Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
Indirect defense mechanisms: Their role in invasive plant success
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Indirect defense mechanisms (such as increased nectar production) attract, rather than repel, herbivores. Rudgers, 2004, concluded that indirect defense mechanisms experience evolutionary pressure both from the herbivores that attack the plant population, and from the predators that attack those herbivores. Because multiple interactions within three trophic levels are at play, indirect defense mechanisms may respond more rapidly to environmental change, potentially causing relatively rapid change within a plant population. Rudgers, 2004, conducted experiments on wild cotton (Gossypium thurberi). Wild cotton produces extrafloral nectar on its leaves. The nectar attracts predatory ants that feed on the nectar and also attack herbivores on the cotton plants. Using insecticide to reduce the ants caused “herbivory reduced plant growth and seed production.” By killing the ants that fed on the nectar, this increased the survival of herbivores that fed on the cotton plant, which resulted in greater damage to leaves and reduced cotton flower and seed reproduction. The research showed that “predatory ants can act as agents of selection on extrafloral nectary traits” in wild cotton.
Tallow trees also produce a nectar on the leaves that has no function in reproduction. Rather, the nectar attracts ants that in turn discourage herbivores from eating the leaves. Thus, this extrafloral nectar production (EFN) is an indirect and induced defense mechanism.
Carrillo, Wang, Ding, Klootwyk, and Siemann (2012) tested the EICA hypothesis in tallow trees by comparing the trees’ response to leaf damage in their natural habitat in China, with their response to leaf damage in an introduced habitat in Texas. They found that extrafloral nectar production, a response induced by leaf damage (normally caused by herbivores), was greater within the Chinese native population than within the Texas introduced population. This result supports the hypothesis that trees in a non-natural habitat do not have to expend as much energy resources on defense mechanisms, and thus have more energy to expend on growth and reproduction, thus giving them a potentially competitive advantage over native species in the same habitat. Carrillo, et al., 2012, also found that the genetic trait that enables the tree to produce extrafloral nectar remains within the non-native population, thus potentially giving the tree the ability to defend against biological controls that may be introduced later. Thus, while this indirect defense mechanism is reduced within non-native populations, it has the potential to become fully activated in the event of the introduction of a biological control. However, the research in this field remains limited, and there are very few studies comparing induced defense mechanisms with constitutive defense mechanisms.
References:
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Indirect defense mechanisms: Their role in invasive plant success
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Indirect defense mechanisms (such as increased nectar production) attract, rather than repel, herbivores. Rudgers, 2004, concluded that indirect defense mechanisms experience evolutionary pressure both from the herbivores that attack the plant population, and from the predators that attack those herbivores. Because multiple interactions within three trophic levels are at play, indirect defense mechanisms may respond more rapidly to environmental change, potentially causing relatively rapid change within a plant population. Rudgers, 2004, conducted experiments on wild cotton (Gossypium thurberi). Wild cotton produces extrafloral nectar on its leaves. The nectar attracts predatory ants that feed on the nectar and also attack herbivores on the cotton plants. Using insecticide to reduce the ants caused “herbivory reduced plant growth and seed production.” By killing the ants that fed on the nectar, this increased the survival of herbivores that fed on the cotton plant, which resulted in greater damage to leaves and reduced cotton flower and seed reproduction. The research showed that “predatory ants can act as agents of selection on extrafloral nectary traits” in wild cotton.
Tallow trees also produce a nectar on the leaves that has no function in reproduction. Rather, the nectar attracts ants that in turn discourage herbivores from eating the leaves. Thus, this extrafloral nectar production (EFN) is an indirect and induced defense mechanism.
Carrillo, Wang, Ding, Klootwyk, and Siemann (2012) tested the EICA hypothesis in tallow trees by comparing the trees’ response to leaf damage in their natural habitat in China, with their response to leaf damage in an introduced habitat in Texas. They found that extrafloral nectar production, a response induced by leaf damage (normally caused by herbivores), was greater within the Chinese native population than within the Texas introduced population. This result supports the hypothesis that trees in a non-natural habitat do not have to expend as much energy resources on defense mechanisms, and thus have more energy to expend on growth and reproduction, thus giving them a potentially competitive advantage over native species in the same habitat. Carrillo, et al., 2012, also found that the genetic trait that enables the tree to produce extrafloral nectar remains within the non-native population, thus potentially giving the tree the ability to defend against biological controls that may be introduced later. Thus, while this indirect defense mechanism is reduced within non-native populations, it has the potential to become fully activated in the event of the introduction of a biological control. However, the research in this field remains limited, and there are very few studies comparing induced defense mechanisms with constitutive defense mechanisms.
References:
- Carrillo, J., Wang, Y., Ding, J., Klootwyk, K., & Siemann, E. (June, 2012). Decreased indirect defense in the invasive tree, Triadica sebifera. Plant Ecology, 213, (6), 945-954.
- Rudgers, J.A. (Jan., 2004). Enemies of herbivores can shape plant traits: Selection in a facultative ant-plant mutualism. Ecology, 85 (1), 192-205.
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