Abstract

According to the Shifting Defense Hypothesis, invasive plants should trade-off their costly quantitative defense to cheaper qualitative defense and growth due to the lack of natural specialist enemies and the presence of generalist enemies in the introduced areas. Several studies showed that plant genotypes from the invasive areas had a better qualitative defense than genotypes from the native area but only a few studies have focused on the quantitative defenses and tolerance ability. We compared structural defenses, tolerance and growth between invasive and native plant populations from different continents using the model plant Jacobaea vulgaris. We examined several microscopical structure traits, toughness, amount of cell wall proteins, growth and root-shoot ratio, which is a proxy for tolerance. The results show that invasive Jacobaea vulgaris have thinner leaves, lower leaf mass area, lower leaf cell wall protein contents and a lower root-shoot ratio than native genotypes. It indicates that invasive genotypes have poorer structural defense and tolerance to herbivory but potentially higher growth compared to native genotypes. These findings are in line with the Evolution of Increased Competitive Ability hypothesis and Shifting Defense Hypothesis. We also show that the invasiveness of this species in three geographically separated regions is consistently associated with the loss of parts of its quantitative defense and tolerance ability. The simultaneous change in quantitative defense and tolerance of the same magnitude and direction in the three invasive regions can be explained by parallel evolution. We argue that such parallel evolution might be attributed to the absence of natural enemies rather than adaptation to local abiotic factors, since climate conditions among these three regions were different. Understanding such evolutionary changes helps to understand why plant species become invasive and might be important for biological control.

Highlights

  • Invasive species are defined as non-native species that successfully establish and spread when introduced beyond their native range (Williamson 1996; Pysek et al 2004)

  • The Evolution of Increased Competitive Ability hypothesis (EICA) predicts that the escape from specialist herbivores leads to an evolutionary change in invasive plants in energy allocation from defense to growth which can give invasive plants a higher competitive ability to outcompete local plant species (Blossey and Notzold 1995)

  • We examined the microscopical structures of leaf traits, the amount of cell wall proteins, leaf toughness, leaf thickness (LMA) and tolerance in native and invasive plant genotypes of common ragwort, Jacobaea vulgaris, our model plant

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Summary

Introduction

Invasive species are defined as non-native species that successfully establish and spread when introduced beyond their native range (Williamson 1996; Pysek et al 2004). The first theories on why plants become invasive emphasize the absence of natural enemies after introduction of plants in a new area (Williamson 1996; Keane and Crawley 2002; Mitchell and Power 2003). When plants are introduced into a new habitat, they leave their co-evolved natural enemies behind. This release from detrimental herbivore pressure potentially resulting in a fast increase in distribution and abundance (the enemy release hypothesis (ERH), (Keane and Crawley 2002). The Evolution of Increased Competitive Ability hypothesis (EICA) predicts that the escape from specialist herbivores leads to an evolutionary change in invasive plants in energy allocation from defense to growth which can give invasive plants a higher competitive ability to outcompete local plant species (Blossey and Notzold 1995)

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