Plant invasions, generalist herbivores, and novel defense weapons

Abstract

Abstract. One commonly accepted mechanism for biological invasions is that species, after introduction to a new region, leave behind their natural enemies and therefore increase in distribution and abundance. However, which enemies are escaped remains unclear. Escape from specialist invertebrate herbivores has been examined in detail, but despite the profound effects of generalist herbivores in natural communities their potential to control invasive species is poorly understood. We carried out parallel laboratory feeding bioassays with generalist invertebrate herbivores from the native (Europe) and from the introduced (North America) range using native and nonnative tetraploid populations of the invasive spotted knapweed, Centaurea stoebe. We found that the growth of North American generalist herbivores was far lower when feeding on C. stoebe than the growth of European generalists. In contrast, North American and European generalists grew equally well on European and North American tetraploid C. stoebe plants, lending no support for an evolutionary change in resistance of North American tetraploid C. stoebe populations against generalist herbivores. These results suggest that biogeographical differences in the response of generalist herbivores to novel plant species have the potential to affect plant invasions.

Exotic Plant Invasion in the Context of Plant Defense against Herbivores

“…Ecosystem-scale interactions between plant resistance to herbivores and exotic invasion can influence their community level functions (Levine et al, 2004;Oduor et al, 2011). Differences in the production of chemicals in the native versus introduced ranges needs to be linked to plant defense and invasion success of exotic plants (see Inderjit et al, 2011b;Schaffner et al, 2011). Further work on the genomics of allelochemicals in native and introduced ranges would help to understand how exotic plants engineer their success in introduced ranges.…”

“…A shift in defense traits saves resources that can be allocated to the production of less-costly chemicals to defend themselves against generalist herbivores and providing competitive advantage against other plant species (Schaffner et al, 2011). In some situations, defense chemicals may also act as allelopathic chemicals.…”

“…Leger and Forister (2005) found that the invasive populations of Eschscholzia californica were less-preferred choice of generalist herbivore Trichoplusia ni and Orgyia vetusta found in California than native populations of E. californica, which supports an alternate hypothesis that exotic plants evolve higher production of defense compounds to resist generalist herbivores. Schaffner et al (2011) compared the relative growth of European and North American generalists (T. ni and Heliothis virescens) when feeding on the North American or European C. stoebe. The growth rates of North American generalists were much lower when fed on C. stoebe than European generalists.…”

“…The ERH generally deemphasizes the importance of generalist herbivores in the invasion process (Schaffner et al, 2011). Regardless of enemy release from insects (Cincotta et al, 2009) or soil biota Reinhart and Callaway, 2004;Inderjit and van der Putten, 2010), generalist herbivores appear to be important for some invasions.…”

“…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 (Mü ller-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 Nö tzold, 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).…”