A review of the phytochemical support for the shifting defence hypothesis

Abstract

Several theories have been developed to explain why invasive species are very successful and develop into pest species in their new area. The shifting defence hypothesis (SDH) argues that invasive plant species quickly evolve towards new defence levels in the invaded area because they lack their specialist herbivores but are still under attack by local (new) generalist herbivores. The SDH predicts that plants should increase their cheap, toxic defence compounds and lower their expensive digestibility reducing compounds. As a net result resources are saved that can be allocated to growth and reproduction giving these plants a competitive edge over the local plant species. We conducted a literature study to test whether toxic defence compounds in general are increased in the invaded area and if digestibility reducing compounds are lowered. We specifically studied the levels of pyrrolizidine alkaloids, a toxin which is known for its beneficial and detrimental impact against specialists and generalists, respectively. Digestibility reducers did not show a clear trend which might be due to the small number of studies and traits measured. The meta analysis showed that toxic compounds in general and pyrrolizidine alkaloid levels specifically, increased significantly in the invaded area, supporting the predictions of the SDH that a fast evolution takes place in the allocation towards defence.

Enemies lost: parallel evolution in structural defense and tolerance to herbivory of invasive Jacobaea vulgaris

“…By doing so they can decrease their costly quantitative defenses against the absent specialist herbivores. As a net result the plants in the invasive area are well defended and resources can be saved for growth and reproduction which can give these plants a competitive edge over the local plant species (Doorduin and Vrieling 2011). This evolutionary shift of quantitative defense to qualitative defense in the invasive area is called the Shifting Defense Hypothesis (SDH) (Müller-Schärer et al 2004;Joshi and Vrieling 2005;Doorduin and Vrieling 2011).…”

Diversity of pyrrolizidine alkaloids in native and invasive Senecio pterophorus (Asteraceae): Implications for toxicity

“…The role of PAs as plant chemical defenses against phytophagous insects has been widely documented (Hartmann, 1999). Indeed, PAs may have evolved in response to the consumption pressure from invertebrate herbivores (Doorduin and Vrieling, 2011;Hartmann, 1999).…”

“…Intraspecific divergence in plant chemistry could have ecological and evolutionary consequences on the invaded ecosystems by modifying the interactions between plants and herbivores and facilitating invasion success (Keane and Crawley, 2002). Genetically determined concentrations of plant toxins, including PAs, were higher in native populations compared with introduced populations of several plant species (Doorduin and Vrieling, 2011). However, whether plant toxicity is increased in the invaded areas is yet unclear.…”

Exotic Plant Invasion in the Context of Plant Defense against Herbivores

“…specialist herbivores that use defense compounds as feeding cues or oviposition stimulants are very well adapted to and not affected by these compounds. The SDH predicts that plants in the introduced range through release from ecological costs via specialists lower the production of costly chemicals involved in reducing insect digestibility (a quantitative defense trait) and increase the production of lessexpensive toxic compounds (a qualitative defense trait; Supplemental Box S1; Doorduin and Vrieling, 2011). For defense chemicals such as alkaloids or glucosinolates (qualitative defense traits) defensegrowth trade-offs are more ambiguous.…”

“…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).…”