Photo: Wild Mustard, southwest Montana. © 2020 Delena Norris-Tull
Evolutionary Reduced Competitive Ability (ERCA) Hypothesis: Its role in biological invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Invasive, non-native species “do not share an evolutionary history with the community they invade, and one might expect greater allelopathic effects in such [invaded] systems” (Prati and Bossdorf, 2004). But Bossdorf, et al., 2004, have proposed that, in contrast to the enemy release hypothesis, in some species there may actually be selective pressure against non-native plants thriving in the new environment. They call this the “evolutionary reduced competitive ability” hypothesis. “If there is less [interspecific] competition in the invasive range and competitive ability involves traits that have a fitness cost, then selection might act against [those traits], thereby reducing intraspecific interactions too.”
The following greenhouse experiment helped them to develop this hypothesis. Garlic mustard (Alliaria petolata) is a plant native to Europe that has become an invasive and is a threat to native flora in North American forests in the northern US and southern Canada. Bossdorf, et al., 2004, collected garlic mustard seeds from eight European and eight U.S. invaded populations, to study intraspecific competition. They germinated the seeds in petri dishes filled with sterilized sand and compost. The seedlings were planted in trays with sand and potting soil. After five months, each population was replanted and grown in monoculture with the following treatments: some pots had either one plant or two plants; some were put in pairwise mixture with each of the other populations, with a total of 456 pots and 864 plants. In other words, plants were either paired with plants from their own continent or from the opposite continent. They measured the leaf lengths of the plants, and calculated a sum of squared leaf lengths, as a predictor for biomass.
Then the plants were grown for two years. 413 plants survived. The plants were harvested at fruit maturity. Bossdorf, et al., 2004, measured plant height and counted the number of fruits (siliques). They dried and weighed the plants and siliques to attain the biomass. “To estimate the precision of different fitness estimates [they] randomly chose 10 siliques on each plant, weighed them, and then counted and weighed the seeds.” They determined that silique biomass was a “reasonable predictor for fitness” of seed output.
Bossdorf, et al., 2004, found that, “Single plants produced 67% more aboveground biomass, 66% more silique biomass, and 99% more siliques than plants in pairs, suggesting that there was strong [intraspecific] competition for soil resources… Plant fitness also strongly depended on the population of origin and its interaction with density. Thus, fitness had a genetic component.” They found that “native plants were significantly taller than plants from the invasive range…. Plants of native origin growing in pairs were significantly taller (+22%) and produced significantly more siliques (+48%)… In addition, [native plants] reduced the fitness of target [American] plants with respect to silique number and silique biomass more than did [American plants].”
Bossdorf, et al., 2004, concluded that “under optimal, competition-free conditions there were no differences between native and invasive populations of Alliaria petiolata. When plants were competing against conspecifics, however, native populations outperformed those from the invasive range. This completely contradicts the EICA hypothesis.” “A potential caveat of this study could be that we assume no adaptive evolution… in the introduced range… We cannot exclude the possibility that local adaptation to other environment factors is responsible for the observed differences.”
Bossdorf, et al., 2004, stated that, “Reduced competitive ability [in the introduced range] could be the consequence of a genetic bottleneck with subsequent inbreeding depression or random changes through genetic drift…. However, we think that this explanation is rather unlikely as Alliaria has… low within-population genetic variation in both its introduced and native ranges.”
Bossdorf, et al., 2004, finish with, “Another explanation for reduced competitive ability in invasive populations would be directional selection. If there are fewer or weaker competitors in the [invaded habitats]…, and at the same time resource competition involves traits [in Alliaria] with a fitness cost,… then there might be selection against [those traits] in the invasive range. Furthermore, if plants in invasive populations have usually more intra- than interspecific neighbours, an Evolutionary Reduced Competitive Ability (ERCA) may increase stand-level fitness by reducing intraspecific interactions, too. It is conceivable that ERCA allows invasive populations… to use the savings not spent for resource competition in other processes that may contribute to their invasion success, such as plasticity, tolerance to herbivory, or allelopathy.”
Few studies have been conducted that “compared the allelopathic effects of an invasive species on competitors from the native and invaded range.” Prati and Bossdorf, 2004, carried out the following experiment. Prati and Bossdorf, 2004, tested the allelopathic response of North American and European Geum species to the root exudate from garlic mustard. The exudate has several putative allelopathic chemicals that could inhibit growth in neighboring plants.
Prati and Bossdorf, 2004, collected garlic mustard seeds from three different locations on each continent. They planted the mustard seeds in pots with sand and compost, in a greenhouse. They added activated carbon to half of the pots. The carbon did not inhibit the growth of garlic mustard. After a year of growth, they removed the substrate from the pots, and placed the substrate in petri dishes, in which they germinated two different species of Geum seeds. Half of the petri dishes had Geum laciniatum (a native North American species), and half had Geum urbanum (a native European species). Activated carbon was added to half the dishes.
They found that the garlic mustard substrate had a negative impact on the germination of co-occurring species. The two Geum species differed in their sensitivity to the allelochemicals in the substrate. They found that the carbon had a slight to moderate mitigating effect on the response to the allelochemicals, which demonstrated that the mustard plants had, in fact, exuded allelopathic chemicals to the substrate.
For the North American Geum laciniatum, there was no difference in the response to carbon, between substrates from either European and North American garlic mustard. Germination of the North American Geum laciniatum increased more than that of Geum urbanum, when carbon was added to the petri dishes, demonstrating that allelochemicals had moderately inhibited Geum laciniatum growth, more so than the European Geum. This result may support the novel weapons hypothesis.
But the European Geum urbanum results were complex. This species germinated better in substrate from the mustards from invaded North American populations. And carbon slightly decreased germination of Geum in substrate from mustard from North American populations. This suggests that the mustard in the introduced populations has reduced levels of allelochemicals. Carbon increased germination of Geum urbanum in substrate from the European mustard, suggesting that in European populations, Geum urbanum is suppressed by garlic mustard. This suggests that mustards from the European populations have higher levels of allelechemicals. These results would seem to support the evolutionary reduced competitive ability hypothesis.
These complex results indicate that Geum sensitivity to allelochemicals is species-specific. Prati and Bossdorf, 2004, concluded that field tests are needed to learn more about the complex interactions and the role of allelopathy between invasive and native species.
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Evolutionary Reduced Competitive Ability (ERCA) Hypothesis: Its role in biological invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Invasive, non-native species “do not share an evolutionary history with the community they invade, and one might expect greater allelopathic effects in such [invaded] systems” (Prati and Bossdorf, 2004). But Bossdorf, et al., 2004, have proposed that, in contrast to the enemy release hypothesis, in some species there may actually be selective pressure against non-native plants thriving in the new environment. They call this the “evolutionary reduced competitive ability” hypothesis. “If there is less [interspecific] competition in the invasive range and competitive ability involves traits that have a fitness cost, then selection might act against [those traits], thereby reducing intraspecific interactions too.”
The following greenhouse experiment helped them to develop this hypothesis. Garlic mustard (Alliaria petolata) is a plant native to Europe that has become an invasive and is a threat to native flora in North American forests in the northern US and southern Canada. Bossdorf, et al., 2004, collected garlic mustard seeds from eight European and eight U.S. invaded populations, to study intraspecific competition. They germinated the seeds in petri dishes filled with sterilized sand and compost. The seedlings were planted in trays with sand and potting soil. After five months, each population was replanted and grown in monoculture with the following treatments: some pots had either one plant or two plants; some were put in pairwise mixture with each of the other populations, with a total of 456 pots and 864 plants. In other words, plants were either paired with plants from their own continent or from the opposite continent. They measured the leaf lengths of the plants, and calculated a sum of squared leaf lengths, as a predictor for biomass.
Then the plants were grown for two years. 413 plants survived. The plants were harvested at fruit maturity. Bossdorf, et al., 2004, measured plant height and counted the number of fruits (siliques). They dried and weighed the plants and siliques to attain the biomass. “To estimate the precision of different fitness estimates [they] randomly chose 10 siliques on each plant, weighed them, and then counted and weighed the seeds.” They determined that silique biomass was a “reasonable predictor for fitness” of seed output.
Bossdorf, et al., 2004, found that, “Single plants produced 67% more aboveground biomass, 66% more silique biomass, and 99% more siliques than plants in pairs, suggesting that there was strong [intraspecific] competition for soil resources… Plant fitness also strongly depended on the population of origin and its interaction with density. Thus, fitness had a genetic component.” They found that “native plants were significantly taller than plants from the invasive range…. Plants of native origin growing in pairs were significantly taller (+22%) and produced significantly more siliques (+48%)… In addition, [native plants] reduced the fitness of target [American] plants with respect to silique number and silique biomass more than did [American plants].”
Bossdorf, et al., 2004, concluded that “under optimal, competition-free conditions there were no differences between native and invasive populations of Alliaria petiolata. When plants were competing against conspecifics, however, native populations outperformed those from the invasive range. This completely contradicts the EICA hypothesis.” “A potential caveat of this study could be that we assume no adaptive evolution… in the introduced range… We cannot exclude the possibility that local adaptation to other environment factors is responsible for the observed differences.”
Bossdorf, et al., 2004, stated that, “Reduced competitive ability [in the introduced range] could be the consequence of a genetic bottleneck with subsequent inbreeding depression or random changes through genetic drift…. However, we think that this explanation is rather unlikely as Alliaria has… low within-population genetic variation in both its introduced and native ranges.”
Bossdorf, et al., 2004, finish with, “Another explanation for reduced competitive ability in invasive populations would be directional selection. If there are fewer or weaker competitors in the [invaded habitats]…, and at the same time resource competition involves traits [in Alliaria] with a fitness cost,… then there might be selection against [those traits] in the invasive range. Furthermore, if plants in invasive populations have usually more intra- than interspecific neighbours, an Evolutionary Reduced Competitive Ability (ERCA) may increase stand-level fitness by reducing intraspecific interactions, too. It is conceivable that ERCA allows invasive populations… to use the savings not spent for resource competition in other processes that may contribute to their invasion success, such as plasticity, tolerance to herbivory, or allelopathy.”
Few studies have been conducted that “compared the allelopathic effects of an invasive species on competitors from the native and invaded range.” Prati and Bossdorf, 2004, carried out the following experiment. Prati and Bossdorf, 2004, tested the allelopathic response of North American and European Geum species to the root exudate from garlic mustard. The exudate has several putative allelopathic chemicals that could inhibit growth in neighboring plants.
Prati and Bossdorf, 2004, collected garlic mustard seeds from three different locations on each continent. They planted the mustard seeds in pots with sand and compost, in a greenhouse. They added activated carbon to half of the pots. The carbon did not inhibit the growth of garlic mustard. After a year of growth, they removed the substrate from the pots, and placed the substrate in petri dishes, in which they germinated two different species of Geum seeds. Half of the petri dishes had Geum laciniatum (a native North American species), and half had Geum urbanum (a native European species). Activated carbon was added to half the dishes.
They found that the garlic mustard substrate had a negative impact on the germination of co-occurring species. The two Geum species differed in their sensitivity to the allelochemicals in the substrate. They found that the carbon had a slight to moderate mitigating effect on the response to the allelochemicals, which demonstrated that the mustard plants had, in fact, exuded allelopathic chemicals to the substrate.
For the North American Geum laciniatum, there was no difference in the response to carbon, between substrates from either European and North American garlic mustard. Germination of the North American Geum laciniatum increased more than that of Geum urbanum, when carbon was added to the petri dishes, demonstrating that allelochemicals had moderately inhibited Geum laciniatum growth, more so than the European Geum. This result may support the novel weapons hypothesis.
But the European Geum urbanum results were complex. This species germinated better in substrate from the mustards from invaded North American populations. And carbon slightly decreased germination of Geum in substrate from mustard from North American populations. This suggests that the mustard in the introduced populations has reduced levels of allelochemicals. Carbon increased germination of Geum urbanum in substrate from the European mustard, suggesting that in European populations, Geum urbanum is suppressed by garlic mustard. This suggests that mustards from the European populations have higher levels of allelechemicals. These results would seem to support the evolutionary reduced competitive ability hypothesis.
These complex results indicate that Geum sensitivity to allelochemicals is species-specific. Prati and Bossdorf, 2004, concluded that field tests are needed to learn more about the complex interactions and the role of allelopathy between invasive and native species.
Next Section:
Summary Thoughts on Research
To return to any of the summaries of research on the success of invasive species, click on any of these links: