Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
Evolution of Increased Competitive Ability hypothesis (EICA): Its role in the Success or Failure of Invasive Plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
If the EICA hypothesis is valid, then one would expect to find that “the low abundance or absence of herbivores in the new environments could cause rapid evolutionary change through selection for reduced anti-herbivore defences. Improved competitive ability through a shift in allocation from defence to growth has been predicted by the Evolution of Improved Competitive Ability… hypothesis” (Joshi & Vrieling, 2005). Joshi and Vrieling tested this idea by conducting garden experiments on Senecio jacobea, a native of Eurasia that has become an invasive species in New Zealand, Australia, and North America. Senecio jacobea is a perennial species that is attacked by more than 70 herbivores in its native range. “In the native area, specialist herbivores and defence mechanisms in S. jacobea, especially the production of pyrrolizidine alkaloids (PAs), secondary compounds toxic to non-adapted [generalist] herbivores and livestock, have been extensively studied and related to the occurrence of specialist insect herbivores.” In the introduced range in the USA, about 42 species of generalist arthropods have been recorded as feeding on Senecio jacobea.
Joshi and Vrieling, 2005, compared herbivory by generalist and specialist insects, in both native and invasive populations of Senecio jacobea. They compared Senecio jacobea plants from invasive populations in New Zealand, Australia, and North America, with Senecio jacobea plants from the native range. In the vegetative stage within introduced populations, they found no reduction in the production of PAs, the main defence chemical. In fact, “plants from invasive populations even produced on average 90% more PAs than plants from native populations.” There was no difference in the production of PAs in the leaves or flower heads, between any of the invasive populations, nor between invasive and native populations. The native European generalist herbivore, cabbage armyworm, Mamestra brassicae, had lower survival on plants from invasive populations than on the plants from native populations. The plants from invasive populations (from all ranges) had fewer leaves attacked and lower feeding damage than did the plants from native populations. The generalist beet armyworm, Spodoptera exigua, laid more eggs on native plants rather than on plants from invasive populations. Both generalist herbivores performed better on (created more damage on) plants from native populations than on invasive populations.
The native specialist herbivore that Joshi and Vrieling, 2005, tested was the Cinnabar moth, Tyria jacobaeae, which is attracted to and known to sequester PAs. In contrast to the generalist insects, the Cinnabar moth attacked more plants from invasive populations than from native populations. “In addition to lower levels of PAs and therefore lower attractiveness, native plants must have had other defences against Tyria that were lost in invasive plants.”
Joshi and Vrieling, 2005, found that the Senecio jacobea plants from invasive populations grew larger than the plants from native populations, as expected by the EICA hypothesis. Even though invasive plants were able to produce higher levels of PAs, they were also able to divert some resources to greater investment in growth and reproduction. “In the invasive populations where specialist biological control agents had been introduced 25-30 years ago…, there was a trend of PA-levels to decrease again.”
“The results…suggest that rapid evolution of decreased defence against specialist herbivores…, and higher protection against generalist herbivores are factors contributing to the invasion success of Senecio jacobaeae. In the native range, defence traits of this species are maintained at intermediate levels by opposing selection pressures of specialist and generalist herbivores. In introduced areas, however, invasive plants have often escaped specialist insect herbivores and are exposed to a natural enemy complex that is dominated by generalists only. Therefore, the observed shift towards higher defences against generalist herbivores and lower defences against specialist herbivores,… was the expected evolutionary response… If the defence chemicals against generalists are less ‘expensive’ than the defence chemicals against the specialists, such an evolutionary shift in defence stategy in invasive populations may result in a net gain of resources for the plant, and may explain the increased growth and reproduction of invasive populations” (Joshi & Vrieling, 2005).
Next Section:
Role of Microbes
For summaries of additional research on the success of invasive species:
Evolution of Increased Competitive Ability hypothesis (EICA): Its role in the Success or Failure of Invasive Plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
If the EICA hypothesis is valid, then one would expect to find that “the low abundance or absence of herbivores in the new environments could cause rapid evolutionary change through selection for reduced anti-herbivore defences. Improved competitive ability through a shift in allocation from defence to growth has been predicted by the Evolution of Improved Competitive Ability… hypothesis” (Joshi & Vrieling, 2005). Joshi and Vrieling tested this idea by conducting garden experiments on Senecio jacobea, a native of Eurasia that has become an invasive species in New Zealand, Australia, and North America. Senecio jacobea is a perennial species that is attacked by more than 70 herbivores in its native range. “In the native area, specialist herbivores and defence mechanisms in S. jacobea, especially the production of pyrrolizidine alkaloids (PAs), secondary compounds toxic to non-adapted [generalist] herbivores and livestock, have been extensively studied and related to the occurrence of specialist insect herbivores.” In the introduced range in the USA, about 42 species of generalist arthropods have been recorded as feeding on Senecio jacobea.
Joshi and Vrieling, 2005, compared herbivory by generalist and specialist insects, in both native and invasive populations of Senecio jacobea. They compared Senecio jacobea plants from invasive populations in New Zealand, Australia, and North America, with Senecio jacobea plants from the native range. In the vegetative stage within introduced populations, they found no reduction in the production of PAs, the main defence chemical. In fact, “plants from invasive populations even produced on average 90% more PAs than plants from native populations.” There was no difference in the production of PAs in the leaves or flower heads, between any of the invasive populations, nor between invasive and native populations. The native European generalist herbivore, cabbage armyworm, Mamestra brassicae, had lower survival on plants from invasive populations than on the plants from native populations. The plants from invasive populations (from all ranges) had fewer leaves attacked and lower feeding damage than did the plants from native populations. The generalist beet armyworm, Spodoptera exigua, laid more eggs on native plants rather than on plants from invasive populations. Both generalist herbivores performed better on (created more damage on) plants from native populations than on invasive populations.
The native specialist herbivore that Joshi and Vrieling, 2005, tested was the Cinnabar moth, Tyria jacobaeae, which is attracted to and known to sequester PAs. In contrast to the generalist insects, the Cinnabar moth attacked more plants from invasive populations than from native populations. “In addition to lower levels of PAs and therefore lower attractiveness, native plants must have had other defences against Tyria that were lost in invasive plants.”
Joshi and Vrieling, 2005, found that the Senecio jacobea plants from invasive populations grew larger than the plants from native populations, as expected by the EICA hypothesis. Even though invasive plants were able to produce higher levels of PAs, they were also able to divert some resources to greater investment in growth and reproduction. “In the invasive populations where specialist biological control agents had been introduced 25-30 years ago…, there was a trend of PA-levels to decrease again.”
“The results…suggest that rapid evolution of decreased defence against specialist herbivores…, and higher protection against generalist herbivores are factors contributing to the invasion success of Senecio jacobaeae. In the native range, defence traits of this species are maintained at intermediate levels by opposing selection pressures of specialist and generalist herbivores. In introduced areas, however, invasive plants have often escaped specialist insect herbivores and are exposed to a natural enemy complex that is dominated by generalists only. Therefore, the observed shift towards higher defences against generalist herbivores and lower defences against specialist herbivores,… was the expected evolutionary response… If the defence chemicals against generalists are less ‘expensive’ than the defence chemicals against the specialists, such an evolutionary shift in defence stategy in invasive populations may result in a net gain of resources for the plant, and may explain the increased growth and reproduction of invasive populations” (Joshi & Vrieling, 2005).
Next Section:
Role of Microbes
For summaries of additional research on the success of invasive species: