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Ragan M. Callaway - One of the best experts on this subject based on the ideXlab platform.
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Enemy Release from the effects of generalist granivores can facilitate bromus tectorum invasion in the great basin desert
Ecology and Evolution, 2019Co-Authors: Jacob E Lucero, Urs Schaffner, Ghorbanali Asadi, Alireza Bagheri, Toshpulot F Rajabov, Ragan M. CallawayAbstract:: The Enemy Release Hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non-native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region-specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that Enemy Release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on Enemy Release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.
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a cross continental test of the Enemy Release Hypothesis leaf herbivory on acer platanoides l is three times lower in north america than in its native europe
Biological Invasions, 2009Co-Authors: Jonathan M. Adams, Ragan M. Callaway, Wei Fang, Don Cipollini, Elizabeth NewellAbstract:Acer platanoides (Norway maple) is a widespread native tree species in Europe. It has been introduced to North America where it has often established dense stands in both secondary woodlands and relatively undisturbed mature woodlands. In Europe A. platanoides is also extending its original range, but generally seems to exist at much lower densities. One explanation for the ‘aggressiveness’ of invasive plants such as A. platanoides is that they have left behind pests and diseases which limit their population densities in their native lands (the Enemy Release Hypothesis or ERH). To assess the ERH for Norway maple, a large network of collaborators assessed leaf herbivory rates in populations throughout Europe and North America. We found significantly lower total leaf herbivory (1.6% ± 0.19, n = 21 vs. 7.4% ± 1.94, n = 34) and lower fungal damage (1.0% ± 0.35, n = 13 vs. 3.7% ± 0.85, n = 34) in North America than in Europe over a 2 year period, which is consistent with the predictions of the Enemy Release Hypothesis. Across years, the average total leaf herbivory was significantly correlated with average annual temperature of the site (P 0.05). Furthermore, only populations in Europe showed very high levels of herbivory (e.g., nine sites had total leaf herbivory ranging from 10.0 to 51.2% in at least 1 year) or leaf fungal damage (only one site in North America showed high levels of fungal damage in 1 year), suggesting the possibility of more frequent episodic outbreaks in the native range. Leaf herbivory and fungal damage are only two aspects of consumer pressure and we do not know whether the differences reported here are enough to actually elicit Release from top-down population control, but such large scale biogeographic differences in herbivory contribute towards understanding exotic invasions.
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soil biota facilitate exotic acer invasions in europe and north america
Ecological Applications, 2004Co-Authors: Kurt O Reinhart, Ragan M. CallawayAbstract:The primary Hypothesis for successful exotic plant invasions is that the invaders have escaped the specialist consumers that control them (Enemy Release Hypothesis). However, few studies have rigorously tested this assertion with biogeographical experiments or considered the effects of soil organisms. We tested the Enemy Release Hypothesis and the enhanced role of mutualisms by comparing density patterns of the North American Acer negundo and European A. platanoides trees in their native and nonnative ranges. Invaders that have escaped their natural enemies are predicted to attain greater densities in nonnative than native ranges. To determine whether interactions with soil biota could explain the population distributions observed in the field, we compared the effects of sterilized and nonsterilized soil associated with Acer and non-Acer trees in native and nonnative ranges on the growth of seedlings in the greenhouse. In the field study, distances from focal trees to the nearest Acer conspecifics were 56– 77% less in their nonnative ranges than in their native ranges. In the greenhouse experiment, the effect of soil biota also differed between native and nonnative ranges of Acers. Relative to sterilized controls, soil associated with conspecifics and heterospecifics from the native ranges decreased the total biomass and relative change in height of Acer seedlings by 35% and 40%, respectively. Soil associated with conspecifics in the nonnative ranges decreased the biomass and relative change in height of Acer seedlings by 112% and 64%, respectively; but the soil associated with heterospecifics in the nonnative ranges increased biomass and relative change in height of Acer seedlings by 13% and 37%, respectively. Our results suggest that invasion of Acers is enhanced by soil biota associated with dominant native species and that the soil biota effect becomes more inhibitory as the Acers establish. The relative difference in soil biota effects between ranges supports the Enemy Release Hypothesis but also suggests that mutualists are relatively more beneficial to Acers in their nonnative ranges than in their native ranges. Mutualisms may be relatively more beneficial in nonnative ranges because the invader has escaped from the negative effect of natural enemies that may attenuate the positive effect of mutualists.
Leif Vøllestad - One of the best experts on this subject based on the ideXlab platform.
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gyrodactylus spp diversity in native and introduced minnow phoxinus phoxinus populations no support for the Enemy Release Hypothesis
Parasites & Vectors, 2016Co-Authors: Ruben Alexander Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:Translocation of native species and introduction of non-native species are potentially harmful to the existing biota by introducing e.g. diseases, parasites and organisms that may negatively affect the native species. The Enemy Release Hypothesis states that parasite species will be lost from host populations when the host is introduced into new environments. We tested the Enemy Release Hypothesis by comparing 14 native and 29 introduced minnow (Phoxinus phoxinus) populations in Norway with regard to the ectoparasitic Gyrodactylus species community and load (on caudal fin). Here, we used a nominal logistic regression on presence/absence of Gyrodactylus spp. and a generalized linear model on the summed number of Gyrodactylus spp. on infected populations, with individual minnow heterozygosity (based on 11 microsatellites) as a covariate. In addition, a sample-based rarefaction analysis was used to test if the Gyrodactylus-species specific load differed between native and introduced minnow populations. An analysis of molecular variance was performed to test for hierarchical population structure between the two groups and to test for signals of population bottlenecks the two-phase model in the Wilcoxon signed-rank test was used. To test for demographic population expansion events in the introduced minnow population, we used the kg-test under a stepwise mutation model. The native and introduced minnow populations had similar species compositions of Gyrodactylus, lending no support to the Enemy Release Hypothesis. The two minnow groups did not differ in the likelihood of being infected with Gyrodactylus spp. Considering only infected minnow populations it was evident that native populations had a significantly higher mean abundance of Gyrodactylus spp. than introduced populations. The results showed that homozygotic minnows had a higher Gyrodactylus spp. infection than more heterozygotic hosts. Using only infected individuals, the two minnow groups did not differ in their mean number of Gyrodactylus spp. However, a similar negative association between heterozygosity and abundance was observed in the native and introduced group. There was no evidence for demographic bottlenecks in the minnow populations, implying that introduced populations retained a high degree of genetic variation, indicating that the number of introduced minnows may have been large or that introductions have been happening repeatedly. This could partly explain the similar species composition of Gyrodactylus in the native and introduced minnow populations. In this study it was observed that native and introduced minnow populations did not differ in their species community of Gyrodactylus spp., lending no support to the Enemy Release Hypothesis. A negative association between individual minnow host heterozygosity and the number of Gyrodactylus spp. was detected. Our results suggest that the Enemy Release Hypothesis does not necessarily limit fish parasite dispersal, further emphasizing the importance of invasive fish species dispersal control.
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Additional file 1: Table S1. of Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for “the Enemy Release” Hypothesis
2016Co-Authors: Ruben Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:A table of other fish species in the sampled localities. All the Norwegian minnow (Phoxinus phoxinus) populations used in the study and the other fish species in the same locality. The species of fish are: AC = Arctic charr (Salvelinus alpinus) BL = bleak (Alburnus alburnus), BR = bream (Abramis brama), BT = brown trout (Salmo trutta), BU = burbot (Lota lota), CC = crucian carp (Carassius carassius), CH = chub (Leuciscus cephalus), EE = eel (Anguilla anguilla), GR = grayling (Thymallus thymallus), PE = perch (Perca fluviatilis), PI = pike (Esox lucius), RU = ruffe (Gymnocephalus cernuus), SS = Siberian sculpin (Cottus poecilopus), VE = vendace (Coregonus albula), WB = white bream (Blicca bjoerkna), WF = European whitefish, SM = smelt (Osmerus eperlanus), ID = ide (Leuciscus idus), DA = dace (Leuciscus leuciscus), 9-SB = nine-spined stickleback (Pungitius pungitius) and 3-SB = tree-spined stickleback (Gasterosteus aculeatus). (DOC 192 kb
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Additional file 2: of Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for “the Enemy Release” Hypothesis
2016Co-Authors: Ruben Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:A table of tests result of demographic bottleneck. All the Norwegian minnow (Phoxinus phoxinus) populations used in the study. Information from tests of demographic bottleneck using Bottleneck 1.2.0.2. (Piry et al. 1999) with the two-phase model (TPM) and the Wilcoxon sign-rank test, and demographic expansion (where the intra-locus k-test identify signals of recent population expansion, while the inter-locus g-test identify signals of more ancient population expansion) using Kg-test (Reich et al. 1999). Footnotes: *Samples used in the rarefaction analysis using EstimateS 8.2.0 (Colwell 2011) where more than 5 Gyrodactylus spp. individuals were observed in the population. This number of observed Gyrodactylus spp. individuals is needed to calculate rarefaction in EstimateS 8.2.0. # None of tests were significant. (DOC 116 kb
Kjartan Østbye - One of the best experts on this subject based on the ideXlab platform.
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gyrodactylus spp diversity in native and introduced minnow phoxinus phoxinus populations no support for the Enemy Release Hypothesis
Parasites & Vectors, 2016Co-Authors: Ruben Alexander Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:Translocation of native species and introduction of non-native species are potentially harmful to the existing biota by introducing e.g. diseases, parasites and organisms that may negatively affect the native species. The Enemy Release Hypothesis states that parasite species will be lost from host populations when the host is introduced into new environments. We tested the Enemy Release Hypothesis by comparing 14 native and 29 introduced minnow (Phoxinus phoxinus) populations in Norway with regard to the ectoparasitic Gyrodactylus species community and load (on caudal fin). Here, we used a nominal logistic regression on presence/absence of Gyrodactylus spp. and a generalized linear model on the summed number of Gyrodactylus spp. on infected populations, with individual minnow heterozygosity (based on 11 microsatellites) as a covariate. In addition, a sample-based rarefaction analysis was used to test if the Gyrodactylus-species specific load differed between native and introduced minnow populations. An analysis of molecular variance was performed to test for hierarchical population structure between the two groups and to test for signals of population bottlenecks the two-phase model in the Wilcoxon signed-rank test was used. To test for demographic population expansion events in the introduced minnow population, we used the kg-test under a stepwise mutation model. The native and introduced minnow populations had similar species compositions of Gyrodactylus, lending no support to the Enemy Release Hypothesis. The two minnow groups did not differ in the likelihood of being infected with Gyrodactylus spp. Considering only infected minnow populations it was evident that native populations had a significantly higher mean abundance of Gyrodactylus spp. than introduced populations. The results showed that homozygotic minnows had a higher Gyrodactylus spp. infection than more heterozygotic hosts. Using only infected individuals, the two minnow groups did not differ in their mean number of Gyrodactylus spp. However, a similar negative association between heterozygosity and abundance was observed in the native and introduced group. There was no evidence for demographic bottlenecks in the minnow populations, implying that introduced populations retained a high degree of genetic variation, indicating that the number of introduced minnows may have been large or that introductions have been happening repeatedly. This could partly explain the similar species composition of Gyrodactylus in the native and introduced minnow populations. In this study it was observed that native and introduced minnow populations did not differ in their species community of Gyrodactylus spp., lending no support to the Enemy Release Hypothesis. A negative association between individual minnow host heterozygosity and the number of Gyrodactylus spp. was detected. Our results suggest that the Enemy Release Hypothesis does not necessarily limit fish parasite dispersal, further emphasizing the importance of invasive fish species dispersal control.
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Additional file 1: Table S1. of Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for “the Enemy Release” Hypothesis
2016Co-Authors: Ruben Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:A table of other fish species in the sampled localities. All the Norwegian minnow (Phoxinus phoxinus) populations used in the study and the other fish species in the same locality. The species of fish are: AC = Arctic charr (Salvelinus alpinus) BL = bleak (Alburnus alburnus), BR = bream (Abramis brama), BT = brown trout (Salmo trutta), BU = burbot (Lota lota), CC = crucian carp (Carassius carassius), CH = chub (Leuciscus cephalus), EE = eel (Anguilla anguilla), GR = grayling (Thymallus thymallus), PE = perch (Perca fluviatilis), PI = pike (Esox lucius), RU = ruffe (Gymnocephalus cernuus), SS = Siberian sculpin (Cottus poecilopus), VE = vendace (Coregonus albula), WB = white bream (Blicca bjoerkna), WF = European whitefish, SM = smelt (Osmerus eperlanus), ID = ide (Leuciscus idus), DA = dace (Leuciscus leuciscus), 9-SB = nine-spined stickleback (Pungitius pungitius) and 3-SB = tree-spined stickleback (Gasterosteus aculeatus). (DOC 192 kb
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Additional file 2: of Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for “the Enemy Release” Hypothesis
2016Co-Authors: Ruben Pettersen, Kjartan Østbye, Johannes Holmen, Leif VøllestadAbstract:A table of tests result of demographic bottleneck. All the Norwegian minnow (Phoxinus phoxinus) populations used in the study. Information from tests of demographic bottleneck using Bottleneck 1.2.0.2. (Piry et al. 1999) with the two-phase model (TPM) and the Wilcoxon sign-rank test, and demographic expansion (where the intra-locus k-test identify signals of recent population expansion, while the inter-locus g-test identify signals of more ancient population expansion) using Kg-test (Reich et al. 1999). Footnotes: *Samples used in the rarefaction analysis using EstimateS 8.2.0 (Colwell 2011) where more than 5 Gyrodactylus spp. individuals were observed in the population. This number of observed Gyrodactylus spp. individuals is needed to calculate rarefaction in EstimateS 8.2.0. # None of tests were significant. (DOC 116 kb
Menno Schilthuizen - One of the best experts on this subject based on the ideXlab platform.
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A review and meta-analysis of the Enemy Release Hypothesis in plant-herbivorous insect systems A review and meta-analysis of the Enemy Release Hypothesis in plant -herbivorous insect systems
2020Co-Authors: Kim Meijer, Menno Schilthuizen, Leo Beukeboom, Christian SmitAbstract:A suggested mechanism for the success of introduced non-native species is the Enemy Release Hypothesis (ERH). Many studies have tested the predictions of the ERH using the community approach (native and non-native species studied in the same habitat) or the biogeographical approach (species studied in their native and non-native range), but results are highly variable, possibly due to large variety of study systems incorporated. We therefore focused on one specific system: plants and their herbivorous insects. We performed a systematic review and compiled a large number (68) of datasets from studies comparing herbivorous insects on native and non-native plants using the community or biogeographical approach. We performed a meta-analysis to test the predictions from the ERH for insect diversity (number of species), insect load (number of individuals) and level of herbivory for both the community and biogeographical approach. For both the community and biogeographical approach insect diversity was significantly higher on native than on non-native plants. Insect load tended to be higher on native than nonnative plants at the community approach only. Herbivory was not different between native and non-native plants at the community approach, while there was too little data available for testing the biogeographical approach. Our meta-analysis generally supports the predictions from the ERH for both the community and biogeographical approach, but also shows that the outcome is importantly determined by the response measured and approach applied. So far, very few studies apply both approaches simultaneously in a reciprocal manner while this is arguably the best way for testing the ERH. Abstract A suggested mechanism for the success of introduced non-native species is the Enemy Release Hypothesis (ERH). Many studies have tested the predictions of the ERH using the community approach (native and non-native species studied in the same habitat) or the biogeographical approach (species studied in their native and non-native range), but results are highly variable, possibly due to large variety of study systems incorporated. We therefore focused on one specific system: plants and their herbivorous insects. We performed a systematic review and compiled a large number (68) of datasets from studies comparing herbivorous insects on native and nonnative plants using the community or biogeographical approach. We performed a meta-analysis to test the predictions from the ERH for insect diversity (number of species), insect load (number of individuals) and level of herbivory for both the community and biogeographical approach. For both the community and biogeographical approach insect diversity was significantly higher on native than on non-native plants. Insect load tended to be higher on native than non-native plants at the community approach only. Herbivory was not different between native and non-native plants at the community approach, while there was too little data available for testing the biogeographical approach. Our meta-analysis generally supports the predictions from the ERH for both the community and biogeographical approach, but also shows that the outcome is importantly determined by the response measured and approach applied. So far, very few studies apply both approaches simultaneously in a reciprocal manner while this is arguably the best way for testing the ERH
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Distributed under Creative Commons CC-BY 4.0 OPEN ACCESS A review and meta-analysis of the Enemy Release Hypothesis in plant-herbivorous insect systems
2020Co-Authors: Kim Meijer, Menno Schilthuizen, Leo Beukeboom, Christian SmitAbstract:ABSTRACT A suggested mechanism for the success of introduced non-native species is the Enemy Release Hypothesis (ERH). Many studies have tested the predictions of the ERH using the community approach (native and non-native species studied in the same habitat) or the biogeographical approach (species studied in their native and non-native range), but results are highly variable, possibly due to large variety of study systems incorporated. We therefore focused on one specific system: plants and their herbivorous insects. We performed a systematic review and compiled a large number (68) of datasets from studies comparing herbivorous insects on native and non-native plants using the community or biogeographical approach. We performed a meta-analysis to test the predictions from the ERH for insect diversity (number of species), insect load (number of individuals) and level of herbivory for both the community and biogeographical approach. For both the community and biogeographical approach insect diversity was significantly higher on native than on non-native plants. Insect load tended to be higher on native than non-native plants at the community approach only. Herbivory was not different between native and non-native plants at the community approach, while there was too little data available for testing the biogeographical approach. Our metaanalysis generally supports the predictions from the ERH for both the community and biogeographical approach, but also shows that the outcome is importantly determined by the response measured and approach applied. So far, very few studies apply both approaches simultaneously in a reciprocal manner while this is arguably the best way for testing the ERH. Subjects Ecolog
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a review and meta analysis of the Enemy Release Hypothesis in plant herbivorous insect systems
PeerJ, 2016Co-Authors: Kim Meijer, Menno Schilthuizen, Leo Beukeboom, Christian SmitAbstract:A suggested mechanism for the success of introduced non-native species is the Enemy Release Hypothesis (ERH). Many studies have tested the predictions of the ERH using the community approach (native and non-native species studied in the same habitat) or the biogeographical approach (species studied in their native and non-native range), but results are highly variable, possibly due to large variety of study systems incorporated. We therefore focused on one specific system: plants and their herbivorous insects. We performed a systematic review and compiled a large number (68) of datasets from studies comparing herbivorous insects on native and non-native plants using the community or biogeographical approach. We performed a meta-analysis to test the predictions from the ERH for insect diversity (number of species), insect load (number of individuals) and level of herbivory for both the community and biogeographical approach. For both the community and biogeographical approach insect diversity was significantly higher on native than on non-native plants. Insect load tended to be higher on native than non-native plants at the community approach only. Herbivory was not different between native and non-native plants at the community approach, while there was too little data available for testing the biogeographical approach. Our meta-analysis generally supports the predictions from the ERH for both the community and biogeographical approach, but also shows that the outcome is importantly determined by the response measured and approach applied. So far, very few studies apply both approaches simultaneously in a reciprocal manner while this is arguably the best way for testing the ERH.
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RESEARCH ARTICLE Phytophagous Insects on Native and Non-Native Host Plants: Combining the Community Approach and the Biogeographical Approach
2016Co-Authors: Hidde Zemel, Christian Smit, Leo Beukeboom, Satoshi Chiba, Menno SchilthuizenAbstract:During the past centuries, humans have introduced many plant species in areas where they do not naturally occur. Some of these species establish populations and in some cases be-come invasive, causing economic and ecological damage. Which factors determine the success of non-native plants is still incompletely understood, but the absence of natural en-emies in the invaded area (Enemy Release Hypothesis; ERH) is one of the most popular ex-planations. One of the predictions of the ERH, a reduced herbivore load on non-native plants compared with native ones, has been repeatedly tested. However, many studies have either used a community approach (sampling from native and non-native species in the same community) or a biogeographical approach (sampling from the same plant spe-cies in areas where it is native and where it is non-native). Either method can sometimes lead to inconclusive results. To resolve this, we here add to the small number of studies that combine both approaches. We do so in a single study of insect herbivory on 47 woody plant species (trees, shrubs, and vines) in the Netherlands and Japan. We find higher herbivore diversity, higher herbivore load and more herbivory on native plants than on non-native plants, generating support for the Enemy Release Hypothesis
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A review and meta-analysis of the Enemy Release Hypothesis in plant–herbivorous insect systems
PeerJ Inc., 2016Co-Authors: Kim Meijer, Menno Schilthuizen, Leo Beukeboom, Christian SmitAbstract:A suggested mechanism for the success of introduced non-native species is the Enemy Release Hypothesis (ERH). Many studies have tested the predictions of the ERH using the community approach (native and non-native species studied in the same habitat) or the biogeographical approach (species studied in their native and non-native range), but results are highly variable, possibly due to large variety of study systems incorporated. We therefore focused on one specific system: plants and their herbivorous insects. We performed a systematic review and compiled a large number (68) of datasets from studies comparing herbivorous insects on native and non-native plants using the community or biogeographical approach. We performed a meta-analysis to test the predictions from the ERH for insect diversity (number of species), insect load (number of individuals) and level of herbivory for both the community and biogeographical approach. For both the community and biogeographical approach insect diversity was significantly higher on native than on non-native plants. Insect load tended to be higher on native than non-native plants at the community approach only. Herbivory was not different between native and non-native plants at the community approach, while there was too little data available for testing the biogeographical approach. Our meta-analysis generally supports the predictions from the ERH for both the community and biogeographical approach, but also shows that the outcome is importantly determined by the response measured and approach applied. So far, very few studies apply both approaches simultaneously in a reciprocal manner while this is arguably the best way for testing the ERH
Jacob E Lucero - One of the best experts on this subject based on the ideXlab platform.
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Enemy Release from the effects of generalist granivores can facilitate bromus tectorum invasion in the great basin desert
Ecology and Evolution, 2019Co-Authors: Jacob E Lucero, Urs Schaffner, Ghorbanali Asadi, Alireza Bagheri, Toshpulot F Rajabov, Ragan M. CallawayAbstract:: The Enemy Release Hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non-native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region-specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that Enemy Release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on Enemy Release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.