Photo: Russian Olive, southwest Montana. © 2020 Delena Norris-Tull
Constitutive defense mechanisms
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Constitutive defense mechanisms are those traits that are always expressed within a plant, unlike induced mechanisms, which are activated rapidly in response to an environmental stimulus (such as herbivory). Wang, Siemann, Wheeler, Zhu, Gu, and Ding (2012) studied two constitutive defense traits, tannin levels and flavonoid levels in leaves, in tallow trees.
The Chinese tallow tree (Triadica sebifera), native to China and Japan, is invasive in the southeastern United States, where it has displaced native trees and created some monospecific stands (stands composed entirely of tallow trees). The tree was first introduced to the United States in Georgia and South Carolina in the late 1700s. It was introduced in Texas, Florida, and Louisiana in the early 1900s (Carrillo, Wang, Ding, Klootwyk, & Siemann, 2012) for ornamental and agricultural purposes. The tree is listed as a noxious invasive species in Texas, Louisiana, Florida, and Mississippi.
Tallow tree populations in their introduced range experience “low herbivore loads, have low allocation to defense against leaf chewing herbivores, high growth rates, and earlier reproductive maturity compared to those [tallow trees] from the native range” (Carillo, et al, 2012).
When comparing tannin levels in native tallow tree populations with tannin levels in the introduced habitat, Wang, et al., 2012, found tannin levels to be higher in the native habitat, as would be expected with the EICA hypothesis. But they found higher levels of flavonoids within the introduced populations, seemingly in violation of the hypotheses. However, flavonoids possess several qualities beneficial to tallow trees, including UV resistance, fungal and nematode resistance, and drought resistance, thus the extra energy needed to produce them gives trees a competitive advantage. Each trait of each specific flavonoid compound requires research to determine its specific function in the plant’s ability to compete. Little such research has been conducted, thus, there remains no solid hypothesis to explain the differential effects that different flavonoids may have in enabling non-native species to be successful in the introduced range. Future researchers also need to examine the costs and trade-offs between multiple chemical compounds within a single plant species. When the tallow tree reduces its tannin levels in the introduced range, is it reallocating those resources to increased flavonoid levels, as well as to increased growth and reproductive capacity, as suspected by Wang, et al., 2012?
Direct defense mechanisms tend to repel herbivores. Lignins and tannins are believed to be effective at repelling specialists (herbivores that only eat a few plant species), as they reduce the digestibility of the plant. Toxins such as alkaloids and flavonoids are believed to be more effective at repelling generalists (herbivores that can eat a wide variety of plant species) because generalists tend to lack the traits found in specialists that enable specialists to detoxify or sequester the toxic compounds (Müller-Scharer, Schaffner, & Steinger, 2004). However, the research in this area is limited and the results are mixed.
Wang, et al., 2012, found that a specialist caterpillar grew bigger on tallow trees with lower levels of tannins (trees growing in the introduced habitat), as would be predicted by Müller-Scharer, et al., 2004. (Refer to the chapter on “Biological Controls” for details of their review of the research on evolution in invasive species and its implications for biological control). But one of the species of generalist caterpillars Wang, et al., 2012, tested also grew bigger on the leaves of trees with lower tannin levels, while the other generalist caterpillar tested did not vary in growth rate between low and high tannin levels. Again, limited research in the field has resulted in a limited ability to develop theory to explain the relationship between specialist versus generalist herbivores and the host plant.
Most direct defense mechanisms do not change rapidly within a population. Evolutionary pressure away from using resources to create these direct defense mechanisms may take a relatively long time to cause changes within the DNA of an introduced population.
Next Section:
Evolution of Increased Competitive Ability
For summaries of additional research on the success of invasive species:
Constitutive defense mechanisms
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Constitutive defense mechanisms are those traits that are always expressed within a plant, unlike induced mechanisms, which are activated rapidly in response to an environmental stimulus (such as herbivory). Wang, Siemann, Wheeler, Zhu, Gu, and Ding (2012) studied two constitutive defense traits, tannin levels and flavonoid levels in leaves, in tallow trees.
The Chinese tallow tree (Triadica sebifera), native to China and Japan, is invasive in the southeastern United States, where it has displaced native trees and created some monospecific stands (stands composed entirely of tallow trees). The tree was first introduced to the United States in Georgia and South Carolina in the late 1700s. It was introduced in Texas, Florida, and Louisiana in the early 1900s (Carrillo, Wang, Ding, Klootwyk, & Siemann, 2012) for ornamental and agricultural purposes. The tree is listed as a noxious invasive species in Texas, Louisiana, Florida, and Mississippi.
Tallow tree populations in their introduced range experience “low herbivore loads, have low allocation to defense against leaf chewing herbivores, high growth rates, and earlier reproductive maturity compared to those [tallow trees] from the native range” (Carillo, et al, 2012).
When comparing tannin levels in native tallow tree populations with tannin levels in the introduced habitat, Wang, et al., 2012, found tannin levels to be higher in the native habitat, as would be expected with the EICA hypothesis. But they found higher levels of flavonoids within the introduced populations, seemingly in violation of the hypotheses. However, flavonoids possess several qualities beneficial to tallow trees, including UV resistance, fungal and nematode resistance, and drought resistance, thus the extra energy needed to produce them gives trees a competitive advantage. Each trait of each specific flavonoid compound requires research to determine its specific function in the plant’s ability to compete. Little such research has been conducted, thus, there remains no solid hypothesis to explain the differential effects that different flavonoids may have in enabling non-native species to be successful in the introduced range. Future researchers also need to examine the costs and trade-offs between multiple chemical compounds within a single plant species. When the tallow tree reduces its tannin levels in the introduced range, is it reallocating those resources to increased flavonoid levels, as well as to increased growth and reproductive capacity, as suspected by Wang, et al., 2012?
Direct defense mechanisms tend to repel herbivores. Lignins and tannins are believed to be effective at repelling specialists (herbivores that only eat a few plant species), as they reduce the digestibility of the plant. Toxins such as alkaloids and flavonoids are believed to be more effective at repelling generalists (herbivores that can eat a wide variety of plant species) because generalists tend to lack the traits found in specialists that enable specialists to detoxify or sequester the toxic compounds (Müller-Scharer, Schaffner, & Steinger, 2004). However, the research in this area is limited and the results are mixed.
Wang, et al., 2012, found that a specialist caterpillar grew bigger on tallow trees with lower levels of tannins (trees growing in the introduced habitat), as would be predicted by Müller-Scharer, et al., 2004. (Refer to the chapter on “Biological Controls” for details of their review of the research on evolution in invasive species and its implications for biological control). But one of the species of generalist caterpillars Wang, et al., 2012, tested also grew bigger on the leaves of trees with lower tannin levels, while the other generalist caterpillar tested did not vary in growth rate between low and high tannin levels. Again, limited research in the field has resulted in a limited ability to develop theory to explain the relationship between specialist versus generalist herbivores and the host plant.
Most direct defense mechanisms do not change rapidly within a population. Evolutionary pressure away from using resources to create these direct defense mechanisms may take a relatively long time to cause changes within the DNA of an introduced population.
Next Section:
Evolution of Increased Competitive Ability
For summaries of additional research on the success of invasive species: