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, updated November 2024.
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).
Beaton, et al. (2011), offer further evidence in support of the theory. Lespedeza cuneata plants were introduced from Japan into the USA in 1896. Several additional cultivars were planted in the USA in later years, to be used as soil stabilizer and forage and hay crop.
Beaton, et al., carried out unique experiments comparing the growth of seeds of Lespedeza cuneata from three different origins. They collected modern seeds of plants from their native environment in Japan (native plants). They collected modern seeds of the invasive Lespedeza cuneata in Illinois (invasive plants). And they were able to locate seeds from the seedbank of Lespedeza cuneata that had been harvested from the Arlington Experimental Farm in 1930, and had been maintained in the USDA Germplasm Resources Information Network (GRIN). They called these seeds "ancestral."
Plants from the three origins were grown in a greenhouse. Eggs of the corn earworm, Helicoverpa zea,a generalist herbivore that is one of the main herbivores on L. cuneata, were hatched and grown for a week prior to being placed on the plants. Plants from different origins were positioned randomly in the greenhouse, so that they would compete with each other for resources. Water and fertilizer were kept to a minimum to ensure competition for soil resources.
Beaton, et al., compared both interspecific competition and intraspecific competition between and among plants from each of the three origins. They compared above ground biomass data from plants after five months growth.
To measure relative investment in plant defenses, Beaton, et al., "assessed constitutive and inducible resistance using bioassays with H. zea. Seed germination was accomplished as in the competition study, except seeds were placed in a growth chamber to eliminate potential damage from any herbivores residing in the greenhouse...Plants were fertilized weekly. After three weeks, 54 seedlings of each of the three genotypes... were transplanted into...pots. Four weeks later, 45 plants of each genotype were subjected to herbivory by H. zea caterpillars." Comparisons were made between plants exposed to zero, one, two or three young caterpillars. After three days, they conducted a visual estimate of the degree of herbivory per plant. In addition to plant biomass, caterpillar weight was also measured.
Beaton, et al., stated, "As predicted by the EICA hypothesis, we found that the invasive genotype outcompeted the ancestral and native genotypes... Specifically, aboveground biomass of the invasive genotype was greater than both the native and ancestral genotype and was unaffected by the identity of the competitor, whereas both the native and ancestral genotypes had reduced aboveground biomass when they were paired with the invasive genotype relative ot other competitors."
Constitutive defenses are those expressed continuously. For example, L. cuneata has numerous toxic chemicals that repel herbivores, and leaves and stems that are high in tannins. Induced defenses are stimulated only after attack by an herbivore. Induced defenses require less resources to maintain. So Beaton, et al., predicted that the invasive genotype would have greater caterpillar growth.
"Constitutive resistance levels were assessed by measuring the growth of caterpillars on plants that had not received any herbivory damage..., while inducibility levels were assessed by measuring the growth of caterpillars on plants that have received some herbivory damage."
As predicted, "The caterpillars grown on the undamaged invasive genotype were larger than caterpillars growing on the undamaged ancestral and native genotypes,... indicating that both ancestral and native L. cuneata genotypes had stronger constitutive resistance than the invasive genotype... Further, only the invasive genotype exhibited induced resistance against herbivores,... as indicated by a significant negative relationship between the degree of herbivore damage and caterpillar body mass."
Franks, et al. (May 2008), challenge the predictability of the EICA theory. They conducted a seed growing study using seeds from the Paperbark tree, Melaleuca quinquenervia, a native to Australia and New Caledonia, that was introduced to Florida in the 1800s and has caused significant ecological damage in the Everglades. Two species of biocontrol insects from the native range in Australia had been released in Florida, Oxyops vitioasa, a leaf-chewing weevil, in 1997, and Boreioglycaspis melaleucae, a sap-feeding psyllid, in 2002.
In 1999, Franks, et al., collected Melaleuca seeds from various locations within both the native and introduced ranges of the species. They germinated the seeds in a greenhouse, and transplanted them to the field. They established 24 split plots with 120 seedlings each. One split plot of each whole plot was randomly assigned a herbivore exclusion treatment.
They compared each plant every 2-3 months, for plant attractiveness and resistance to herbivores (measured by presence or absence of insects, and presence or absence of damage), competitive ability, and fitness (leaf area, plant height, and biomass). They carried out genotype analysis of the plants, examining genetic correlations for each measured trait.
They found that there was selection for decreased resistance to herbivory. "The patterns of selection on growth and defense we found... accurately predict the lack of evolutionary changes in Melaleuca following its introduction into its new range... there was strong selection on increased growth and weak selection on decreased resistance and... the patterns of selection generally did not differ for the native or introduced genotypes nor depending on the presence or absence of herbivores. Thus, it would not be expected that evolutionary changes in growth or resistance would occur following release from natural enemies."
"While there was evidence for selection on herbivory resistance and competitive ability traits, the patterns of selection generally did not differ depending on the source population or the presence of herbivores and did not match predictions based on the EICA hypothesis."
Zheng, et al. (October 22, 2015), conducted common garden experiments to test the effects of plant competitors. "Ageratina adenophora plants from both native and invasive ranges competed directly and competed with native residents from both invasive (China) and native (Mexico) ranges respectively. Invasive A. adenophora plants were more competitive than their conspecifics from native populations when competing with natives from China (interspecific competition), but not when competing with natives from Mexico. Invasive A. adenophora plants also showed higher competitive ability when grown in high-density monoculture communities of plants from the same population (intrapopulation competition). In contrast, invasive A. adenophora plants showed lower competitive ability when competing with plants from native populations (intraspecific competition)." Their results "indicated that in the invasive range A. adenophora has evolved to effectively cope with co-occurring natives and high density environments, contributing to invasion success. Here, (they) showed the significant effects of competitors, which should be considered carefully when testing the EICA hypothesis."
For further discussion on the EICA theory, within this website, see: Enemy Release Hypothesis
References:
Next Sections on research on the success of invasive species:
Link to 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, updated November 2024.
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).
Beaton, et al. (2011), offer further evidence in support of the theory. Lespedeza cuneata plants were introduced from Japan into the USA in 1896. Several additional cultivars were planted in the USA in later years, to be used as soil stabilizer and forage and hay crop.
Beaton, et al., carried out unique experiments comparing the growth of seeds of Lespedeza cuneata from three different origins. They collected modern seeds of plants from their native environment in Japan (native plants). They collected modern seeds of the invasive Lespedeza cuneata in Illinois (invasive plants). And they were able to locate seeds from the seedbank of Lespedeza cuneata that had been harvested from the Arlington Experimental Farm in 1930, and had been maintained in the USDA Germplasm Resources Information Network (GRIN). They called these seeds "ancestral."
Plants from the three origins were grown in a greenhouse. Eggs of the corn earworm, Helicoverpa zea,a generalist herbivore that is one of the main herbivores on L. cuneata, were hatched and grown for a week prior to being placed on the plants. Plants from different origins were positioned randomly in the greenhouse, so that they would compete with each other for resources. Water and fertilizer were kept to a minimum to ensure competition for soil resources.
Beaton, et al., compared both interspecific competition and intraspecific competition between and among plants from each of the three origins. They compared above ground biomass data from plants after five months growth.
To measure relative investment in plant defenses, Beaton, et al., "assessed constitutive and inducible resistance using bioassays with H. zea. Seed germination was accomplished as in the competition study, except seeds were placed in a growth chamber to eliminate potential damage from any herbivores residing in the greenhouse...Plants were fertilized weekly. After three weeks, 54 seedlings of each of the three genotypes... were transplanted into...pots. Four weeks later, 45 plants of each genotype were subjected to herbivory by H. zea caterpillars." Comparisons were made between plants exposed to zero, one, two or three young caterpillars. After three days, they conducted a visual estimate of the degree of herbivory per plant. In addition to plant biomass, caterpillar weight was also measured.
Beaton, et al., stated, "As predicted by the EICA hypothesis, we found that the invasive genotype outcompeted the ancestral and native genotypes... Specifically, aboveground biomass of the invasive genotype was greater than both the native and ancestral genotype and was unaffected by the identity of the competitor, whereas both the native and ancestral genotypes had reduced aboveground biomass when they were paired with the invasive genotype relative ot other competitors."
Constitutive defenses are those expressed continuously. For example, L. cuneata has numerous toxic chemicals that repel herbivores, and leaves and stems that are high in tannins. Induced defenses are stimulated only after attack by an herbivore. Induced defenses require less resources to maintain. So Beaton, et al., predicted that the invasive genotype would have greater caterpillar growth.
"Constitutive resistance levels were assessed by measuring the growth of caterpillars on plants that had not received any herbivory damage..., while inducibility levels were assessed by measuring the growth of caterpillars on plants that have received some herbivory damage."
As predicted, "The caterpillars grown on the undamaged invasive genotype were larger than caterpillars growing on the undamaged ancestral and native genotypes,... indicating that both ancestral and native L. cuneata genotypes had stronger constitutive resistance than the invasive genotype... Further, only the invasive genotype exhibited induced resistance against herbivores,... as indicated by a significant negative relationship between the degree of herbivore damage and caterpillar body mass."
Franks, et al. (May 2008), challenge the predictability of the EICA theory. They conducted a seed growing study using seeds from the Paperbark tree, Melaleuca quinquenervia, a native to Australia and New Caledonia, that was introduced to Florida in the 1800s and has caused significant ecological damage in the Everglades. Two species of biocontrol insects from the native range in Australia had been released in Florida, Oxyops vitioasa, a leaf-chewing weevil, in 1997, and Boreioglycaspis melaleucae, a sap-feeding psyllid, in 2002.
In 1999, Franks, et al., collected Melaleuca seeds from various locations within both the native and introduced ranges of the species. They germinated the seeds in a greenhouse, and transplanted them to the field. They established 24 split plots with 120 seedlings each. One split plot of each whole plot was randomly assigned a herbivore exclusion treatment.
They compared each plant every 2-3 months, for plant attractiveness and resistance to herbivores (measured by presence or absence of insects, and presence or absence of damage), competitive ability, and fitness (leaf area, plant height, and biomass). They carried out genotype analysis of the plants, examining genetic correlations for each measured trait.
They found that there was selection for decreased resistance to herbivory. "The patterns of selection on growth and defense we found... accurately predict the lack of evolutionary changes in Melaleuca following its introduction into its new range... there was strong selection on increased growth and weak selection on decreased resistance and... the patterns of selection generally did not differ for the native or introduced genotypes nor depending on the presence or absence of herbivores. Thus, it would not be expected that evolutionary changes in growth or resistance would occur following release from natural enemies."
"While there was evidence for selection on herbivory resistance and competitive ability traits, the patterns of selection generally did not differ depending on the source population or the presence of herbivores and did not match predictions based on the EICA hypothesis."
Zheng, et al. (October 22, 2015), conducted common garden experiments to test the effects of plant competitors. "Ageratina adenophora plants from both native and invasive ranges competed directly and competed with native residents from both invasive (China) and native (Mexico) ranges respectively. Invasive A. adenophora plants were more competitive than their conspecifics from native populations when competing with natives from China (interspecific competition), but not when competing with natives from Mexico. Invasive A. adenophora plants also showed higher competitive ability when grown in high-density monoculture communities of plants from the same population (intrapopulation competition). In contrast, invasive A. adenophora plants showed lower competitive ability when competing with plants from native populations (intraspecific competition)." Their results "indicated that in the invasive range A. adenophora has evolved to effectively cope with co-occurring natives and high density environments, contributing to invasion success. Here, (they) showed the significant effects of competitors, which should be considered carefully when testing the EICA hypothesis."
For further discussion on the EICA theory, within this website, see: Enemy Release Hypothesis
References:
- Beaton, L.L., van Zandt, P.A., Esselman, E.J., & Knight, T.M. (2011). Comparison of the herbivore defense and competitive ability of ancestral and modern genotypes of an invasive plant, Lespedeza cuneata. Oikos, 120, 1413-1419.
- Franks, S.J., Pratt, P.D., Dray, F.A., & Simms, E.L. (May 2008). Selection on herbivory resistance and growth rate in an invasive plant. The American Naturalist, 171(5), 678-691.
- Joshi, J., Vrieling, K., & Rejmanek, M. (July, 2005). The enemy release and EICA hypothesis revisited: Incorporating the fundamental difference between specialist and generalist herbivores. Ecology Letters, 8 (7), 704–714.
- Zheng,Y., Feng, Y., Valiente-Banuet, A., Li, Y., Liao, Z., Zhang, J., & Chen, Y. (October 22, 2015). Are invasive plants more competitive than native conspecifics? Patterns vary with competitors. Scientific Reports, 5, 1-8.
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