Beyond EICA: understanding post-establishment evolution requires a broader evaluation of potential selection pressures

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

Do Trade-Offs Have Explanatory Power for the Evolution of Organismal Interactions?

“…These experiments typically measure growth and fecundity as response variables, with fewer studies quantifying differences in herbivore performance (e.g., Rapo et al 2010), the host's tolerance to herbivory (e.g., Zou et al 2008), or defensive chemistry (e.g., Cano et al 2009). In fact, only 26% of studies aimed at testing the EICA hypothesis measured both growth and defense traits (Atwood and Meyerson 2011). Given the variety of mechanisms by which investment in defense might limit plant growth such as autotoxicity from defensive compounds or physical limitations related to structural defenses (Poorter and de Jong 1999) Nonetheless, progress has been made.…”

“…Empirical evaluations of EICA are most often conducted in a common garden by direct comparisons of phenotypic differences between native and invasive genotypes (Bossdorf et al 2005;Atwood and Meyerson 2011). These experiments typically measure growth and fecundity as response variables, with fewer studies quantifying differences in herbivore performance (e.g., Rapo et al 2010), the host's tolerance to herbivory (e.g., Zou et al 2008), or defensive chemistry (e.g., Cano et al 2009).…”

Herbivory of native and exotic North-American prairie grasses by nymph Melanoplus grasshoppers

“…Such inconsistency may be due to measurement of only a single plant trait or using non-standard metrics of herbivore defense, such as, for example, uncommon data conversions or chemical pathways which might differ among plants (Atwood and Meyerson 2011). In addition, different experimental designs across studies, such as conducting experiments in the field or greenhouse, or experiments with clipped leaves alone might limit the data integration across studies (Atwood and Meyerson 2011). Moreover, results from laboratory or common garden experiments alone might not reflect all possible plant responses which occur naturally and therefore should be interpreted carefully (Motheral and Orrock 2010).…”

“…In our study, we also address the issue of inconsistency in studies used to test the invasion hypotheses described above by comparing leaf damage between native and exotic plants (e.g., Chun et al 2010;Atwood and Meyerson 2011;Inderjit 2012). Such inconsistency may be due to measurement of only a single plant trait or using non-standard metrics of herbivore defense, such as, for example, uncommon data conversions or chemical pathways which might differ among plants (Atwood and Meyerson 2011). In addition, different experimental designs across studies, such as conducting experiments in the field or greenhouse, or experiments with clipped leaves alone might limit the data integration across studies (Atwood and Meyerson 2011).…”

Seven years of NeoBiota – the times, were they a changin’?

“…They used hypotheses in invasion biology and improved tests of these by introducing a simple mathematical framework to quantify the invasiveness of species. Also the work of Atwood and Meyerson (2011) was based on favourite hypotheses in invasion biology. They argue that the lack of consensus across studies that test EICA (evolution of increased competitive ability; Blossey and Nötzold 1995) may be in part due to the lack of consistent definitions and varying experimental designs.…”