Differences in growth and herbivory damage of native and invasive kudzu (Peuraria montana var. lobata) populations grown in the native range

The invasion success of exotic plants is often attributed to escape from natural enemies in their introduced ranges and subsequent evolutionary change in resource allocation from defense to growth and reproduction. We tested this idea by comparing resistance, tolerance, and growth between native (China) and invasive (US) populations of kudzu (Peuraria montana var. lobata) exposed to natural herbivores in the native range. The percentage of foliar damage was much higher in invasive populations than in native populations, indicating that plants from invasive populations had lower resistance to herbivory. Regression of total mass on percentage of foliar damage showed no significant differences in tolerance to herbivory between native and invasive populations. However, stem diameter and mass were significantly greater in invasive populations than in native populations. Our results may suggest geographic variation in herbivory damage and plant growth among kudzu native and invasive populations, but the role of herbivores influencing kudzu invasion requires further investigation.     

Decreased resistance and increased tolerance to native herbivores of the invasive plant Sapium sebiferum

Release from natural enemies may favor invasive plants evolving traits associated with reduced herbivore‐resistance and faster‐growth in introduced ranges. Given a genetic trade‐off between resistance and tolerance, invasive plants could also become more tolerant to herbivory than conspecifics in the native range. We conducted a field common garden study in the native range of Sapium sebiferum using seeds from native Chinese populations and invasive North American populations to compare their growth and herbivory resistance. We also performed a cage‐pot experiment to compare their resistance and tolerance to Bikasha collaris beetles that are specialist feeders on S. sebiferum trees in China. Results of the common garden study showed that Sapium seedlings of invasive populations relative to native populations were more frequently attacked by native herbivores. Growth and leaf damage were significantly higher for invasive populations than for native populations. Growth of invasive populations was not significantly affected by insecticide spray, but insecticide spray benefited that of native populations. In the bioassay trial, beetles preferentially consumed leaf tissue of invasive populations compared to native populations when beetles had a choice between them. Regression of percent leaf damage on biomass showed that invasive populations tolerated herbivory more effectively than native populations. Our results suggest that S. sebiferum from the introduced range had lower resistance but higher tolerance to specialist herbivores. Both defense strategies could have evolved as a response to the escape from natural enemies in the introduced range.     

Increased reproductive capacity and physical defense but decreased tannin content in an invasive plant

Abstract Plant invasions create novel plant–insect interactions. The EICA (evolution of increased competitive ability) hypothesis proposes that invasive plants will reallocate resources from defense to growth and/or reproduction because they have escaped from their co‐evolved insect natural enemies. Testing multiple herbivory by monophagous and oligophagous herbivores and simultaneous measurement of various plant traits will provide new insights into the evolutionary change of invasive plants. In this context, we conducted a common garden experiment to compare plant growth and reproduction, chemical and physical defense, and plant responses to herbivory by different types of herbivores between invasive North American populations and native East Asian populations of mile‐a‐minute weed, Persicaria perfoliata. We found that invasive mile‐a‐minute exhibited lower biomass, flowered earlier and had greater reproductive output than plants from the native range. Compared with native populations, plants from invasive populations had lower tannin content, but exhibited higher prickle density on nodes and leaves. Thus our results partially support the EICA hypothesis. When exposed to the monophagous insect, Rhinoncomimus latipes and the oligophagous insects, Gallerucida grisescens and Smaragdina nigrifrons, more damage by herbivory was found on invasive plants than on natives. R. latipes, G. grisescens and S. nigrifrons had strong, moderate and weak impacts on the growth and reproduction of mile‐a‐minute, respectively. The results indicate that mile‐a‐minute may have evolved a higher reproductive capacity in the introduced range, and this along with a lack of oligophagous and monophagous herbivores in the new range may have contributed to its invasiveness in North America.     

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.     

Increased competitive ability and herbivory tolerance in the invasive plant <Emphasis Type="Italic">Sapium sebiferum</Emphasis>

The evolution of increased competitive ability (EICA) hypothesis predicts that release from natural enemies in the introduced range favors exotic plants evolving to have greater competitive ability and lower herbivore resistance than conspecifics from the native range. We tested the EICA hypothesis in a common garden experiment with Sapium sebiferum in which seedlings from native (China) and invasive (USA) populations were grown in all pairwise combinations in the native range (China) in the presence of herbivores. When paired seedlings were from the same continent, shoot mass and leaf damage per seedling were significantly greater for plants from invasive populations than those from native populations. Despite more damage from herbivores, plants from invasive populations still outperformed those from native populations when they were grown together. Increased competitive ability and higher herbivory damage of invasive populations relative to native populations of S. sebiferum support the EICA hypothesis. Regression of biomass against percent leaf damage showed that plants from invasive populations tolerated herbivory more effectively than those from native populations. The results of this study suggest that S. sebiferum has become a faster-growing, less herbivore-resistant, and more herbivore-tolerant plant in the introduced range. This implies that increased competitive ability of exotic plants may be associated with evolutionary changes in both resistance and tolerance to herbivory in the introduced range. Understanding these evolutionary changes has important implications for biological control strategies targeted at problematic invaders.     

Are invaders moving targets? The generality and persistence of advantages in size, reproduction, and enemy release in invasive plant species with time since introduction

Successful plant invasions are often attributed to increased plant size, reproduction, or release from natural enemies, but the generality and persistence of these patterns remains widely debated. Meta-analysis was used to quantitatively assess invasive plant performance and release from enemy damage and how these change with residence time and geographic distribution. Invasive plants were compared either in their introduced and home ranges or with native congeners in the introduced range. Invasive plants in the introduced range were generally larger, allocated more to reproduction, and had lower levels of herbivore damage compared with conspecifics in the home range; pathogen attack, however, varied widely. In congener comparisons, invasive and native plants did not differ in size or herbivory, but invaders did allocate less to reproduction and had lower levels of pathogen damage. Time since introduction was a significant nonlinear predictor of enemy release for both herbivores and pathogens, with initial release in recently arrived species and little to no release after 50 to 200 years. Geographic distribution was also a significant nonlinear predictor of enemy release. The observed nonlinear relationships are consistent with dynamic invasions and may define targets for eradication efforts if these patterns hold up for individual species.     

Geographic patterns of herbivory and resource allocation to defense, growth, and reproduction in an invasive biennial, Alliaria petiolata

We investigated geographic patterns of herbivory and resource allocation to defense, growth, and reproduction in an invasive biennial, Alliaria petiolata, to test the hypothesis that escape from herbivory in invasive species permits enhanced growth and lower production of defensive chemicals. We quantified herbivore damage, concentrations of sinigrin, and growth and reproduction inside and outside herbivore exclusion treatments, in field populations in the native and invasive ranges. As predicted, unmanipulated plants in the native range (Hungary, Europe) experienced greater herbivore damage than plants in the introduced range (Massachusetts and Connecticut, USA), providing evidence for enemy release, particularly in the first year of growth. Nevertheless, European populations had consistently larger individuals than US populations (rosettes were, for example, eightfold larger) and also had greater reproductive output, but US plants produced larger seeds at a given plant height. Moreover, flowering plants showed significant differences in concentrations of sinigrin in the invasive versus native range, although the direction of the difference was variable, suggesting the influence of environmental effects. Overall, we observed less herbivory, but not increased growth or decreased defense in the invasive range. Geographical differences in performance and leaf chemistry appear to be due to variation in the environment, which could have masked evolved differences in allocation.     

Competitive ability,not tolerance,may explain success of invasive plants over natives

When entering a new community, introduced species leave behind members of their native community while simultaneously forming novel biotic interactions. Escape from enemies during the process of introduction has long been hypothesized to drive the increased performance of invasive species. However, recent studies and quantitative syntheses find that invaders often receive similar, or even more, damage from enemies than do native species. Therefore, invasives may be those more tolerant to enemy damage, or those able to maintain competitive ability in light of enemy damage. Here, we investigate whether tolerance and competitive ability could contribute to invasive plant success. We determined whether invasive plants were more competitive than native or noninvasive exotic species in both the presence and absence of simulated herbivory. We found competition and herbivory additively reduced individual performance, and affected the performance of native, invasive, and noninvasive exotic species’ to the same degree. However, invasives exerted stronger competitive effects on an abundant native species (Elymus canadensis) in both the presence and absence of herbivory. Therefore, while invasive species responded similarly to competition and simulated herbivory, their competitive effects on natives may contribute to their success in their introduced range.     

Response to enemies in the invasive plant Lythrum salicaria is genetically determined

Background and Aims

The enemy release hypothesis assumes that invasive plants lose their co-evolved natural enemies during introduction into the new range. This study tested, as proposed by the evolution of increased competitive ability (EICA) hypothesis, whether escape from enemies results in a decrease in defence ability in plants from the invaded range. Two straightforward aspects of the EICA are examined: (1) if invasives have lost their enemies and their defence, they should be more negatively affected by their full natural pre-invasion herbivore spectrum than their native conspecifics; and (2) the genetic basis of evolutionary change in response to enemy release in the invasive range has not been taken sufficiently into account.

Methods

Lythrum salicaria (purple loosestrife) from several populations in its native (Europe) and invasive range (North America) was exposed to all above-ground herbivores in replicated natural populations in the native range. The experiment was performed both with plants raised from field-collected seeds as well as with offspring of these where maternal effects were removed.

Key Results

Absolute and relative leaf damage was higher for introduced than for native plants. Despite having smaller height growth rate, invasive plants attained a much larger final size than natives irrespective of damage, indicating large tolerance rather than effective defence. Origin effects on response to herbivory and growth were stronger in second-generation plants, suggesting that invasive potential through enemy release has a genetic basis.

Conclusions

The findings support two predictions of the EICA hypothesis – a genetically determined difference between native and invasive plants in plant vigour and response to enemies – and point to the importance of experiments that control for maternal effects and include the entire spectrum of native range enemies.     

喜旱莲子草对模拟全天增温的可塑性: 引入地和原产地种群的比较

植物可以通过关键功能性状的表型可塑性来适应气候变暖背景下环境温度的增加。表型可塑性增强进化假说(evolution of increased phenotypic plasticity hypothesis)认为外来植物在引入地进化出了更强的表型可塑性。以往对该假说的验证多集中于外来植物对光照、水分、养分、邻体以及天敌等的可塑性进化, 而对增温条件下植物生长和功能性状可塑性进化的研究相对较少。仅有的几项研究多集中在温带地区, 且多集中于研究植物生长相关的性状, 而对植物的抗性和草食作用对增温的响应的关注相对较少。本研究采用同质园实验比较了喜旱莲子草(Alternanthera philoxeroides)引入地(中国)和原产地(阿根廷)各8个种群的生物量、功能性状和草食作用在热带地区(广州市增城区)对模拟全天增温2℃的响应差异。结果表明: (1)模拟全天增温显著降低了喜旱莲子草总生物量(-7.8%)、贮藏根生物量(-12.8%)、分枝强度(-11.6%)和茎端取食率(-34.4%)。(2)模拟全天增温造成的引入地种群总生物量降低幅度大于原产地种群; 模拟全天增温使引入地种群的比茎长和茎端取食率降低, 而原产地种群则相反。(3)无论是否模拟全天增温, 引入地种群的贮藏根生物量(+31.5%)、分枝强度(+38.5%)、比茎长(+30.2%)、根冠比(+24.5%)和比叶面积(+20.0%)均高于原产地种群, 茎端取食率则低于原产地种群(-35.8%)。这些结果表明, 热带地区全天增温2℃对喜旱莲子草是一种胁迫; 引入地种群的生物量对模拟全天增温2℃的响应更强, 而其株形相关性状(比茎长)和草食作用(茎端取食率)对模拟全天增温的可塑性方向与原产地种群相反。由于引入地种群在热带地区模拟全天增温条件下生物量的下降和草食作用的增加明显高于原产地种群, 因此在未来全球气候变暖的背景下, 热带地区温度升高可能不利于喜旱莲子草种群多度的增加。     

Effects of generalist herbivory on resistance and resource allocation by the invasive plant,Phytolacca americana

Successful invasions by exotic plants are often attributed to a loss of co‐evolved specialists and a re‐allocation of resources from defense to growth and reproduction. However, invasive plants are rarely completely released from insect herbivory because they are frequently attacked by generalists in their introduced ranges. The novel generalist community may also affect the invasive plant's defensive strategies and resource allocation. Here, we tested this hypothesis using American pokeweed (Phytolacca americana L.), a species that has become invasive in China, which is native to North America. We examined resistance, tolerance, growth and reproduction of plant populations from both China and the USA when plants were exposed to natural generalist herbivores in China. We found that leaf damage was greater for invasive populations than for native populations, indicating that plants from invasive ranges had lower resistance to herbivory than those from native ranges. A regression of the percentage of leaf damage against mass showed that there was no significant difference in tolerance between invasive and native populations, even though the shoot, root, fruit and total mass were larger for invasive populations than for native populations. These results suggest that generalist herbivores are important drivers mediating the defensive strategies and resource allocation of the invasive American pokeweed.     

No impact of a native beetle on exotic plant performance and competitive ability due to plant compensation

The novel associations between invasive plants and their natural enemies in the introduced range have recently received increasing attention; however, the effects of novel enemies on exotic plant performance and competition with native species remain poorly explored. Here, we tested the impact of herbivory by a native beetle, Cassida piperata, on the performance of the exotic species Alternanthera philoxeroides and competition with a native congener, Alternanthera sessilis, using common garden experiments in central China. We found A. philoxeroides was able to fully compensate for intense herbivory by C. piperata. Herbivory by C. piperata that released at the average density in this region had no impact on competition between the native and exotic plant species. Our results indicate that herbivory by novel enemies may not reduce exotic plant performance due to plant compensation. However, high tolerance to herbivory may not confer a competitive advantage for exotic species compared to less tolerant native competitors if the herbivore damage is below a certain threshold. Thus, it is necessary to assess the impact of novel enemies on exotic plant performance and competition with native plants along gradients of insect densities. This may lead to a better understanding of how best to exploit the role of native herbivores in facilitating or slowing plant invasions.     

History of exposure to herbivores increases the compensatory ability of an invasive plant

Release from natural enemies is frequently cited as an important factor contributing to plant invasions. But such effects are likely to be temporary—native herbivores can form new plant-herbivore associations and co-evolved insects might reach the new range. While the potential effects of the initial enemy release have been well studied, the consequences of any resumption of herbivory are poorly understood. Alternanthera philoxeroides is one of the most widespread invasive plants in China and is attacked both by a specialist herbivore introduced from the native range, Agasicles hygrophila, and a native beetle Cassida piperata Hope which has formed a new association. However, these insects are not found throughout the invaded range. To test the effect of the history of population exposure to herbivory on compensatory ability, plants were cultured from 14 populations around China that differed in whether A. hygrophila or C. piperata were present. Treatment plants were exposed to herbivory by A. hygrophila for a week until 50% of the leaf area was defoliated, then grown for 80 days. Plants from populations with prior exposure to herbivory (of any kind) accumulated more root mass than populations without prior exposure, indicating that prior exposure to insects can stimulate plant compensation to herbivory. We would recommend that potential changes in plant tolerance in response to prior exposure to herbivory are considered in invasive plant management plans that employ bio-control agents.     

Introduced plants of the invasive Solidago gigantea (Asteraceae) are larger and grow denser than conspecifics in the native range

Introduced plant species that became successful invaders appear often more vigorous and taller than their conspecifics in the native range. Reasons postulated to explain this better performance in the introduced range include more favourable environmental conditions and release from natural enemies and pathogens. According to the Evolution of Increased Competitive Ability hypothesis (EICA hypothesis) there is a trade‐off between investment into defence against herbivores and pathogens, and investment into a stronger competitive ability. In this study, we conducted field surveys to investigate whether populations of the invasive perennial Solidago gigantea Ait (Asteraceae) differ with respect to growth and size in the native and introduced range, respectively. We assessed size and morphological variation of 46 populations in the native North American range and 45 populations in the introduced European range. Despite considerable variation between populations within continents, there were pronounced differences between continents. The average population size, density and total plant biomass were larger in European than in American populations. Climatic differences and latitude explained only a small proportion of the total variation between the two continents. The results show that introduced plants can be very distinct in their growth form and size from conspecifics in the native range. The apparently better performance of this invasive species in Europe may be the result of changed selection pressures, as implied by the EICA hypothesis.     

High chemical diversity of a plant species is accompanied by increased chemical defence in invasive populations

Mechanisms contributing to the invasive success of plants are still only partly understood. A main assumption is that an escape from specialized enemies in introduced ranges allows a reduction of chemical defences resulting in an increase in growth and reproduction and thus increased competitive ability of introduced plants. Not only variation in concentration but also variation in composition of chemical compounds between individuals may be a key advantage for plants introduced to novel areas impeding adaptation of different plant antagonists. To investigate quantitative and qualitative variation of putative defence compounds and investment of resources in growth, we conducted a common garden experiment in the native range with seeds of 13 native and 9 introduced populations of Tanacetum vulgare, an aromatic plant forming different chemotypes. After 3.5 months, plants of introduced populations had similar biomass but more stems and higher concentrations of volatile secondary compounds (mainly terpenes) than plants of native populations. Both native and invasive T. vulgare populations exhibited high chemotypic variation with nine chemotypes occurring on both continents, whereas several were found exclusively either in plants originating from the native (n = 10) or invasive (n = 2) range. Due to the known negative effects of many mono- and sesquiterpenes on various organisms, we propose that high concentrations of these secondary compounds accompanied by high chemotypic diversity may facilitate the invasion success of a plant species.     

Enemy release at range edges: do invasive species escape their herbivores as they expand into new areas?

Aims We test the hypothesis that invasive plant species at their range edges experience lower herbivory and allocate less to defense at the edge of an expanding range edge than from more central parts of their distribution, during secondary invasion in a new range. Invasive plants are often able to spread rapidly through new areas. The success of invasive species in new ranges is frequently attributed to enemy release in these new areas and associated evolutionary changes minimizing allocation to defense in favor of growth and reproduction. Enemy release could also explain rapid advances of invasive species upon arriving in new habitats. If invasive species accumulate enemies over time in a new location, then these species may experience a release from their enemies at expanding range fronts. Enemy release at these range fronts may accelerate range expansion.Methods We used populations of four woody invasive species within the invaded range, and four native control species. We quantified leaf herbivory and leaf physical defense traits at both range central and range edge locations, over two 1-month sampling periods, sampled 7 months apart.Important findings Herbivory at the range edge did not differ to the range center but patterns were not consistent across species. There was a trend for lower herbivory at the range edge for Lantana camara, which was reflected in lower leaf toughness. Overall, leaf toughness was greater at the range edge location across invasive and control species. Physical defenses were different among range locations in a few species, though most species show the same trend, suggesting higher herbivory pressures at the range edge location or differences may be due to climatic factors. Leaves of L. camara were significantly less tough at range edges, suggesting that some species can potentially escape their enemies at range edges. However, our results overall do not support the hypothesis that plants at the edge of their ranges experience reduced impact from their enemies.     

Reduced tolerance to simulated herbivory on clonal organs in alien genotypes: a multi-species experiment with native and introduced origins

Clonal growth may increase the likelihood for alien plants becoming invasive, as it is an efficient foraging and spatial exploration strategy. Here, we investigated the effect of artificial herbivory on organs of clonal growth and its potential to drive post-introduction evolutionary change. Based on the assumption that tolerance traits are costly and that clonal alien species may benefit from investing freed resources into growth, fecundity or nutrient acquisition, we tested the hypothesis of lower tolerance to herbivory on organs of clonal growth in alien plants. In a common-garden experiment we studied divergence in plants from native German and alien New Zealand populations of six species with different clonal growth forms. A nutrient treatment testing the plant’s acquisition abilities, was combined with artificial herbivory on clonal organs. We investigated origin-dependent differences in sexual reproduction, plant growth and the production of clonal organs. For aboveground and clonal organ biomass, alien plants showed lower tolerance to artificial herbivory on clonal organs than native plants. In the combined herbivory and nutrient treatment, alien plants of four species grew fewer clonal organs when compared to the nutrient treatment alone. Alien plants of the other two species produced more clonal organs, regardless of treatment. All species revealed significant differences in flower production between origins, with five of them producing more flowers on alien than on native plants. The results support the hypothesis that a release of herbivory on clonal organs has lead to subtle evolutionary changes in tolerance of alien plants and to a species-dependent increase in plant vigour, clonal growth and/or sexual reproduction that may enhance their invasive success.     

When do herbivores affect plant invasion? Evidence for the natural enemies and biotic resistance hypotheses

Two venerable hypotheses, widely cited as explanations for either the success or failure of introduced species in recipient communities, are the natural enemies hypothesis and the biotic resistance hypothesis. The natural enemies hypothesis posits that introduced organisms spread rapidly because they are liberated from their co‐evolved predators, pathogens and herbivores. The biotic resistance hypothesis asserts that introduced species often fail to invade communities because strong biotic interactions with native species hinder their establishment and spread. We reviewed the evidence for both of these hypotheses as they relate to the importance of non‐domesticated herbivores in affecting the success or failure of plant invasion.
To evaluate the natural enemies hypothesis, one must determine how commonly native herbivores have population‐level impacts on native plants. If native herbivores seldom limit native plant abundance, then there is little reason to think that introduced plants benefit from escape from these enemies. Studies of native herbivore‐native plant interactions reveal that plant life‐history greatly mediates the strength with which specialist herbivores suppress plant abundance. Relatively short‐lived plants that rely on current seed production for regeneration are most vulnerable to herbivory that reduces seed production. As such, these plants may gain the greatest advantage from escaping their specialist enemies in recipient communities. In contrast, native plants that are long lived or that possess long‐lived seedbanks may not be kept “in check” by native herbivores. For these species, escape from native enemies may have little to do with their success as exotics; they are abundant both where they are native and introduced.
Evidence for native herbivores providing biotic resistance to invasion by exotics is conflicting. Our review reveals that: 1) introduced plants can attract a diverse assemblage of native herbivores and that 2) native herbivores can reduce introduced plant growth, seed set and survival. However, the generality of these impacts is unclear, and evidence that herbivory actually limits or reduces introduced plant spread is scarce. The degree to which native herbivores provide biotic resistance to either exotic plant establishment or spread may be greatly determined by their functional and numerical responses to exotic plants, which we know little about. Generalist herbivores, through their direct effects on seed dispersal and their indirect effects in altering the outcome of native–non‐native plant competitive interactions, may have more of a facilitative than negative effect on exotic plant abundance.     

Loss of specificity: native but not invasive populations of Triadica sebifera vary in tolerance to different herbivores

During introduction, invasive plants can be released from specialist herbivores, but may retain generalist herbivores and encounter novel enemies. For fast-growing invasive plants, tolerance of herbivory via compensatory regrowth may be an important defense against generalist herbivory, but it is unclear whether tolerance responses are specifically induced by different herbivores and whether specificity differs among native and invasive plant populations. We conducted a greenhouse experiment to examine the variation among native and invasive populations of Chinese tallow tree, Triadica sebifera, in their specificity of tolerance responses to herbivores by exposing plants to herbivory from either one of two generalist caterpillars occurring in the introduced range of Triadica. Simultaneously, we measured the specificity of another defensive trait, extrafloral nectar (EFN) production, to detect potential tradeoffs between resistance and tolerance of herbivores. Invasive populations had higher aboveground biomass tolerance than native populations, and responded non-specifically to either herbivore, while native populations had significantly different and specific aboveground biomass responses to the two herbivores. Both caterpillar species similarly induced EFN in native and invasive populations. Plant tolerance and EFN were positively correlated or had no relationship and biomass in control and herbivore-damaged plants was positively correlated, suggesting little costs of tolerance. Relationships among these vegetative traits depended on herbivore type, suggesting that some defense traits may have positive associations with growth-related processes that are differently induced by herbivores. Importantly, loss of specificity in invasive populations indicates subtle evolutionary changes in defenses in invasive plants that may relate to and enhance their invasive success.     

Increased resistance to generalist herbivores in invasive populations of the California poppy (Eschscholzia californica)

Escape from enemies in the native range is often assumed to contribute to the successful invasion of exotic species. Following optimal defence theory, which assumes a trade‐off between herbivore resistance and plant growth, some have predicted that the success of invasive species could be the result of the evolution of lower resistance to herbivores and increased allocation of resources to growth and reproduction. Lack of evidence for ubiquitous costs of producing plant toxins, and the recognition that invasive species may escape specialist, but not generalist enemies, has led to a new prediction: invasive species may escape ecological trade‐offs associated with specialist herbivores, and evolve increased, rather than decreased, production of defensive compounds that are effective at deterring generalist herbivores in the introduced range. We tested the performance of two generalist lepidopteran herbivores, Trichoplusia ni and Orgyia vetusta, when raised on diets of native and invasive populations of the California poppy, Eschscholzia californica. Pupae of T. ni were significantly larger when reared on native populations. Similarly, caterpillars of O. vetusta performed significantly better when raised on native populations, indicating that invasive populations of the California poppy are more resistant to herbivores than native populations. The chance of successful establishment of some non‐indigenous plant species may be increased by retaining resistance to generalist herbivores, and in some cases, invasive species may be able to escape ecological trade‐offs in their new range and evolve, as we observed, even greater resistance to generalist herbivores than native plants.