For or against: the importance of variation in growth rate for testing the EICA hypothesis

Abstract

The evolution of increased competitive ability (EICA) hypothesis proposes that invasive species evolve decreased defense and increased competitive ability following natural enemy release. Previous studies have found evidence both for and against EICA. The resource-enemy release hypothesis (R-ERH) suggests that fast-growing species may experience stronger enemy release than slow-growing species. On the basis of R-ERH, the prediction of EICA will be held true for slow-growing genotypes, i.e., the slow-growing genotypes from the introduced range will be less resistant to herbivory and grow faster than those from the home range; while the EICA will not be held for fast-growing genotypes, i.e., there will be no significant differences in growth and defense traits between the introduced and native fast-growing genotypes. We tested these predictions preliminarily using five populations of the invasive plant Alternanthera philoxeroides. This species has two varieties in its home range, which showed a distinct growth-defense strategy: the northern A. p. var. acutifolia (Apa) had higher growth rate but lower resistance, while the southern A. p. var. obtusifolia (Apo) had lower growth rate but higher resistance level. Our results suggest that the EICA hypothesis is consistent with the slow-growing Apo, but not with the fast-growing Apa. We suggest that evolutionary changes in growth or resistance following enemy release are influenced by variation in growth rate within an invasive alien plant. These findings have important implications for the EICA hypothesis, and may partially explain why previous studies have found evidence both for and against EICA.Keywords Alien plant Á Biological invasions Á Growth-defense trade-off Á Growth rate hypothesis Á Resource-enemy release hypothesis (R-ERH)

Invasion Success in a Marginal Habitat: An Experimental Test of Competitive Ability and Drought Tolerance in Chromolaena odorata

“…The well-studied EICA (Evolution of Increased Competitive Ability) hypothesis states that invasive species can re-allocate resources from defence to growth in the absence of natural enemies [32] and numerous studies supporting and rejecting this hypothesis have been published [33], [34], [35]. A recent analysis [36] shows that the hypothesis might hold for slow-growing species only and not for fast-growing species, such as C. odorata . This is further supported by a recent study that did not find evidence for decreased tolerance to herbivory in C. odorata [37].…”

Contrasting effects of specialist and generalist herbivores on resistance evolution in invasive plants

“…, Pan et al. ). Therefore, it is important to consider both resistance traits and resistance effects when testing the Shifting Defense hypothesis.…”

“…Consequently, whether evolution of resistance traits could translate into evolution of resistance effects (i.e., damage by and performance of herbivores) is unknown. Several experimental studies that evaluated damage by and/or performance of herbivores in native and introduced populations of invasive species found equivocal results (Wolfe et al 2004, Joshi and Vrieling 2005, Ridenour et al 2008, Pan et al 2012. Therefore, it is important to consider both resistance traits and resistance effects when testing the Shifting Defense hypothesis.…”

Geographical diversification of growth–defense strategies in an invasive plant

“…Recently, Blumenthal () developed an interpretive framework suggesting that enemy release and increased resource availability may act in concert to cause successful invasion among plant species (resource–enemy release hypothesis, R‐ERH). To the best of our knowledge, this potentially important hypothesis has not been tested formally (Pan et al, ). Although the R‐ERH (and the resource availability hypothesis) is largely concerned with variations in growth–defense strategies among plant species that differ in inherent growth rates, a genetically based variation in growth–defense strategies within plant species (i.e.…”

“…Empirical studies have generally supported this hypothesis, although many exceptions exist (for a review, see Crawley, ). However, this interactive effect between enemy pressure and resource availability has been largely unappreciated in invasion ecology (but see Blumenthal, ; Zhang & Jiang, ; Pan et al, ).…”