Increase in mate availability without loss of self-incompatibility in the invasive species Senecio inaequidens (Asteraceae)

The evolution of the strength of self-incompatibility in Senecio inaequidens , a native of South Africa was investigated in relation to its invasion in Europe. Levels of self-incompatibility were estimated with hand-pollinations in five populations in greenhouse conditions. One population came from the native range of the species and four populations were sampled in Europe from two independent transects of colonization with old and recent populations. Contrary to Baker's law predictions, our results suggest that the species has a sporophytic self-incompatible system maintained in all populations. We suggest that the ability of S. inaequidens to colonize new sites with a self-incompatibility system is promoted by its ecological characteristics (perenniality, extended reproductive period, massive seed production, generalist pollinators). In addition, we found that mate availability was increased (1) in the introduced range compared to the native range, (2) in marginal versus central European populations. Possible explanations for this surprising result are discussed.     

Palatability to a generalist herbivore,defence and growth of invasive and native <Emphasis Type="Italic">Senecio</Emphasis> species: testing the evolution of increased competitive ability hypothesis

This paper tests the prediction that introduced plants may become successful invaders because they experience evolutionary changes in growth and defence in their new range [evolution of increased competitive ability hypothesis (EICA)]. Interspecific and intraspecific binary feeding choices were offered to the snail Helix aspersa. The choices were between: (1) plants of the invasive Senecio inaequidens and Senecio pterophorus derived from populations in the introduced range (Europe) and plants of three indigenous species (Senecio jacobea, Senecio vulgaris and Senecio malacitanus) from populations in Europe; (2) plants of the invasive S. inaequidens and S. pterophorus from populations in the introduced range (Europe) and from populations in the native range (South Africa). We did not find a clear pattern of preference for indigenous or alien species of Senecio. However, we found that European invasive populations of S. inaequidens and S. pterophorus were less palatable than South African native populations. Moreover, in contrast to the predictions of the EICA hypothesis, the invasive genotypes of both species also showed a higher total concentration of pyrrolizidine alkaloids, and in the case of S. inaequidens we also found higher growth than in native genotypes. Our results are discussed with respect to the refinement of the EICA hypothesis that takes into account the difference between specialist and generalist herbivores and between qualitative and quantitative defences. We conclude that invasive populations of S. inaequidens and S. pterophorus are less palatable than native populations, suggesting that genetic differentiation associated with founding may occur and contribute to the plants’ invasion success by selecting the best-defended genotypes in the introduced range.     

The making of a rapid plant invader: genetic diversity and differentiation in the native and invaded range of Senecio inaequidens

To become invasive, exotic species have to succeed in the consecutive phases of introduction, naturalization, and invasion. Each of these phases leaves traces in genetic structure, which may affect the species’ success in subsequent phases. We examined this interplay of genetic structure and invasion dynamics in the South African Ragwort (Senecio inaequidens), one of Europe’s fastest plant invaders. We used AFLP and microsatellite markers to analyze 19 native African and 32 invasive European populations. In combination with historic data, we distinguished invasion routes and traced them back to the native source areas. This revealed that different introduction sites had markedly different success in the three invasion phases. Notably, an observed lag‐phase in Northern Germany was evidently not terminated by factors increasing the invasiveness of the resident population but by invasive spread from another introduction centre. The lineage invading Central Europe was introduced to sites in which winters are more benign than in the native source region. Subsequently, this lineage spread into areas in which winter temperatures match the native climate more closely. Genetic diversity clearly increases with population age in Europe and less clearly decreases with spread rate up to population establishment. This indicates that gene flow along well‐connected invasion routes counteracted losses of genetic diversity during rapid spread. In summary, this study suggests that multiple introductions, environmental preadaptation and high gene flow along invasion routes contributed to the success of this rapid invader. More generally, it demonstrates the benefit of combining genetic, historical, and climatic data for understanding biological invasions.     

Reduced seed predation after invasion supports enemy release in a broad biogeographical survey

The Enemy Release (ER) hypothesis predicts an increase in the plant invasive capacity after being released from their associated herbivores or pathogens in their area of origin. Despite the large number of studies on biological invasions addressing this hypothesis, tests evaluating changes in herbivory on native and introduced populations and their effects on plant reproductive potential at a biogeographical level are relatively rare. Here, we tested the ER hypothesis on the South African species Senecio pterophorus (Asteraceae), which is native to the Eastern Cape, has expanded into the Western Cape, and was introduced into Australia (>70–100 years ago) and Europe (>30 years ago). Insect seed predation was evaluated to determine whether plants in the introduced areas were released from herbivores compared to plants from the native range. In South Africa, 25 % of the seedheads of sampled plants were damaged. Plants from the introduced populations suffered lower seed predation compared to those from the native populations, as expected under the ER hypothesis, and this release was more pronounced in the region with the most recent introduction (Europe 0.2 % vs. Australia 15 %). The insect communities feeding on S. pterophorus in Australia and Europe differed from those found in South Africa, suggesting that the plants were released from their associated fauna after invasion and later established new associations with local herbivore communities in the novel habitats. Our study is the first to provide strong evidence of enemy release in a biogeographical survey across the entire known distribution of a species.     

Ploidy level and origin of the European invasive weed Senecio inaequidens (Asteraceae)

Native to South-Africa, species of the Senecio inaequidens complex are presently invasive in Europe, Australia and South-America. Previously, different ploidy levels have been found in these different areas, with only tetraploid individuals reported in Europe, and only diploids in South-Africa and Australia. In the present study chromosome counts and flow cytometry were used to survey DNA ploidy levels in a large sample of 66 native and 21 European invasive populations. One Mexican individual was also added to the study. We found only tetraploid individuals occurring in Europe, whereas both ploidy levels, diploid and tetraploid, were found in South-Africa. Moreover, based on genome size, we suggest that two largely allopatric varieties of diploids exist in South-Africa. The Mexican individual was diploid. We suggest that European tetraploid individuals come from South-Africa and hypothesize that a hybridization event between the two DNA types of diploids occurred in the Lesotho area. The taxonomic difficulties surrounding species of theS. inaequidens complex are briefly discussed.     

Reproductive timing as a constraint on invasion success in the Ring-necked parakeet (<Emphasis Type="Italic">Psittacula krameri</Emphasis>)

Climate similarity favors biological invasion, but a match between seasonality in the novel range and the timing of life cycle events of the invader also influences the outcome of species introduction. Yet, phenology effects on invasion success have generally been neglected. Here we study whether a phenological mismatch limits the non-native range of a globally successful invader, the Ring-necked parakeet, in Europe. Given the latitudes at which parakeets have established across Europe, they breed earlier than expected based on breeding dates from the native Asian range. Moreover, comparing the breeding dates of European populations to those of parakeets in the native Asian range, to five native breeding bird species in Europe and to the start of the growing season of four native European trees shows that the discrepancy between expected and actual breeding phenology is greater in northern Europe. In northern European populations, this temporal mismatch appears to have negative effects on hatching success, and on population growth rates in years that are colder than average in the first six months. Phenological mismatch also can explain why parakeets from African populations (that are more likely to breed in autumn) have been poor invaders compared to parakeets from Asia. These lines of evidence support the hypothesis that the reproductive phenology of the Ring-necked parakeet can be a limiting factor for establishment and range expansion in colder climates. Our results provide growing support for the hypothesis that the match between climate seasonality and timing of reproduction (or other important life cycle events) can affect the establishment success, invasive potential and distribution range of introduced non-native species, beyond the mere effect of climate similarity.     

Evolution of starvation resistance in an invasive insect species,Aethina tumida (Coleoptera: Nitidulidae)

Starvation resistance, or the ability to survive periods without food, can shed light on selection pressure imposed by food scarcity, including chances to invade new regions as a result of human transport. Surprisingly, little information is known about starvation resistance for invasive insect species. Given that native and invasive populations differ in starvation resistance, this would suggest different selection scenarios and adaptive shifts fostering invasion success. Here, we show striking differences in starvation resistance of adult small hive beetles Aethina tumida (SHB) between native and invasive populations. In the laboratory, starvation resistance of freshly emerged laboratory‐reared and field‐collected adult females and males was evaluated in the beetle's native African range and in their invasive North American range. SHB in their native African range survived longer than SHB in their invasive North American range. Across ranges, females survived longer than males. Field‐collected SHB survived in Africa longer than freshly emerged ones, but not in the invasive range. This suggests no selection for starvation resistance in the invasive range, possibly due to differences between African and European‐derived honey bee hosts facilitating a trade‐off scenario between reproduction and starvation resistance. The ability of adult females to survive up to two months without food appears to be one factor contributing to the invasion success of this species. Assuming food availability is usually high in the invasive ranges, and trade‐offs between starvation resistance and fecundity/reproduction are common, it seems as if selection for starvation resistance during transport could set up potential trade‐offs that enhance reproduction after invasion. It would be interesting to see if this is a possible general pattern for invasive insect species.     

The genetic ghost of an invasion past: colonization and extinction revealed by historical hybridization in Senecio

Hybridization is an important evolutionary factor in the diversification of many plant and animal species. Of particular interest is that historical hybridization resulting in the origin of new species or introgressants has occurred between species now geographically separated by great distances. Here, we report that Senecio massaicus, a tetraploid species native to Morocco and the Canary Islands, contains genetic material of two distinct, geographically separated lineages: a Mediterranean lineage and a mainly southern African lineage. A time-calibrated internal transcribed spacer phylogeny indicates that the hybridization event took place up to 6.18 Ma. Because the southern African lineage has never been recorded from Morocco or the Canary Islands, we hypothesize that it reached this area in the distant past, but never became permanently established. Interestingly, the southern African lineage includes S. inaequidens, a highly invasive species that has recently become widespread throughout Europe and was introduced at the end of the 19th century as a 'wool alien'. Our results suggest that this more recent invasion of Europe by S. inaequidens represents the second arrival of this lineage into the region.     

A striking example of the founder effect in the mollusc Littorina saxatilis

Two South African populations of Littorina saxatilis were examined by starch-gel electrophoresis at 16 enzyme loci and compared with 13 populations of North Atlantic saxatilis from both American and European coasts, and with six British populations of the closely related species Littorina arcana. The South African animals showed a severely reduced heterozygosity (– 0.052) compared with Atlantic populations of saxatilis ( = 0.181), and the mean genetic distance between the two areas was high ( = 0.203) compared with distances within the North Atlantic saxatilis populations (D = 0.034). In fact, the saxatilis from South Africa were genetically more distant from the North Atlantic samples of L. saxatilis than were the arcana from British shores. The reduced genetic heterozygosity and genetic divergence of the South African populations is attributed to founder effects following a postulated recent introduction by man.     

High genetic diversity is not essential for successful introduction

Some introduced populations thrive and evolve despite the presumed loss of diversity at introduction. We aimed to quantify the amount of genetic diversity retained at introduction in species that have shown evidence of adaptation to their introduced environments. Samples were taken from native and introduced ranges of Arctotheca populifolia and Petrorhagia nanteuilii. Using microsatellite data, we identified the source for each introduction, estimated genetic diversity in native and introduced populations, and calculated the amount of diversity retained in introduced populations. These values were compared to those from a literature review of diversity in native, confamilial populations and to estimates of genetic diversity retained at introduction. Gene diversity in the native range of both species was significantly lower than for confamilials. We found that, on average, introduced populations showing evidence of adaptation to their new environments retained 81% of the genetic diversity from the native range. Introduced populations of P. nanteuilii had higher genetic diversity than found in the native source populations, whereas introduced populations of A. populifolia retained only 14% of its native diversity in one introduction and 1% in another. Our literature review has shown that most introductions demonstrating adaptive ability have lost diversity upon introduction. The two species studied here had exceptionally low native range genetic diversity. Further, the two introductions of A. populifolia represent the largest percentage loss of genetic diversity in a species showing evidence of substantial morphological change in the introduced range. While high genetic diversity may increase the likelihood of invasion success, the species examined here adapted to their new environments with very little neutral genetic diversity. This finding suggests that even introductions founded by small numbers of individuals have the potential to become invasive.     

Increased Phenotypic Plasticity to Climate May Have Boosted the Invasion Success of Polyploid Centaurea stoebe

Phenotypic plasticity may allow organisms to cope with altered environmental conditions as e.g. after the introduction into a new range. In particular polyploid organisms, containing more than two sets of chromosomes, may show high levels of plasticity, which could in turn increase their environmental tolerance and invasiveness. Here, we studied the role of phenotypic plasticity in the invasion of Centaurea stoebe (Asteraceae), which in the native range in Europe occurs as diploids and tetraploids, whereas in the introduced range in North America so far only tetraploids have been found. In a common garden experiment at two sites in the native range, we grew half-sibs of the three geo-cytotypes (native European diploids, European tetraploids and invasive North American tetraploids) from a representative sample of 27 populations. We measured the level and the adaptive significance of phenotypic plasticity in eco-physiological and life-history traits in response to the contrasting climatic conditions at the two study sites as well as three different soil conditions in pots, simulating the most crucial abiotic differences between the native and introduced range. European tetraploids showed increased levels of phenotypic plasticity as compared to diploids in response to the different climatic conditions in traits associated with rapid growth and fast phenological development. Moreover, we found evidence for adaptive plasticity in these traits, which suggests that increased plasticity may have contributed to the invasion success of tetraploid C. stoebe by providing an advantage under the novel climatic conditions. However, in invasive tetraploids phenotypic plasticity was similar to that of native tetraploids, indicating no evolution of increased plasticity during invasions. Our findings provide the first empirical support for increased phenotypic plasticity associated with polyploids, which may contribute to their success as invasive species in novel environments.     

Pathways of spread of the introduced ascidian Styela clava (Tunicata) in Northern Europe, as revealed by microsatellite markers

Styela clava, a solitary ascidian native to the NW Pacific, has become a conspicuous member of fouling communities in NW European waters. As its natural dispersal appears to be limited, the wide distribution of S. clava along coasts within its introduced range may be attributed to secondary spread assisted by human activities. Here, we used six microsatellite loci to examine the genetic diversity and extent of gene flow among S. clava populations in its European introduced range. Samples were collected from 21 populations within Europe (N = 808), 4 populations within the USA and two populations within the native range (Japan). Large variation in genetic diversity was observed among the European populations but were not explained either by the geographic distance from the first introduction area (i.e. Plymouth, UK) nor by the time elapsed since the introduction. No founder effect was observed in the introduced populations, except possibly in Puget Sound (USA). At least two different introductions occurred in Europe, identified as distinct genetic clusters: northern Danish populations (resembling one Japanese population), and the rest of Europe; a sample from Shoreham (England) possibly represents a third introduction. In North America, the population from the Atlantic was genetically similar to the majority of European populations, suggesting a European origin for populations on this seaboard, while populations from the Pacific coast were genetically similar to the same Japanese population as the Danish populations.     

Evidence for multiple introductions of Centaurea stoebe micranthos (spotted knapweed,Asteraceae) to North America

Invasive species’ success may depend strongly on the genetic resources they maintain through the invasion process. We ask how many introductions have occurred in the North American weed Centaurea stoebe micranthos (Asteraceae), and explore whether genetic diversity and population structure have changed as a result of introduction. We surveyed individuals from 15 European native range sites and 11 North American introduced range sites at six polymorphic microsatellite loci. No significant difference existed in the total number of alleles or in the number of private alleles found in each range. Shannon–Weaver diversity of phenotype frequencies was also not significantly different between the ranges, while expected heterozygosity was significantly higher in the invasive range. Population structure was similar between the native range and the invasive range, and isolation by distance was not significant in either range. Traditional assignment methods did not allocate any North American individuals to the sampled European populations, while Bayesian assignment methods grouped individuals into nine genetic clusters, with three of them shared between North America and Europe. Invasive individuals tended to have genetically admixed profiles, while natives tended to assign more strongly to a single cluster. Many North American individuals share assignment with Romania and Bulgaria, suggesting two separate invasions that have undergone gene flow in North America. Samples from three other invasive range sites were genetically distinct, possibly representing three other unique introductions. Multiple introductions and the maintenance of high genetic diversity through the introduction process may be partially responsible for the invasive success of C. stoebe micranthos.     

Molecular evidence for multiple introductions of garlic mustard (Alliaria petiolata, Brassicaceae) to North America

Invasive species offer excellent model systems for studying rapid evolutionary change. In this context, molecular markers play an important role because they provide information about pathways of introduction, the amount of genetic variation introduced, and the extent to which founder effects and inbreeding after population bottlenecks may have contributed to evolutionary change. Here, we studied microsatellite variation in eight polymorphic loci among and within 27 native and 26 introduced populations of garlic mustard (Alliaria petiolata), a European herb which is a current serious invader in North American deciduous forests. Overall, introduced populations were genetically less diverse. However, considerable variability was present and when compared to the probable source regions, no bottleneck was evident. Observed heterozygosity was very low and resulted in high inbreeding coefficients, which did not differ significantly between native and introduced populations. Thus, selfing seems to be equally dominant in both ranges. Consequently, there was strong population differentiation in the native (F(ST) = 0.704) and the introduced (F(ST) = 0.789) ranges. The high allelic diversity in the introduced range strongly suggests multiple introductions of Alliaria petiolata to North America. Out of six European regions, the British Isles, northern Europe, and central Europe had significantly higher proportions of alleles, which are common to the introduced range, and are therefore the most probable source regions. The genetic diversity established by multiple introductions, and the lack of inbreeding depression in this highly selfing species, may have contributed to the invasion success of Alliaria petiolata.     

Genomics reveals the history of a complex plant invasion and improves the management of a biological invasion from the South African–Australian biotic exchange

Many plants exchanged in the global redistribution of species in the last 200 years, particularly between South Africa and Australia, have become threatening invasive species in their introduced range. Refining our understanding of the genetic diversity and population structure of native and alien populations, introduction pathways, propagule pressure, naturalization, and initial spread, can transform the effectiveness of management and prevention of further introductions. We used 20,221 single nucleotide polymorphisms to reconstruct the invasion of a coastal shrub, Chrysanthemoides monilifera ssp. rotundata (bitou bush) from South Africa, into eastern Australia (EAU), and Western Australia (WAU). We determined genetic diversity and population structure across the native and introduced ranges and compared hypothesized invasion scenarios using Bayesian modeling. We detected considerable genetic structure in the native range, as well as differentiation between populations in the native and introduced range. Phylogenetic analysis showed the introduced samples to be most closely related to the southern‐most native populations, although Bayesian analysis inferred introduction from a ghost population. We detected strong genetic bottlenecks during the founding of both the EAU and WAU populations. It is likely that the WAU population was introduced from EAU, possibly involving an unsampled ghost population. The number of private alleles and polymorphic SNPs successively decreased from South Africa to EAU to WAU, although heterozygosity remained high. That bitou bush remains an invasion threat in EAU, despite reduced genetic diversity, provides a cautionary biosecurity message regarding the risk of introduction of potentially invasive species via shipping routes.     

Contrasting mitochondrial diversity of European starlings (Sturnus vulgaris) across three invasive continental distributions

European starlings (Sturnus vulgaris) represent one of the most widespread and problematic avian invasive species in the world. Understanding their unique population history and current population dynamics can contribute to conservation efforts and clarify evolutionary processes over short timescales. European starlings were introduced to Central Park, New York in 1890, and from a founding group of about 100 birds, they have expanded across North America with a current population of approximately 200 million. There were also multiple introductions in Australia in the mid‐19th century and at least one introduction in South Africa in the late 19th century. Independent introductions on these three continents provide a robust system to investigate invasion genetics. In this study, we compare mitochondrial diversity in European starlings from North America, Australia, and South Africa, and a portion of the native range in the United Kingdom. Of the three invasive ranges, the North American population shows the highest haplotype diversity and evidence of both sudden demographic and spatial expansion. Comparatively, the Australian population shows the lowest haplotype diversity, but also shows evidence for sudden demographic and spatial expansion. South Africa is intermediate to the other invasive populations in genetic diversity but does not show evidence of demographic expansion. In previous studies, population genetic structure was found in Australia, but not in South Africa. Here we find no evidence of population structure in North America. Although all invasive populations share haplotypes with the native range, only one haplotype is shared between invasive populations. This suggests these three invasive populations represent independent subsamples of the native range. The structure of the haplotype network implies that the native‐range sampling does not comprehensively characterize the genetic diversity there. This study represents the most geographically widespread analysis of European starling population genetics to date.     

Dispersal Pathways and Genetic Differentiation among Worldwide Populations of the Invasive Weed Centaurea solstitialis L. (Asteraceae)

The natural history of introduced species is often unclear due to a lack of historical records. Even when historical information is readily available, important factors of the invasions such as genetic bottlenecks, hybridization, historical relationships among populations and adaptive changes are left unknown. In this study, we developed a set of nuclear, simple sequence repeat markers and used these to characterize the genetic diversity and population structure among native (Eurasian) and non-native (North and South American) populations of Centaurea solstitialis L., (yellow starthistle). We used these data to test hypotheses about the invasion pathways of the species that were based on historical and geographical records, and we make inferences about historical relationships among populations and demographic processes following invasion. We confirm that the center of diversity and the native range of the species is likely the eastern Mediterranean region in the vicinity of Turkey. From this region, the species likely proceeded to colonize other parts of Europe and Asia via a slow, stepwise range expansion. Spanish populations were the primary source of seed to invade South America via human-mediated events, as was evident from historical records, but populations from the eastern Mediterranean region were also important. North American populations were largely derived from South America, but had secondary contributors. We suggest that the introduction history of non-native populations from disparate parts of the native range have allowed not just one, but multiple opportunities first in South America then again in North America for the creation of novel genotypes via intraspecific hybridization. We propose that multiple intraspecific hybridization events may have created especially potent conditions for the selection of a noxious invader, and may explain differences in genetic patterns among North and South America populations, inferred differences in demographic processes, as well as morphological differences previously reported from common garden experiments.     

Pathways of cryptic invasion in a fish parasite traced using coalescent analysis and epidemiological survey

Introduced species have the potential to outperform natives via the introduction of new parasites to which the native ecosystem is vulnerable. Cryptic diversity within an invasive species can obscure invasion patterns and confound proper management measures. The aim of this study is to use coalescent theory based methodology to trace recent routes of invasion in populations of Ligula intestinalis, a globally distributed fish parasite possessing both native and recently introduced populations in North Africa. Molecular analyses of mitochondrial DNA discerned a pronounced genetic divergence between introduced and native populations. Distribution of mitochondrial haplotypes demonstrated common origin of European populations with North African parasites sampled from introduced fish species in Tunisia. To test the suggested pathway of introduction, microsatellite data were examined in a model-based coalescent analysis using the software MIGRATE, where Europe to Tunisia direction of migration was favoured over alternative hypotheses of gene flow. Specificity of Tunisian populations to different host species was assessed in an epidemiologic survey confirming prevailing host-based division between introduced and native parasites in North Africa. This approach combining advanced analysis of molecular markers with host-specificity data allows revealing the evolution of host-parasite interactions following biological invasion and provides basis for devising future management measurements.     

Comparison of phototrophic shell-degrading endoliths in invasive and native populations of the intertidal mussel Mytilus galloprovincialis

The intertidal mussel Mytilus galloprovincialis is a successful invader worldwide. Since its accidental introduction onto the South African west coast in the late 1970s, it has become the most successful marine invasive species in South Africa. One possible explanation for this phenomenon is that M. galloprovincialis suffers less from phototrophic shell-degrading endoliths in its invasive than in its native range. We assessed photoautotrophic endolithic pressure on M. galloprovincialis in native (Portugal) and invasive (South Africa) ranges. Invasive populations were more heavily infested than native populations. In Portugal, only the biggest/oldest mussels displayed endolithic erosion of the shell and the incidence of infestation was greater at higher shore levels where more prolonged exposure to light enhances endolith photosynthesis. In South Africa, even the smallest size classes of mussels were heavily infested throughout the shore. In Portugal, endolithic-induced mortality was observed at only one location, while in South Africa it occurred at all locations and at significantly higher rates than in Portugal. Important sub-lethal effects were detected in infested native mussels, confirming previous studies of invasive populations and suggesting an energy trade-off between shell repair and other physiological constraints. We observed a positive relationship between infestation rates and barnacle colonization on mussel shells, suggesting possible facilitation of barnacle settlement/survival by shell-boring pathogens. Identification of endoliths revealed common species between regions. However, two species were unique in the invasive range while another was unique in the native region. Different levels of endolithic infestation in the invasive and the native range were not explained by the effect of major environmental determinants (Photosynthetically Available Radiation and wave height). The results reject our initial hypothesis, indicating that invasion success of M. galloprovincialis is not simply explained by escape from its natural enemies but results from complex interactions between characteristics of the invaded community and properties of the invader.     

Evidence for the enemy release hypothesis in <Emphasis Type="Italic">Hypericum perforatum</Emphasis>

The enemy release hypothesis (ERH), which has been the theoretical basis for classic biological control, predicts that the success of invaders in the introduced range is due to their release from co-evolved natural enemies (i.e. herbivores, pathogens and predators) left behind in the native range. We tested this prediction by comparing herbivore pressure on native European and introduced North American populations of Hypericum perforatum (St Johns Wort). We found that introduced populations occur at larger densities, are less damaged by insect herbivory and suffer less mortality than populations in the native range. However, overall population size was not significantly different between ranges. Moreover, on average plants were significantly smaller in the introduced range than in the native range. Our survey supports the contention that plants from the introduced range experience less herbivore damage than plants from the native range. While this may lead to denser populations, it does not result in larger plant size in the introduced versus native range as postulated by the ERH.