driving evolution in the invaded range.
Highlights
It has been commonly observed that life-history traits of nonnative plant species vary across habitats in native and introduced ranges, most conspicuously as either increased growth (Crawley 1987; Thebaud and Simberloff 2001) or decreased growth (Crawley 1987; Siemann and Rogers 2003a; Bossdorf et al 2004)
We examined previous work regarding post-establishment evolution in order to better understand why outcomes across studies have been inconsistent. Because this literature has focused primarily on testing the Evolution of Increased Competitive Ability (EICA) hypothesis, we focused our analysis within the field of evolution and invasive species by examining tests of the EICA
We developed a framework that can be used to move beyond EICA to evaluate a broad range of habitat characteristics that can act as selection pressures driving post-establishment evolution
Summary
It has been commonly observed that life-history traits of nonnative plant species vary across habitats in native and introduced ranges, most conspicuously as either increased growth (Crawley 1987; Thebaud and Simberloff 2001) or decreased growth (Crawley 1987; Siemann and Rogers 2003a; Bossdorf et al 2004). The focus on testing EICA has resulted in the general adoption of the term “competitive ability” as synonymous with “vegetative growth,” as it was interpreted by Blossey and Nötzold (1995) This is an oversimplification since EICA links multiple traits that are influenced by competition through energetic tradeoffs, including growth, defense, and, in more recent work, reproduction (Herms and Mattson 1992; Bazzaz and Grace 1997; Table 2). While EICA is likely an accurate predictor of evolutionary changes in some (but not all) of the species on which it has been tested, the fundamental differences in experimental designs and the use of loosely defined terms such as “competitive ability” in these studies may have hindered a more complete understanding of the applicability of the EICA hypothesis and of post-establishment evolution in general. We focused our recommendations on experimental designs that reduce the potential for confounding factors and increase the ability to integrate results among studies (Table 4)
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