Exotic plant invasion in the context of plant defense against herbivores.

Upon their introduction to new environments, plants may become invasive and suppress native plant species (Gurevitch et al., 2011). Plant-herbivore interactions play an important role in invasion success of an exotic plant. In the introduced ranges, exotic plants are believed to be largely free from specialist herbivore pressure, a hypothesis known as the enemy release hypothesis (ERH; Keane and Crawley, 2002). Exotic plants are also less likely to be attacked by generalist herbivores in introduced ranges (Müller-Schärer et al., 2004; Schaffner et al., 2011). In the absence of specialist herbivores in introduced ranges, resources previously allocated to plant defense in the native range might get reallocated to: (1) growth or reproduction of the exotic plant, so leading to enhanced competitiveness, as suggested by the evolution of increased competitive ability hypothesis (EICA; Blossey and Nötzold, 1995; Bossdorf et al., 2005; Ridenour et al., 2008), (2) the production of less-costly chemical compounds such as terpenes and glucosinolates, as predicted by the shift in defense hypothesis (SDH; Doorduin and Vrieling, 2011), and/or (3) the higher production of allelochemicals novel for invaded communities, as assumed by the novel weapons hypothesis (NWH; Callaway and Ridenour, 2004; Supplemental Box S1). Biogeographical approaches, comparisons of the ecological traits of species in native and introduced ranges, have proved useful for testing different hypotheses on defense-growth tradeoffs. Herbivores in native and introduced ranges may influence plant performance to varying degrees but empirical evidence on the impact of specialist and generalist herbivores on the biogeographic differences in the production of chemicals is lacking. Since plant defense against herbivores is influenced by genetic and environmental interaction (Ballhorn et al., 2011), biogeographic comparisons of resource allocation to plant defense and competition traits would help us to better understand the influence of plant defense against herbivores in exotic plant invasion (Fig. 1). In this Update, I discuss evidence for each of the above hypotheses on defense-growth trade-offs and whether these hypotheses are supported or new hypotheses are needed to understand exotic plant invasion in the context of plant defense against herbivores.