Photo: Native prairie, East Texas. © 2017 Delena Norris-Tull
The Role of Diversity in Invasive Success or Failure
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
How Ecosystems Maintain Diversity
The study of the impact of invasive species is based on the assumption that most invasive non-native species out-perform their neighboring native species, and that they spread more rapidly in the non-native range than in their native environment. Most hypotheses, regarding factors that facilitate the success of invasive species, assume “that success is acquired as a result of novel ecological and environmental dynamics” the species encounters in the introduced range (Levine, Adler, & Yelenik, Sept., 2004).
In reality, relatively few non-native species become invasive in their introduced ranges. So what, then, are the factors that cause non-native species to become invasive, and what factors restrict the spread of non-native species?
Levine, et al., 2004, conducted a meta-analysis on biotic resistance research, research that examines the factors that contribute to successful non-native plant invasions. Quite a bit of that research has focused on biotic resistance, defined as “the reduction in invasion success caused by the resident [native plant] community…. Biotic resistance can arise from any effect of resident species on colonizing invaders, including predation, herbivory, and disease… In addition, abiotic factors, such as high temperature or salinity can also make an ecosystem difficult to colonize…. An understanding of biotic resistance might be used to predict which communities are most susceptible to invasions or where invasions are most likely to occur… Similarly, restorative ecologists need to design communities that will best resist invasion (Levine, et al., 2004).”
Of the 52 research studies Levine, et al., 2004, reviewed in their meta-analysis, 24 examined competition as a factor in resistance to non-native plant invasion. These studies “revealed strong and significant effects of resident [native] competitors on both the establishment and individual performance of exotic invaders.” The higher density of native plants in the study plots did show an effect in reducing the success of the invader species. The studies also indicated that “for invader performance, but not establishment, [invasive] grasses, forbs and woody plants differed significantly in their susceptibility to biotic resistance from competition [from native plants], with grasses and forbs showing greater sensitivity than woody species (Levine, et al, 2004).”
Levine, et al., 2004, found that, of the seven studies on the effects of species diversity, the research does support the conclusion that higher native plant diversity in an ecosystem constrains invasion by non-native plants species. “Resident species diversity had a significant negative effect on both invader establishment and individual performance” (Levine, et al., 2004). But even high plant diversity within a community has not been shown to entirely prevent infiltration by invader species. However, only one study effectively tackled that question by introducing an invader not already present in the study habitat (a treatment that would be unethical in most contexts). Other evidence indicates that, “The most diverse natural communities contain the greatest number of exotic species” (Levine, et al., 2004). For example, there are many more exotic species established in the highly diverse California coastal areas and foothill grasslands, than in the harsh environments of the Western States desert, chaparral, and alpine environments.
Levine, et al., 2004, point out that “it is still possible that in the absence of the diversity effect, many more invaders would be found in these communities.” Unfortunately, once a non-native species has invaded, if it is able to outproduce the native plants in seed production, it may continue further invasion. The research indicates that successful invaders are often better at seed production than the native plant community.
Levine, et al., 2004, examined 12 studies, including 27 experiments, of “the impact of native herbivores in natural ecosystems on the establishment and performance of exotic invaders.” Most of the studies examined mammalian herbivores, or the combined effect of mammals and insects, two studies examined only insect herbivores, and one study examined birds as seed predators. All but three of the studies concluded that “herbivory has a significant and strong negative effect on invader establishment.” Herbaceous invading plant species were more susceptible to herbivory than woody plants. But, “although herbivory had a strong negative effect on survivorship and individual plant performance,… whether this was strong enough to reverse the course of an invasion is difficult to infer… Even when herbivory negatively affects an invader population, its relative abundance increases if herbivory impacts other species to a greater extent” (Levine, et al., 2004). Eight of the 27 experiments “showed that herbivory could reduce invader establishment or fecundity to zero.” These were studies on mammalian herbivory. However, it is unlikely that even mammalian herbivores could completely prevent or eliminate an invader species. The studies also indicated that “herbivory is more destructive to perennials than annuals because perennials have a longer period of vulnerability prior to reproduction.”
Levine, et al., 2004, also examined research on soil fungal communities on invasive plant success. All but one of the nine experiments they reviewed were conducted in greenhouses, and most “examined performance measures (biomass, number of leaves and growth) rather than establishment.” In some studies, “native fungal communities increased plant biomass.” In some studies, fungi had differential effects on invader species, in different treatments. Levine, et al., 2004, found these greenhouse experiments difficult to generalize to environments in the field. Even when examining the seven studies on soil or foliar fungal pathogens, the results were variable. “The effect of fungal pathogens in individual cases is strong, but on average, positive and negative effects cancel out.” Based on the limited research available, Levine, et al., 2004, agreed with “the emerging view that depending on the identity of the plant and fungal species, mycorrhizal fungi can be either mutualistic or parasitic.”
Levine, et al., 2004, found “no significant difference between the reduction in invader establishment and individual performance caused by competition vs. herbivory.”
Overall, Levine, et al., 2004, concluded that the available research does support the Enemies Release Hypothesis, described below, more so than the idea that biotic factors within the native plant community are the dominant factors in resistance to invasion. Abiotic factors (e.g., temperature, precipitation, soil type, salinity) may be more important in regulating invasions, than biotic factors. “If biotic resistance does not deterministically drive colonizing invaders to extinction, it may still cause invasions to fail if the invaders have a limited number of arriving seeds… Although in theory the invader would eventually colonize, limited seed arrival coupled with biotic resistance could dramatically alter the course of an invasion… Interactions between different sources of biotic resistance and between biotic and abiotic factors have the potential to more strongly inhibit invasions than each ecological process in isolation” (Levine, et al., 2004). They suggested that more research is needed to examine the role of abiotic factors, and the interplay between biotic and abiotic factors, in invasive success or failure.
Levine, et al., 2004, concluded their meta-analysis with a call for much more research. They point out that, “If species interactions do not completely repel invaders, but rather, constrain their abundance within communities, new questions gain prominence. Among the most interesting is what factors allow native species to persist with invaders once the latter have established… Only a small fraction of invaders ever reach high abundance or exert large impacts… Understanding the ecological dynamics enabling native species to persist in these communities is critical to preserving their diversity.”
For more research on the role of diversity in preventing invasions, click on any of these LINKS:
For summaries of other research on the success of invasive species, click on any of these links:
The Role of Diversity in Invasive Success or Failure
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
How Ecosystems Maintain Diversity
The study of the impact of invasive species is based on the assumption that most invasive non-native species out-perform their neighboring native species, and that they spread more rapidly in the non-native range than in their native environment. Most hypotheses, regarding factors that facilitate the success of invasive species, assume “that success is acquired as a result of novel ecological and environmental dynamics” the species encounters in the introduced range (Levine, Adler, & Yelenik, Sept., 2004).
In reality, relatively few non-native species become invasive in their introduced ranges. So what, then, are the factors that cause non-native species to become invasive, and what factors restrict the spread of non-native species?
Levine, et al., 2004, conducted a meta-analysis on biotic resistance research, research that examines the factors that contribute to successful non-native plant invasions. Quite a bit of that research has focused on biotic resistance, defined as “the reduction in invasion success caused by the resident [native plant] community…. Biotic resistance can arise from any effect of resident species on colonizing invaders, including predation, herbivory, and disease… In addition, abiotic factors, such as high temperature or salinity can also make an ecosystem difficult to colonize…. An understanding of biotic resistance might be used to predict which communities are most susceptible to invasions or where invasions are most likely to occur… Similarly, restorative ecologists need to design communities that will best resist invasion (Levine, et al., 2004).”
Of the 52 research studies Levine, et al., 2004, reviewed in their meta-analysis, 24 examined competition as a factor in resistance to non-native plant invasion. These studies “revealed strong and significant effects of resident [native] competitors on both the establishment and individual performance of exotic invaders.” The higher density of native plants in the study plots did show an effect in reducing the success of the invader species. The studies also indicated that “for invader performance, but not establishment, [invasive] grasses, forbs and woody plants differed significantly in their susceptibility to biotic resistance from competition [from native plants], with grasses and forbs showing greater sensitivity than woody species (Levine, et al, 2004).”
Levine, et al., 2004, found that, of the seven studies on the effects of species diversity, the research does support the conclusion that higher native plant diversity in an ecosystem constrains invasion by non-native plants species. “Resident species diversity had a significant negative effect on both invader establishment and individual performance” (Levine, et al., 2004). But even high plant diversity within a community has not been shown to entirely prevent infiltration by invader species. However, only one study effectively tackled that question by introducing an invader not already present in the study habitat (a treatment that would be unethical in most contexts). Other evidence indicates that, “The most diverse natural communities contain the greatest number of exotic species” (Levine, et al., 2004). For example, there are many more exotic species established in the highly diverse California coastal areas and foothill grasslands, than in the harsh environments of the Western States desert, chaparral, and alpine environments.
Levine, et al., 2004, point out that “it is still possible that in the absence of the diversity effect, many more invaders would be found in these communities.” Unfortunately, once a non-native species has invaded, if it is able to outproduce the native plants in seed production, it may continue further invasion. The research indicates that successful invaders are often better at seed production than the native plant community.
Levine, et al., 2004, examined 12 studies, including 27 experiments, of “the impact of native herbivores in natural ecosystems on the establishment and performance of exotic invaders.” Most of the studies examined mammalian herbivores, or the combined effect of mammals and insects, two studies examined only insect herbivores, and one study examined birds as seed predators. All but three of the studies concluded that “herbivory has a significant and strong negative effect on invader establishment.” Herbaceous invading plant species were more susceptible to herbivory than woody plants. But, “although herbivory had a strong negative effect on survivorship and individual plant performance,… whether this was strong enough to reverse the course of an invasion is difficult to infer… Even when herbivory negatively affects an invader population, its relative abundance increases if herbivory impacts other species to a greater extent” (Levine, et al., 2004). Eight of the 27 experiments “showed that herbivory could reduce invader establishment or fecundity to zero.” These were studies on mammalian herbivory. However, it is unlikely that even mammalian herbivores could completely prevent or eliminate an invader species. The studies also indicated that “herbivory is more destructive to perennials than annuals because perennials have a longer period of vulnerability prior to reproduction.”
Levine, et al., 2004, also examined research on soil fungal communities on invasive plant success. All but one of the nine experiments they reviewed were conducted in greenhouses, and most “examined performance measures (biomass, number of leaves and growth) rather than establishment.” In some studies, “native fungal communities increased plant biomass.” In some studies, fungi had differential effects on invader species, in different treatments. Levine, et al., 2004, found these greenhouse experiments difficult to generalize to environments in the field. Even when examining the seven studies on soil or foliar fungal pathogens, the results were variable. “The effect of fungal pathogens in individual cases is strong, but on average, positive and negative effects cancel out.” Based on the limited research available, Levine, et al., 2004, agreed with “the emerging view that depending on the identity of the plant and fungal species, mycorrhizal fungi can be either mutualistic or parasitic.”
Levine, et al., 2004, found “no significant difference between the reduction in invader establishment and individual performance caused by competition vs. herbivory.”
Overall, Levine, et al., 2004, concluded that the available research does support the Enemies Release Hypothesis, described below, more so than the idea that biotic factors within the native plant community are the dominant factors in resistance to invasion. Abiotic factors (e.g., temperature, precipitation, soil type, salinity) may be more important in regulating invasions, than biotic factors. “If biotic resistance does not deterministically drive colonizing invaders to extinction, it may still cause invasions to fail if the invaders have a limited number of arriving seeds… Although in theory the invader would eventually colonize, limited seed arrival coupled with biotic resistance could dramatically alter the course of an invasion… Interactions between different sources of biotic resistance and between biotic and abiotic factors have the potential to more strongly inhibit invasions than each ecological process in isolation” (Levine, et al., 2004). They suggested that more research is needed to examine the role of abiotic factors, and the interplay between biotic and abiotic factors, in invasive success or failure.
Levine, et al., 2004, concluded their meta-analysis with a call for much more research. They point out that, “If species interactions do not completely repel invaders, but rather, constrain their abundance within communities, new questions gain prominence. Among the most interesting is what factors allow native species to persist with invaders once the latter have established… Only a small fraction of invaders ever reach high abundance or exert large impacts… Understanding the ecological dynamics enabling native species to persist in these communities is critical to preserving their diversity.”
For more research on the role of diversity in preventing invasions, click on any of these LINKS:
- Fluctuation Dependent Mechanisms
- Competition-based coexistence mechanisms
- Niche Differences
- Species Richness
For summaries of other research on the success of invasive species, click on any of these links: