Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
Enemy Release Hypothesis (ERH): 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.
Do invasive species succeed in the new environment because of some trait(s) that is inherent in the species when they arrive in the new habitat, or do they develop/evolve new or enhanced competitive advantages within the new environment?
A number of researchers have posed that plant and animal species become invasive due to the lack of natural predators in their new habitat. When placed in a new environment, the plant or animal species is ‘released’ from its natural enemies, and thus can thrive. Ridenour, Vivanco, Feng, Horiuchi, and Callaway (2008) note that the enemy release hypothesis is based on the idea that “exotic [plant] invaders explode in abundance because they are no longer suppressed by the specialist herbivore consumers they evolved with in their natural environments.”
Ziska and Runion (2007, p. 264) point out that invasive plants have a number of biological similarities that help explain why they become problems. Plants become problems in agriculture because of their superior “colonization of disturbed environments, vigorous growth, prodigious seed production, and seed longevity.” In addition, some invasives are able to spread vegetatively, in other words, without seed production. Non-crop species become a problem if they can successfully compete with crop species.
The Enemy Release hypothesis is based on the following: Plants have a limited amount of energy resources (in the form of carbohydrates) to devote to (1) growth, (2) reproduction, and (3) defense from herbivore predation. Because there are no natural herbivore predators in the new ecological community, Blossey and Nötzold, 1995, proposed that introduced plant species do not have to waste resources defending against predators. Thus, they can redirect resources to growth and reproduction. This would, over time, result in the “evolution of increased competitive ability” (EICA). If this hypothesis is correct, this increased competitive ability could be a major factor in the plant’s success within the new habitat.
Treharne, 1989, found that non-native plants that become invasive have a greater genetic diversity and therefore greater physiological plasticity when compared with many crops. Schierenbeck, et al., 1994, found that invasive plant genotypes can be larger or more reproductive than the same species in its native habitat.
Research in support of the Enemy Release hypothesis, or the IECA hypothesis, has shown mixed results, and has mainly focused on growth and reproduction and studies of direct defense mechanisms (e.g., mechanical defenses, such as spines and thorns, or biochemical defenses, such as alkaloids, lignins, flavonoids, and tannins). Some studies (Joshi and Vrieling, 2005; Ridenour, et al., 2008) have shown that some invasive plant species have a higher level of chemical defenses in the introduced habitat than in their native habitat, which is the opposite of what one would expect if the IECA hypothesis is correct. Both Centaurea maculosa and Senecio jacobaea appear to have a competitive advantage because they expend more resources on defense in the introduced habitat than in their native habitats.
References:
Next Sections on the research on the success of invasive species:
Enemy Release Hypothesis (ERH): 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.
Do invasive species succeed in the new environment because of some trait(s) that is inherent in the species when they arrive in the new habitat, or do they develop/evolve new or enhanced competitive advantages within the new environment?
A number of researchers have posed that plant and animal species become invasive due to the lack of natural predators in their new habitat. When placed in a new environment, the plant or animal species is ‘released’ from its natural enemies, and thus can thrive. Ridenour, Vivanco, Feng, Horiuchi, and Callaway (2008) note that the enemy release hypothesis is based on the idea that “exotic [plant] invaders explode in abundance because they are no longer suppressed by the specialist herbivore consumers they evolved with in their natural environments.”
Ziska and Runion (2007, p. 264) point out that invasive plants have a number of biological similarities that help explain why they become problems. Plants become problems in agriculture because of their superior “colonization of disturbed environments, vigorous growth, prodigious seed production, and seed longevity.” In addition, some invasives are able to spread vegetatively, in other words, without seed production. Non-crop species become a problem if they can successfully compete with crop species.
The Enemy Release hypothesis is based on the following: Plants have a limited amount of energy resources (in the form of carbohydrates) to devote to (1) growth, (2) reproduction, and (3) defense from herbivore predation. Because there are no natural herbivore predators in the new ecological community, Blossey and Nötzold, 1995, proposed that introduced plant species do not have to waste resources defending against predators. Thus, they can redirect resources to growth and reproduction. This would, over time, result in the “evolution of increased competitive ability” (EICA). If this hypothesis is correct, this increased competitive ability could be a major factor in the plant’s success within the new habitat.
Treharne, 1989, found that non-native plants that become invasive have a greater genetic diversity and therefore greater physiological plasticity when compared with many crops. Schierenbeck, et al., 1994, found that invasive plant genotypes can be larger or more reproductive than the same species in its native habitat.
Research in support of the Enemy Release hypothesis, or the IECA hypothesis, has shown mixed results, and has mainly focused on growth and reproduction and studies of direct defense mechanisms (e.g., mechanical defenses, such as spines and thorns, or biochemical defenses, such as alkaloids, lignins, flavonoids, and tannins). Some studies (Joshi and Vrieling, 2005; Ridenour, et al., 2008) have shown that some invasive plant species have a higher level of chemical defenses in the introduced habitat than in their native habitat, which is the opposite of what one would expect if the IECA hypothesis is correct. Both Centaurea maculosa and Senecio jacobaea appear to have a competitive advantage because they expend more resources on defense in the introduced habitat than in their native habitats.
References:
- Blossey, B., & Nötzold, R. (Oct., 1995). Evolution of increased competitive ability in invasive nonindigenous plants: A hypothesis. Journal of Ecology, 83 (5), 887-889.
- 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.
- Ridenour, W.M., & Callaway, R.M. (Oct., 2004). Novel weapons: Invasion success and the evolution of increased competitive ability. Frontiers in Ecology and the Environment, 2(8): 436- 443.
- Treharne, K. (1989). The implications of the “greenhouse effect” for fertilizers and agrochemicals. In The greenhouse effect and UK agriculture, Bennet, R.C. (Ed.) Ministry of Agriculture, Fisheries and Food, U.K., pages 67-78.
- Ziska, L. H., & Runion, G.B. (2007). Future weed, pest, and disease problems for plants. In Newton, P.C.D., Carran, R.A., Edwards, G.R., & Niklaus, P.A. (Eds.), Agroecosystems in a changing climate, pp. 261-287. Boca Raton, FL: CRC Press.
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