Native Conspecifics Research Articles

Invasive freshwater snails are less sensitive to population density than native conspecifics.

Understanding how and why some species or lineages become invasive is critically important for effectively predicting and mitigating biological invasions. Here, we address an important unanswered question in invasion biology: do key life-history traits of invasive versus native lineages within a species differ in response to key environmental stressors? We focus on the environmental factor of population density, which is a fundamental characteristic of all populations, and investigate how changes in density affect native versus invasive Potamopyrgus antipodarum (New Zealand mudsnail). P. antipodarum has invaded 39 countries and detrimentally affects invaded environments. Previous studies of native and invasive populations and from laboratory experiments have demonstrated that growth and reproduction of P. antipodarum is sensitive to population density, though whether and how this sensitivity varies across native versus invasive lineages remains uncharacterized. We quantified individual growth rate and reproduction in P. antipodarum from multiple distinct native and invasive lineages across three different population density treatments. The growth of native but not invasive lineages decreased as density increased. There was no differential effect of density treatment on embryo production of invasive versus native snails, but a significantly higher proportion of snails were reproductive in high density compared to intermediate density for invasive lineages. In native lineages, there were no significant differences in the relative frequency of reproductive snails across density treatments. While the extent to which these results from our laboratory study can be extrapolated to the more complex natural world remain unclear, our findings are consistent with a scenario where differential sensitivity to population density could help explain why some lineages become successful invaders. Our findings also align with previous studies that show that invasive P. antipodarum lineages exhibit a relatively wide range of tolerance to environmental stressors.

Feeding Habits and Diet Overlap between Brown Trout Lineages from the Danube Basin of Croatia

Brown trout of non-native lineages have been stocked into Croatian streams and rivers primarily to meet angler demand. The diet of brown trout in the Black Sea Basin of Croatia is poorly understood, and there are no studies examining feeding competition between the Atlantic (AT) and Danube (DA) lineages of brown trout and their hybrids (HY). The aim of this study was to examine the natural diet of brown trout of both lineages and their hybrids and to compare feeding overlap. Canonical correspondence analysis was used to investigate the relationships between feeding habits of fish from different streams and of different genetic origin. The differences in variation of the consumed prey items were analysed by canonical variate analysis, and diet overlap was assessed by the Schoener index. The results indicate that stocked brown trout (AT) adapt rapidly to new habitat and food, as revealed by the consumption of a wide range of available food items and competition for food and space by taking on the feeding behaviour of wild native conspecifics. Diet overlap was also detected between brown trout of the DA and AT lineages. This study highlights the need to implement control measures to preserve and protect the native diversity of this species.

Extension of the EICA hypothesis for invasive Chromolaena odorata

Invasive plants usually occur in limited areas in their native range but may spread into diverse environments in non-native ranges. Therefore, besides increasing competitive ability, evolution may also increase the tolerance of invading plants to environmental stress. In our previous study, we found that evolution facilitates invasive Chromolaena odorata by increasing the allelopathic effect. In this study, we compared drought tolerance of C. odorata between six native and six non-native populations. The results indicated that invasive C. odorata displayed higher drought tolerance than native conspecifics. This study proved that invasive C. odorata had not only evolved increased competitive ability but also drought tolerance, which may have contributed to its wide distribution in non-native ranges.

Shifts in thermal preference of introduced Asian house geckos (Hemidactylus frenatus) in temperate regions of southeastern Australia

Despite its tropical origin, the Asian house gecko (Hemidactylus frenatus) is currently invading higher latitudes around the world. In this study, we investigated whether the introduced geckos in the subtropical/temperate region of southeastern Australia have shifted their thermal biology to cope with colder temperatures. In the lab, we measured the body temperatures of geckos from Thailand and Australia in a cost-free thermal gradient. Native H. frenatus from Thailand displayed a diel pattern of thermoregulation. Geckos maintained higher body temperatures during mid-afternoon and at dusk but selected cooler temperatures during the night. Introduced geckos showed a similar pattern of thermoregulation, but selected lower body temperatures in summer (mean=28.9°C) and winter (mean=25.5°C) than native geckos (mean=31.5°C). While the Asian house geckos from Thailand did not alter their body temperatures after feeding, their conspecifics from southeastern Australia selected body temperatures that were 1.6-3.1°C higher after feeding. In conclusion, our study shows that invasive house geckos in Australia have shifted their preferred body temperatures downwards relative to their native conspecifics in Thailand, presumably as a result of plasticity or natural selection. Our findings suggest that these tropical geckos have adapted to colder regions, and thus, they may spread much further than expected for a tropical ectotherm.

Invasive toads adopt marked capital breeding when introduced to a cooler, more seasonal environment

Abstract Amphibians from cold and seasonal environments show marked capital breeding and sustained resource allocation to growth when compared with conspecifics from warmer, less seasonal environments. Capital breeding fuels reproduction by using only stored energy, and larger sizes and masses confer higher fecundity, starvation resistance and heat and water retention. Invasive populations act as experiments to explore how resources are allocated in novel environments. We investigated resource allocation of the southern African toad Sclerophrys gutturalis in a native source population (Durban) and in an invasive population recently (< 20 years) established in a cooler, more seasonal climate (Cape Town). After dissection, lean structural mass (bones and muscles), gonadal mass, liver mass and body fat percentage were measured in 161 native and invasive animals sampled at the beginning and the end of the breeding season. As expected, female gonadal mass decreased throughout the breeding season only in the invaded range. Thus, invasive female toads adopt a more marked capital breeding strategy than native conspecifics. Conversely, males from both populations appear to be income breeders. Also, male and female toads from the invaded range allocate more resources to growth than their native counterparts. Such a novel allocation strategy might be a response to the low temperatures, reduced rainfall and heightened seasonality encountered by the invasive population.

Biomass reallocation and increased plasticity might contribute to successful invasion of Chromolaena odorata

Invasive plants are often genetically different from their native conspecifics. In the introduced range, the evolution of functional traits or plasticity might occur as an adaptive response to novel environmental pressures. Chromolaena odorata originates from the Americas, but now is a noxiously invasive species in some other tropical and subtropical areas. In this study, plant growth, morphology, photosynthesis and plasticity were compared between C. odorata plants from five introduced and five native populations under three light intensities (low, medium and full light). Compared to native C. odorata, introduced conspecifics had higher height under medium and full light, but lower root to shoot biomass ratio across all light conditions, and lower leaf N under medium light. This indicates genetic differences between native and introduced C. odorata. Furthermore, plasticity was higher for introduced C. odorata plants than for native conspecifics under various light environments. Our results indicate that biomass reallocation (from belowground into aboveground) and stronger plasticity could contribute to successful invasion of introduced C. odorata in frequently disturbed environments.

Release from natural enemies mitigates inbreeding depression in native and invasive Silene latifolia populations.

Inbreeding and enemy infestation are common in plants and can synergistically reduce their performance. This inbreeding ×environment (I × E) interaction may be of particular importance for the success of plant invasions if introduced populations experience a release from attack by natural enemies relative to their native conspecifics. Here, we investigate whether inbreeding affects plant infestation damage, whether inbreeding depression in growth and reproduction is mitigated by enemy release, and whether this effect is more pronounced in invasive than native plant populations. We used the invader Silene latifolia and its natural enemies as a study system. We performed two generations of experimental out‐ and inbreeding within eight native (European) and eight invasive (North American) populations under controlled conditions using field‐collected seeds. Subsequently, we exposed the offspring to an enemy exclusion and inclusion treatment in a common garden in the species’ native range to assess the interactive effects of population origin (range), breeding treatment, and enemy treatment on infestation damage, growth, and reproduction. Inbreeding increased flower and leaf infestation damage in plants from both ranges, but had opposing effects on fruit damage in native versus invasive plants. Inbreeding significantly reduced plant fitness; whereby, inbreeding depression in fruit number was higher in enemy inclusions than exclusions. This effect was equally pronounced in populations from both distribution ranges. Moreover, the magnitude of inbreeding depression in fruit number was lower in invasive than native populations. These results support that inbreeding has the potential to reduce plant defenses in S. latifolia, which magnifies inbreeding depression in the presence of enemies. However, future studies are necessary to further explore whether enemy release in the invaded habitat has actually decreased inbreeding depression and thus facilitated the persistence of inbred founder populations and invasion success.

Genetically based phenotypic differentiation between native and introduced tetraploids of Oxalis pes-caprae

Rapid evolutionary change often plays an important role in determining the success of plant invasions. Oxalis pes-caprae, a geophyte native to South Africa, has become a persistent invasive weed in several areas of the world, being particularly widespread in regions with a Mediterranean climate. The objective of this study was to look for the existence of genetically based phenotypic differences regarding competitive ability between O. pes-caprae native tetraploids and introduced tetraploids recently discovered in the invaded range of the western Mediterranean basin. For this, shoot emergence time, beginning of flowering, chlorophyll fluorescence parameters, aboveground biomass, final offspring bulb production and survival were measured in a greenhouse experiment with plants from both ranges growing alone or in competition with Trifolium repens. Results demonstrated significant differences between introduced and native tetraploids, with plants from the invaded range emerging earlier, beginning flowering later and producing more aboveground biomass and offspring bulbs than South African plants. Furthermore, introduced plants showed an increase in aboveground biomass when grown with T. repens, and affected T. repens growth more severely than their native conspecifics, which may be indicative of a greater competitive ability. These findings provide strong evidence for genetic differentiation between introduced and native tetraploids. It is suggested that founder events and rapid post-introduction adaptive evolution may have contributed, independently or in concert, to this divergence.

Consumption of Invasive Western Mosquitofish Fry by Adult Conspecifics and Native Crayfish

The absence of predators is often invoked to explain the ability of non-native species to successfully invade a habitat, however native species can control or regulate populations of invasive species through predation. To better understand the regulation of invasive Gambusia affinis (Western Mosquitofish), we conducted a laboratory experiment to examine the potential for native Cambarus thomai (Little Brown Mudbug) and adult conspecifics to consume Western Mosquitofish fry. On average, female Western Mosquitofish consumed nearly 3 times the number of fry in 24 h than males. Little Brown Mudbug, consumed some of the Western Mosquitofish fry, but the resulting mosquitofish mortality was not significantly higher than the control (which had 100% survivorship). Our results show that cannibalism is a potentially significant source of mortality in Western Mosquitofish, and thus may be a factor involved in the regulation of their population dynamics. However, native Little Brown Mudbug, while they do consume mosquitofish fry, are probably not a major source of mosquitofish mortality in nature.

Trophic consequences of introduced species: Comparative impacts of increased interspecific versus intraspecific competitive interactions.

Invasive species can cause substantial ecological impacts on native biodiversity. While ecological theory attempts to explain the processes involved in the trophic integration of invaders into native food webs and their competitive impacts on resident species, results are equivocal. In addition, quantifying the relative strength of impacts from non‐native species (interspecific competition) versus the release of native conspecifics (intraspecific competition) is important but rarely completed.Two model non‐native fishes, the globally invasive Cyprinus carpio and Carassius auratus, and the model native fish Tinca tinca, were used in a pond experiment to test how increased intra‐ and interspecific competition influenced trophic niches and somatic growth rates. This was complemented by samples collected from three natural fish communities where the model fishes were present. The isotopic niche, calculated using stable isotope data, represented the trophic niche.The pond experiment used additive and substitutive treatments to quantify the trophic niche variation that resulted from intra‐ and interspecific competitive interactions. Although the trophic niche sizes of the model species were not significantly altered by any competitive treatment, they all resulted in patterns of interspecific niche divergence. Increased interspecific competition caused the trophic niche of T. tinca to shift to a significantly higher trophic position, whereas intraspecific competition caused its position to shift towards elevated δ13C. These patterns were independent of impacts on fish growth rates, which were only significantly altered when interspecific competition was elevated.In the natural fish communities, patterns of trophic niche partitioning between the model fishes was evident, with no niche sharing. Comparison of these results with those of the experiment revealed the most similar results between the two approaches were for the niche partitioning between sympatric T. tinca and C. carpio.These results indicate that trophic niche divergence facilitates the integration of introduced species into food webs, but there are differences in how this manifests between introductions that increase inter‐ and intraspecific competition. In entirety, these results suggest that the initial ecological response to an introduction appears to be a trophic re‐organisation of the food web that minimises the trophic interactions between competing species. A plain language summary is available for this article.

In situ common garden assays demonstrate increased defense against natural fouling in non-native populations of the red seaweed Gracilaria vermiculophylla

The susceptibility of native and non-native populations of the red alga Gracilaria vermiculophylla to fouling was compared in common garden experiments. Native and non-native algae were enclosed into dialysis membrane tubes, and the tubes were exposed to natural fouling. Fouling on the outside of the tubes was mediated by chemical compounds excreted by G. vermiculophylla that diffused through the membranes. Fouling pressure was significantly higher in the Kiel Fjord (non-native range) than in Akkeshi Bay (native range), but, at both sites, tubes containing non-native G. vermiculophylla were less fouled than those with native conspecifics. This is the first in situ evidence that susceptibility to fouling differs between native and non-native populations of an aquatic organism. The technique of enclosing organisms into dialysis tubes represents a simple, efficient and accurate way to test chemical antifouling defenses and could possibly be applied to other organisms.

Anti‐epiphyte defences in the red seaweed Gracilaria vermiculophylla: non‐native algae are better defended than their native conspecifics

Summary Epibiosis in the marine environment is a stressor that may determine invasion success in introduced species. Previous comparisons showed resistance to epibionts can be higher in non‐native than in resident seaweed species, but we do not know whether it is an intrinsic trait of the non‐natives or it has been acquired during the invasion process. To elucidate this question, a comparison between native and non‐native populations of the same species is needed. Resistance against two groups of epiphytes was assessed in living thalli and in artificial substrata coated with surface extracts, both gained from four Asian (native) and four European (non‐native) populations of the red alga Gracilaria vermiculophylla. Two diatom species and two filamentous macroalgae were used as micro‐ and macro‐epiphytes, and one of each type was collected in Asia, while the other came from Europe. Laboratory assays were done in both distributional ranges of G. vermiculophylla and in different seasons. We used G. vermiculophylla from four populations in each range and used a fully crossed design with the factors (i) ‘Origin of Gracilaria’, (ii) ‘Origin of epiphytes’, (iii) ‘Season’ and (iv) ‘Solvent used for extraction’. Both groups of epiphytes, regardless of their origin, attached less to living thalli and to surface extracts from non‐native G. vermiculophylla. Fewer diatoms attached to hexane‐based extracts, while fewer Ceramium filaments settled on extracts gained with dichloromethane. Synthesis. Our results show for the first time that non‐native individuals of a marine organism are better defended against epiphytes than native conspecifics. Furthermore, we found evidence that at least a part of the defence is based on extractable secondary metabolites. We discuss several mechanisms that could explain the increased resistance to epiphytes in non‐native individuals, including the release from enemies in the non‐native range, which could lead to an increase in algal performance during the invasion process. We suggest that an enhanced defence against epiphytes after introduction is one reason for G. vermiculophylla's invasion success. Our observation may also apply to other basibiont–epibiont and host–enemy systems, including plant–plant, plant–animal and animal–animal interactions, in aquatic environments and could be a key feature of bioinvasions.

Are invasive plants more competitive than native conspecifics? Patterns vary with competitors.

Invasive plants are sometimes considered to be more competitive than their native conspecifics, according to the prediction that the invader reallocates resources from defense to growth due to liberation of natural enemies [‘Evolution of Increased Competitive Ability’ (EICA) hypothesis]. However, the differences in competitive ability may depend on the identity of competitors. In order to test the effects of competitors, Ageratina adenophora plants from both native and invasive ranges competed directly, and competed with native residents from both invasive (China) and native (Mexico) ranges respectively. Invasive A. adenophora plants were more competitive than their conspecifics from native populations when competing with natives from China (interspecific competition), but not when competing with natives from Mexico. Invasive A. adenophora plants also showed higher competitive ability when grown in high-density monoculture communities of plants from the same population (intrapopulation competition). In contrast, invasive A. adenophora plants showed lower competitive ability when competing with plants from native populations (intraspecific competition). Our results indicated that in the invasive range A. adenophora has evolved to effectively cope with co-occurring natives and high density environments, contributing to invasion success. Here, we showed the significant effects of competitors, which should be considered carefully when testing the EICA hypothesis.

No difference in the competitive ability of introduced and native Trifolium provenances when grown with soil biota from their introduced and native ranges.

The evolution of increased competitive ability (EICA) hypothesis could explain why some introduced plant species perform better outside their native ranges. The EICA hypothesis proposes that introduced plants escape specialist pathogens or herbivores leading to selection for resources to be reallocated away from defence and towards greater competitive ability. We tested the hypothesis that escape from soil-borne enemies has led to increased competitive ability in three non-agriculturalTrifolium(Fabaceae) species native to Europe that were introduced to New Zealand in the 19th century.Trifoliumperformance is intimately tied to rhizosphere biota. Thus, we grew plants from one introduced (New Zealand) and two native (Spain and the UK) provenances for each of three species in pots inoculated with soil microbiota collected from the rhizosphere beneath conspecifics in the introduced and native ranges. Plants were grown singly and in competition with conspecifics from a different provenance in order to compare competitive ability in the presence of different microbial communities. In contrast to the predictions of the EICA hypothesis, we found no difference in the competitive ability of introduced and native provenances when grown with soil microbiota from either the native or introduced range. Although plants from introduced provenances of two species grew more slowly than native provenances in native-range soils, as predicted by the EICA hypothesis, plants from the introduced provenance were no less competitive than native conspecifics. Overall, the growth rate of plants grown singly was a poor predictor of their competitive ability, highlighting the importance of directly quantifying plant performance in competitive scenarios, rather than relying on surrogate measures such as growth rate.

Non-native populations of an invasive tree outperform their native conspecifics.

Introduced plants often face new environmental conditions in their non-native ranges. To become invasive, they need to overcome several biotic and abiotic filters that may trigger adaptive changes in life-history traits, like post-germination processes. Such early life cycle traits may play a crucial role in the colonization and establishment success of invasive plants. As a previous study revealed that seeds of non-native populations of the woody Siberian elm, Ulmus pumila, germinated faster than those of native populations, we expected growth performance of seedlings to mirror this finding. Here, we conducted a common garden greenhouse experiment using different temperature and watering treatments to compare the biomass production of U. pumila seedlings derived from 7 native and 13 populations from two non-native ranges. Our results showed that under all treatments, non-native populations were characterized by higher biomass production and enhanced resource allocation to aboveground biomass compared to the native populations. The observed enhanced growth performance of non-native populations might be one of the contributing factors for the invasion success of U. pumila due to competitive advantages during the colonization of new sites.

Research highlights for issue 7: the evolution of invasiveness.

With continually increasing global connectivity and wholesale change of landscapes, the need to understand and control the spread of invasive species has never been greater. To do so requires insight into the factors that allow particular species to become invasive to begin with as well as into the evolutionary trajectory those invasive populations follow after establishment in new ranges. One of the more prominent hypotheses explaining the success of invasive species relies on the idea that populations can disperse away from their natural enemies, and thus grow without the constraints of antagonist-mediated control. Recent work by Tiantian Lin et al. builds on this idea by demonstrating parallel evolution of invasive plant populations across three geographically distinct regions towards decreased investment in defense and increased competitive ability. By comparing the competitive ability of native versus invasive genotypes of Common ragwort in either the absence of herbivores or in the presence of either a specialist or a generalist herbivore, the authors were able to show that invasive genotypes outcompete their native conspecifics in both the absence of herbivores and presence of the generalist herbivore, but have lowered defenses against the specialist herbivore (Lin et al. 2015). Another prominent idea in the field is that increased phenotypic plasticity of invasive species underlies their ability to successfully invade new habitats. Despite being a longstanding point of discussion, however, this idea has proven difficult to demonstrate empirically. New work by Russel Lande clarifies that quantitative genetic theory does not necessarily predict increased, long-term plasticity of invasive species. Instead, he argues that the outcome will depend on the optimal phenotype in the native versus colonized environments, as well as the predictability of each environment and the cost of plasticity (Lande 2015). Furthermore, he suggests that increased plasticity will often be hard to observe given the short-lived nature of the associated fitness advantage once successful colonization and adaptation have occurred. Indeed, recent evidence from the invasive Japanese stiltgrass suggests rapid adaptation to local abiotic conditions (Ziska et al. 2015), suggesting that any initial plasticity was likely transient. In line with this idea, a recent hypothesis formalized by Qiao Huang et al. suggests that invasive species may initially be under selection for increased adaptive plasticity due to their release from the stress of their native habitat. The authors develop a model to argue that release from local stressors such as natural enemies would both increase the benefits of plasticity and reduce potential costs, for example through the need to allocate resources to traits other than maintaining the ability to be plastic (Huang et al. 2015). Again, this increased plasticity is likely to be transient as the invasive species adapts to its local environment and, reciprocally, the local biotic environment responds to the presence of the invasive species. A contrasting hypothesis for invasiveness is the idea that introduced species succeed if/when they are able to occupy an open niche in the introduced range. Katrina Dlugosch et al. recently set out to test this hypothesis using the highly invasive yellow starthistle plant, which has spread far outside its native Eurasian range into Australia, Argentina, Chile, and the USA. Using a greenhouse experiment on invasive populations from Californian grasslands, the authors demonstrate that the plant has evolved an increased dependence of water allowing it to reach a higher fitness (Dlugosch et al. 2015). This niche exploitation is possible due to local declines in resident competitors, indicating that loss of biodiversity can open new niches and promote invasion by non-native species. Finally, a recent hypothesis put forward by Jessica Stapley et al. suggests that invasiveness may be driven in part by increased activity of transposable elements allowing rapid adaptation despite the small size (and thus low genetic variation) of most founding populations. The authors describe how environmental and genomic perturbations typical of invasion may favor activation of transposable elements, and therefore increase the rate of introduction of potentially beneficial alleles (Stapley et al. 2015).

Genetic differentiation and phenotypic plasticity in life-history traits between native and introduced populations of invasive maple trees

Genetically based phenotypic differentiation between native and invasive populations of exotic plants has been increasingly documented and commonly invoked to explain the success of some invasive species. Nonetheless, this basic information is lacking for invasive trees although they currently represent a major concern worldwide. Reciprocal common gardens were therefore set up in both native and introduced ranges of two exotic maple trees to assess the contribution of genetic differentiation and phenotypic plasticity to tree invasiveness. Almost 3,000 native and invasive seedlings of Acer negundo and Acer platanoides were planted in Canada and in France and their performances were compared in various life-history traits related to growth, leaf phenology and ecophysiology over 2 and 3 year periods. Invasive populations of A. negundo exhibited strong genetic differentiation in all the traits examined. Compared to their native conspecifics, they grew significantly larger in the introduced range and showed lower survival, reduced maximum assimilation rate and increased leaf area in the two gardens. They also expressed greater plasticity for growth and greater phenological sensitivity to temperature. Native and invasive populations of A. platanoides were plastic across environments but in contrast did not exhibit any genetic differentiation. This cross-continental comparison provides evidence that both genetic differentiation and phenotypic plasticity contribute synergistically to tree invasiveness. The influence of these respective processes depends on stage of invasion and the life-history strategy of each species. Plastic effects are likely more important during colonization and establishment whilst genetic effects may contribute more significantly during the spread of established populations.

飞机草入侵种群与原产地种群生长性状的差异

飞机草(<em>Chromolaena odorata</em>) 是我国热带地区危害严重的外来入侵植物,为揭示适应进化对其成功入侵的贡献,在同质种植园中,比较研究了飞机草10 个入侵地种群与12 个原产地种群生长性状的差异,为排除奠基者效应的可能影响,进一步比较了飞机草10 个入侵地种群与其原产地可能的祖先种群间的差异。结果表明,飞机草10 个入侵地种群的基茎、株高、分枝数、生物量和比叶面积均显著高于12 个原产地种群;与可能的祖先种群相比,飞机草10 个入侵种群的生物量、分枝数和比叶面积仍更高。这些结果表明,在长期的入侵过程中飞机草通过进化提高了资源向生长的分配,支持增强竞争能力的进化假说。;Biological invasions are serious socio-economic and environmental problems. Invasion ecology has become one of the hotspots in recent years. However, the mechanisms underlying exotic plant invasions are still not well elucidated, although many related studies have been conducted worldwide. The Evolution of Increased Competitive Ability (EICA) Hypothesis predicts that invasive plant species may decrease resource allocation to defense and therefore increase allocation to growth in response to enemy release in their new ranges. To test this hypothesis, many studies have been carried out in both invasive and native ranges of many introduced plants. Some of these studies do not provide any evidence for the EICA hypothesis, although more studies support this hypothesis. One of the reasons for the inconsistent results is that almost all related studies compared the differences in performance between plants from invasive and native populations in general, without considering the potential influence of founder effect. To exclude the influence of confounding founder effect, we should compare the difference between the plants from invasive populations and the plants from their source native populations. However, the source populations of invasive species are rarely known with certainty. To investigate the role of post-introduction evolution in successful invasion of <em>Chromolaena odorata </em>(L.) King & Robinson (Asteraceae), we compared the overall differences in growth traits between the plants originating from seeds collected in 10 populations in its invasive range in Asia and the plants originating from seeds collected in 12 populations in its native range in America in a common garden. In order to decrease or even exclude the influence of founder effect, we specifically compared the plants from the 10 invasive populations with the plants from their six putative source populations. <em>Chromolaena odorata</em> is native to North, Central, and South America, but a noxious perennial invasive forb or subshrub throughout the tropics in Africa, Asia, and Oceania. It was introduced into Calcutta, India in the middle of the nineteen century. It was first found in Yunnan, southwest China in 1934. Now it occurs in Yunnan, Guangxi, Guizhou, Hainan, Guangdong, Taiwan, Hongkong, and Macao, becoming one of the most noxious invasive plants in China. <em>Chromolaena odorata</em> harbours more than 240 enemies in its native range and 25% of them are specialists to the invader. However, only few generalists were found for the invader in its native range in China. <em>Chromolaena odorata</em> plants invading in Asia may originate from Florida and Trinidad according to the results of studies with molecular markers. In the common garden, <em>C. odorata</em> plants from the 10 invasive populations were significantly higher in stem diameter, plant height, branch number, total biomass, and specific leaf area than the plants from the 12 native populations. Compared with the plants from the six putative source populations, the plants from the 10 invasive populations were also higher in total biomass, branch number, and specific leaf area. Our results indicate that <em>C. odorata</em> appears to have increased resource allocation to growth through post-introduction evolution, providing more convincing evidence for the EICA hypothesis than general comparisons between invasive plants and their native conspecifics without considering their ancestors.

A test for pre-adapted phenotypic plasticity in the invasive tree Acer negundo L.

Phenotypic plasticity is a key mechanism associated with the spread of exotic plants and previous studies have found that invasive species are generally more plastic than co-occurring species. Comparatively, the evolution of phenotypic plasticity in plant invasion has received less attention, and in particular, the genetic basis of plasticity is largely unexamined. Native from North America, Acer negundo L. is aggressively impacting the riparian forests of southern and eastern Europe thanks to higher plasticity relative to co-occurring native species. We therefore tested here whether invasive populations have evolved increased plasticity since introduction. The performance of 1152 seedlings from 8 native and 8 invasive populations was compared in response to nutrient availability. Irrespective of nutrients, invasive populations had higher growth and greater allocation to above-ground biomass relative to their native conspecifics. More importantly, invasive genotypes did not show increased plasticity in any of the 20 traits examined. This result suggests that the high magnitude of plasticity to nutrient variation of invasive seedlings might be pre-adapted in the native range. Invasiveness of A. negundo could be explained by higher mean values of traits due to genetic differentiation rather than by evolution of increased plasticity.

Competitive dominance of the invasive plant Impatiens glandulifera: using competitive effect and response with a vigorous neighbour

Plant invasiveness was commonly attributed to the invader’s competitive superiority over the native community, but a general pattern supporting this prediction is still lacking. This is particularly enhanced by the fact that competitive dominance and its role in plant invasiveness require the use of scarcely-practiced experimental elements. Here, we used a comprehensive experimental approach to evaluate the competitive superiority of the highly invasive annual Impatiens glandulifera. We used two competition-setting treatments, which independently examine competitive effect versus response of both native and invasive genotypes. As a neighbour species we used Urtica dioica, a vigorous perennial co-occurring with I. glandulifera at both its native and introduced ranges. By examining both components of competitive ability we were able to show that although invasive genotypes exert a weaker competitive pressure compared to their native conspecifics, they are still competitively superior to U. dioica. Our results also suggest that the high competitive ability of I. glandulifera could be attributed it to allelopathic effects on co-occurring native species. We suggest that an appropriate experimental setup, which examines competitive effect and response independently, can provide a more factual evaluation of the competitive ability of invasive plants. Furthermore, the use of a dominant species as a target plant rather than a random species or one with poor competitive ability, renders our results more general, implying that I. glandulifera might exert greater competitive effect on the less robust co-occurring species. We conclude that our approach is highly useful and advocate its application in future tests of invasion success.