Abstract
The evolution of increased competitive ability (EICA) hypothesis and the shifting defense hypothesis (SDH) predict that evolutionary changes occur in a suite of traits related to defense and growth in invasive plant species as result of the absence of specialist herbivores. We tested how this suite of traits changed due to the absence of specialist herbivores in multiple invasive regions that differ in climatic conditions with native and invasive Jacobaea vulgaris in a controlled environment. We hypothesized that invasive J. vulgaris in all invasive regions have i) a higher plant growth and underlying traits, such as photosynthetic capacity, ii) lower regrowth-related traits, such as carbohydrate storage, and iii) an increased plant qualitative defense, such as pyrrolizidine alkaloids (PAs). Our results show that invasive J. vulgaris genotypes have evolved a higher photosynthetic rate and total PA concentration but a lower investment in root carbohydrates, which supports the SDH hypothesis. All the traits changed consistently and significantly in the same direction in all four invasive regions, indicative of a parallel evolution. Climatic and soil variables did differ between ranges but explained only a very small part of the variation in trait values. The latter suggests that climate and soil changes were not the main selective forces on these traits.
Highlights
Invasive plants provide an excellent opportunity for ecologists to study evolutionary changes by considering invasions as large-scale and long-term experiments in which major alterations in selective forces have occurred (Turner et al, 2014)
The major prediction of the evolution of increased competitive ability (EICA) and shifting defense hypothesis (SDH) hypothesis is that the absence of herbivory by specialist herbivores leads to an evolutionary shift in allocation from defense to growth
We found strong evidence that invasive J. vulgaris genotypes grow faster compared to the native genotypes and that the increased growth is due to a higher SLA, atmospheric CO2 concentration per unit leaf area (Asat), and photosynthetic nitrogen use efficiency (PNUE) (Table 1)
Summary
Invasive plants provide an excellent opportunity for ecologists to study evolutionary changes by considering invasions as large-scale and long-term experiments in which major alterations in selective forces have occurred (Turner et al, 2014). Since defenses against herbivores are costly (Koricheva, 2002; Velzen and Etienne, 2015) and many defensive traits are genetically controlled (Fritz and Simms, 1992; Kariñho‐Betancourt et al, 2015), a shift in the herbivore composition towards a guild that is dominated by generalist herbivores is expected to exert an altered selective pressure on invasive plants. Joshi and Vrieling, 2005) argue that a release from specialist herbivores leads to a decreased investment in costly quantitative defenses and an increased investment in less costly qualitative defenses. The release of specialist herbivores, leads to specific evolutionary changes in the allocation pattern to growth, defense, and regrowth after defoliation
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