Novel Weapons: Invasive Success and the Evolution of Increased Competitive Ability

When introduced to new habitats by humans, some plant species become much more dominant. This is primarily attributed to escape from specialist consumers. Release from these specialist enemies is also thought by some to lead to the evolution of increased competitive ability, driven by a decrease in the plant\u27s resource allocation to consumer defense and an increase in allocation to size or fecundity. Here, we discuss a new theory for invasive success – the “novel weapons hypothesis”. We propose that some invaders transform because they possess novel biochemical weapons that function as unusually powerful allelopathic agents, or as mediators of new plant–soil microbial interactions. Root exudates that are relatively ineffective against their natural neighbors because of adaptation, may be highly inhibitory to newly encountered plants in invaded communities. In other words, the novel weapons of some plant invaders provide them with an advantage that may arise from differences in the regional coevolutionary trajectories of plant communities. Furthermore, the selective advantage of possessing a novel weapon may result in rapid evolution of that weapon – for example, the production of greater quantities of allelopathic or antimicrobial root exudates. Direct selection of competitive traits provides an alternative to the “grow versus defend” trade-offs that underpin the theory of the evolution of increased competitive ability. Read More: http://www.esajournals.org/doi/abs/10.1890/1540-9295(2004)002%5B0436%3ANWISAT%5D2.0.CO%3B

No Evidence for Trade-Offs: Centaurea Plants from America are Better Competitors and Defenders

The natural enemies hypothesis has led to a number of ideas by which invaders might evolve superior competitive ability. In this context, we compared growth, reproduction, competitive effect, competitive response, and defense capabilities between invasive North American populations of Centaurea maculosa and populations in Europe, where the species is native. We found that Centaurea from North America were larger than plants from European populations. North American Centaurea also demonstrated stronger competitive effects and responses than European Centaurea. However, competitive superiority did not come at a cost to herbivore defense. North American plants were much better defended against generalist insect herbivores and slightly better defended against specialists. North Americans showed a stronger inhibitory effect on the consumers (resistance) and a better ability to regrow after attack by herbivores (tolerance). Better defense by North Americans corresponded with higher constitutive levels of a biochemical defense compound precursor, tougher leaves, and more leaf trichomes than Europeans. North American F1 progeny of field collected lines retained the traits of larger size and greater leaf toughness suggesting that genetic differences, rather than maternal effects, may be the cause of intercontinental differences, but these sample sizes were small. Our results suggest that the evolution of increased competitive ability may not always be driven by physiological trade-offs between the allocation of energy or resources to growth or to defense. Instead, we hypothesize that Centaurea maculosa experiences strong directional selection on novel competitive and defense traits in its new range

Plant invasions, generalist herbivores, and novel defense weapons

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

Appendix D. Mean total biomass of Centaurea maculosa from European and North American populations: (A) with and without Agapeta zoegana infecting roots and (B) at the end of the experiment with Cyphocleonus achates; controls, and treatment combined.

Mean total biomass of Centaurea maculosa from European and North American populations: (A) with and without Agapeta zoegana infecting roots and (B) at the end of the experiment with Cyphocleonus achates; controls, and treatment combined

Appendix A. Populations used in growth and herbivory experiments.

Populations used in growth and herbivory experiments

Appendix C. Germination rates (percentage) for North American and European populations of Centaurea maculosa.

Germination rates (percentage) for North American and European populations of Centaurea maculosa

Appendix C. Relative growth rate (means ± SE) of European and North American generalist herbivores fed European tetraploid (C. stoebe), North American tetraploid (C. stoebe), or control plants.

Relative growth rate (means ± SE) of European and North American generalist herbivores fed European tetraploid (C. stoebe), North American tetraploid (C. stoebe), or control plants