Evolution Of Increased Competitive Ability Research Articles

Non-MHC immunity genes do not affect parasite load in European invasive populations of common raccoon

Understanding the evolutionary mechanisms behind invasion success enables predicting which alien species and populations are the most predisposed to become invasive. Parasites may mediate the success of biological invasions through their effect on host fitness. The evolution of increased competitive ability (EICA) hypothesis assumes that escape from parasites during the invasion process allows introduced species to decrease investment in immunity and allocate resources to dispersal and reproduction. Consequently, the selective pressure of parasites on host species in the invasive range should be relaxed. We used the case of the raccoon Procyon lotor invasion in Europe to investigate the effect of gastrointestinal pathogen pressure on non-MHC immune genetic diversity of newly established invasive populations. Despite distinct differences in parasite prevalence between analysed populations, we detected only marginal associations between two analysed SNPs and infection intensity. We argue that the differences in parasite prevalence are better explained by detected earlier associations with specific MHC-DRB alleles. While the escape from native parasites seems to allow decreased investment in overall immunity, which relaxes selective pressure imposed on immune genes, a wide range of MHC variants maintained in the invasive range may protect from newly encountered parasites.

Flowers of Impatiens glandulifera as hubs for both pollinators and pathogens

Flower infestation by pathogens may influence pollination effectiveness. At the same time, by sharing infested flowers, pollinators increase transmission of pathogens. In the presented study we identified fungi that colonised flowers of the invasive alien Himalayan balsam Impatiens glandulifera, one of the most nectar rewarding plants in Europe, as well as its pollinators. We determined factors (e.g., plant size, length of flower lower sepal and the width of its entry, air temperature and sun illuminance) that affect pathogen species presence and pollinators numbers. The study was conducted in three regions in Poland differing in time from the I. glandulifera invasion onset. It allowed embedding our results in the context of the evolution of increased competitive ability (EICA) hypothesis. With reference to this hypothesis we tested whether I. glandulifera from the two younger populations are more frequently pollinated than individuals from the old one, which may be a result of the higher infection prevalence in the flowers of individuals from the latter population. Harmful primary pathogens of I. glandulifera (e.g., Botrytis cinerea and Fusarium graminearum) were identified from its flowers. Although the knowledge of the impact of the recorded pathogen species on the pollinators that transmit them is still limited, these pathogens are known to cause devastating diseases of native plant species and to incur significant economic losses in crops. Therefore, the facilitation of their transmission by I. glandulifera in the invaded communities may pose a serious threat both to native biodiversity and nearby crop production. We did not find support for the EICA hypothesis that flower release from pathogens may increase the pollinator’s activity. Bombus hortorum was the most frequent visitor in the youngest surveyed population, while B. pascuorum was most frequent in the two others. So far the dominance of B. hortorum as a pollinator of I. glandulifera has not been recorded. A possible explanation is that flowers in the youngest population, with significantly wider entries than in the two older ones, were more accessible for this large bumblebee. We suggest that the shifts in flower dimensions may result from the evolutionary processes and/or phenotypic plasticity; however, this suggestion needs to be confirmed in further studies. At the same time, it can be expected that exceptionally frequent visits of B. hortorum in flowers of I. glandulifera in the youngest population may contribute to increasing transmission rate of pathogen species to the new native host plants that are particularly associated with this pollinator.

Shifts in immune responses of an invasive alien species: A test of the evolution of increased competitive ability hypothesis using American Eastern gray squirrels in Italy

Based on the Evolution of Increased Competitive Ability (EICA) hypothesis, a reduced investment in immunity, consequent to parasite loss, could partly explain the success of invasive alien species. We investigated variation in parasite load and immune responses of alien Eastern gray squirrels (Sciurus carolinensis) along the invasion wave of an expanding population. We first verified by fecal analyses that 1) parasite abundance decreased moving from the core towards the invasion front. Next, we used multiple measures of immunity to investigate whether, in response to the lower parasite pressure, individuals at the invasion front 2) dampened their costly inflammatory response, and 3) increased their investment in less expensive acquired immunity. We first explored variation in hematological variables related either to the inflammatory or the acquired response. On a subset of individuals, we carried out ex vivo cell cultures to analyse the basal expression of MHC class II genes and the expression of TNF-α genes in response to an immune challenge. Platelet counts and TNF-α expression suggested higher inflammation in individuals living at the invasion core, whereas parameters associated with an acquired response (lymphocyte counts and MHC II expression by spleen cells), conversely, were higher in squirrels at the front. Overall, our results suggest a shift between different immune strategies along the invasion wave, supporting a reduced investment in costly inflammatory responses and an increased investment in acquired immunity in individuals at the expanding edge of the range, which are subjected to high selective pressures for dispersal and reproduction.

The EICA is dead? Long live the EICA!

An important hypothesis for how plants respond to introduction to new ranges is the evolution of increased competitive ability (EICA). EICA predicts that biogeographical release from natural enemies initiates a trade-off in which exotic species in non-native ranges become larger and more competitive, but invest less in consumer defences, relative to populations in native ranges. This trade-off is exceptionally complex because detecting concomitant biogeographical shifts in competitive ability and consumer defence depends upon which traits are targeted, how competition is measured, the defence chemicals quantified, whether defence chemicals do more than defend, whether 'herbivory' is artificial or natural, and where consumers fall on the generalist-specialist spectrum. Previous meta-analyses have successfully identified patterns but have yet to fully disentangle this complexity. We used meta-analysis to reevaluate traditional metrics used to test EICA theory and then expanded on these metrics by partitioning competitive effect and competitive tolerance measures and testing Leaf-Specific Mass in detail as a response trait. Unlike previous syntheses, our meta-analyses detected evidence consistent with the classic trade-off inherent to EICA. Plants from non-native ranges imposed greater competitive effects than plants from native ranges and were less quantitatively defended than plants from native ranges. Our results for defence were not based on complex leaf chemistry, but instead were estimated from tannins, toughness traits and primarily Leaf-Specific Mass. Species specificity occurred but did not influence the general patterns. As for all evidence for EICA-like trade-offs, we do not know if the biogeographical differences we found were caused by trade-offs per se, but they are consistent with predictions derived from the overarching hypothesis. Underestimating physical leaf structure may have contributed to two decades of tepid perspectives on the trade-offs fundamental to EICA.

Evolution of increased competitive ability may explain dominance of introduced species in ruderal communities

AbstractThe evolution of increased competitive ability (EICA) hypothesis encapsulates the importance of evolution and ecology for biological invasions. According to this proposition, leaving specialist herbivores at home frees introduced plant species from investing limited resources in defense to instead use those resources for growth, selecting for individuals with reduced defense, enhanced growth, and, consequently, increased competitive ability. We took a multispecies approach, including ancestral and non‐native populations of seven weeds, as well as seven coexisting local weeds, to explore all three predictions (i.e., lower defense, greater growth, and better ability to compete in non‐native than ancestral populations), the generality as an invasion mechanism for a given system, and community‐level consequences of EICA. We assessed plant defenses by conducting herbivory trials with a generalist herbivore. Therefore, finding that non‐native populations are better defended than ancestral populations would lend support to the shifting defense (SD) hypothesis, an extension of EICA that incorporates the observation that introduced species escape specialists, but encounter generalists. We also manipulated water additions to evaluate how resource availability influences competition in the context of EICA and plant plasticity in our semiarid system. We found that non‐native populations of one study species, Centaurea solstitialis, were better defended, grew faster, and exerted stronger suppression on locals than ancestral populations, offering support to EICA through the SD hypothesis. The other species also displayed variation in trait attributes between ancestral and non‐native populations, but they did not fully comply with the three predictions of EICA. Notably, differences between those populations generally favored the non‐natives. Moreover, non‐native populations were, overall, superior at suppressing locals relative to ancestral populations under low water conditions. There were no differences in plasticity among all three groups. These results suggest that evolutionary change between ancestral and non‐native populations is widespread and could have facilitated invasion in our system. Additionally, although trading growth for shifted defense does not seem to be the main operational path for evolutionary change, it may explain the dominance of some introduced species in ruderal communities. Because introduced species dominate communities in disturbed environments around the world, our results are likely generalizable to other systems.

Differences in leaf traits of Spartina alterniflora between native and invaded habitats: Implication for evolution of alien species competitive ability increase

The enemy release and pre-adaptation of alien species often drive their contemporary evolution across latitudinal gradients within its invasive and native ranges. Leaf functional traits constitute an important part of plant functional traits, reflecting their resource acquisition and usage. Thus, comparing the leaf functional traits (growth and defense traits) and latitude patterns of invasive and native plants could provide leading indicators of contemporary evolutionary and adaptative abilities that explain the success of invasive plants. Spartina alterniflora, a coastal wetland invasive plant native to the Atlantic coast and the Gulf of Mexico, has invaded China along the coast wetland (19–40 °N), similar to the United States (27–43 °N). To understand the leaf trait differences and variation across latitudes among continents of S. alterniflora within its invasive and native ranges and determine which factors drive leaf trait variation across latitudes, we investigated the relationship between leaf traits, related to growth (leaf area and specific leaf area), and defensive traits (leaf thickness and leaf toughness) at 10 invaded locations and 15 native locations. We identified correlations between leaf traits and abiotic conditions such as annual mean temperature, tide range, and soil salinity. Compared with native S. alterniflora, Chinese costal S. alterniflora has a larger leaf area and specific leaf area, less leaf thickness, and lower leaf toughness, suggesting that biosynthetic resources are transferred from defense to competitive traits, and indirectly supporting the Evolution of Increased Competitive Ability (EICA) hypothesis. The leaf area, specific leaf area, thickness, and toughness of S. alterniflora from both ranges had similar quadratic latitudinal relationships. Some of these relationships were stronger in the invasive range than the native range, indicating that the leaf traits of invasive S. alterniflora experienced pre-adaptation and had high phenotypic plasticity. Tide range, soil salinity, and especially temperature formed latitudinal gradients in different geographic regions. By analyzing indicators of leaf functional traits across geographical ranges, this study shows that the escape from natural enemies plays an important role in the successful invasion of S. alterniflora, while pre-adaptation and high phenotypic plasticity promote its rapid expansion along the latitudinal gradients.

Environmental Degradation by Invasive Alien Plants in the Anthropocene: Challenges and Prospects for Sustainable Restoration

Biodiversity, soil, air, and water are the vital life-supporting systems of this planet Earth. However, the deliberate and accidental introduction of invasive alien plants (IAPs) in the Anthropocene majorly due to the global international trade perturbed the homeostasis of our biosphere. IAPs are considered as one of the major drivers of biodiversity loss and ecosystem degradation. The pervasive threats of IAPs to environmental sustainability and biosecurity are further exacerbated under the COVID-19 pandemic. The environmental disturbances resulting from IAPs can be attributed to several mechanisms/hypothesis (e.g., novel weapon (NW), enemy release (ER), and evolution of increased competitive ability (EICA), efficient reproductive attributes, and phenotypic plasticity, etc.) deployed by IAPs. Nevertheless, the interrelationship of IAPs with environmental degradation and restoration remain elusive especially in terms of ecological sustainability. Moreover, there is a dearth of studies which empirically assess the synergies of IAPs spread with other anthropogenic disturbances such as climate and land-use change. In this context, the present review is aimed to depict the impacts of IAPs on environment and also to assess their role as drivers of ecosystem degradation. The restoration prospects targeted to revitalize the associated abiotic (soil and water) and biotic environment (biodiversity) are also discussed in detail. Furthermore, the effects of IAPs on socio-economy, livelihood, and plant-soil microbe interactions are emphasized. On the other hand, the ecosystem services of IAPs such as associated bioresource co-benefits (e.g., bioenergy, phytoremediation, biopolymers, and ethnomedicines) can also be vital in sustainable management prospects. Nevertheless, IAPs-ecological restoration interrelationship needs long-term pragmatic evaluation in terms of ecological economics and ecosystem resilience. The incorporation of ‘hybrid technologies’, integrating modern scientific information (e.g., ‘biorefinery’: conversion of IAPs feedstock to produce bioenergy/biopolymers) with traditional ecological knowledge (TEK) can safeguard the environmental sustainability in the Anthropocene. Importantly, the management of IAPs in concert with circular economy principles can remarkably help achieving the target of UN Sustainable Development Goals and UN-Decade on Ecosystem Restoration.

Two sides of the same coin: Does alien Impatiens balfourii fall into an ecological trap after releasing from enemies?

The evolution of increased competitive ability (EICA) hypothesis assumes that recently introduced populations of alien species are under low pressure from enemies, which allows them to reduce investment into defence and re-allocate the saved resources into the improvement of characteristics related to competitive abilities. We performed EICA tests to check if the low dispersal potential of Impatiens balfourii in Europe results from a limited evolution of its capability to release from enemies or from the low phenotypic plasticity of this trait.We tested habitat preferences of the species and compared the levels of enemy pressure in different habitats. This allowed us to determine if the tendency to occur along roadsides is manifested at the European scale in this species and if this preference is determined by a lower pressure of enemies in that habitat. It cannot be ruled out that occurrence along roads can be maladaptive for the species, and this habitat can potentially constitute an ecological trap due to frequent mowing.The study was carried out in six European populations differing in age and in five types of habitat. Enemy pressure was estimated by assessing leaf damage in adult plants and fungal pathogen load at the seed stage. We tested if the saved reserves are invested in the size and/or fertility of adult plants or in the seed quality. Activity of pollinators in different populations and habitats was also assessed.The majority of the results for adult plants indicate that the younger populations of I. balfourii release from enemy pressure better than the older ones. However, we found only weak support that the reserves previously spent on defence against natural enemies could be re-allocated into larger size or increased fertility. Only in the youngest population from Andorra was it largely confirmed. However, at seed stage, we found no support for the EICA assumptions in any of the populations. Thus, in general, the results of our study provide a fairly weak support for the EICA hypothesis.In comparisons between the habitats, we found that in five of the six populations, the species preferred roadsides, which was probably determined by the lower enemy pressure there. However, it cannot be excluded that this preference can be maladaptive and that this habitat can potentially constitute an ecological trap due to frequent mowing. Notably, the species was also abundant along streams, where the plants are less disturbed by human activities. This finding was particularly pronounced at the invasion forefront in Andorra, where the species was completely absent from roadsides. Therefore, we assume that unless control measures are undertaken, streamsides may be more important for the future invasion of I. balfourii than roadsides in that area.The ornamental value of I. balfourii makes it an obvious candidate to replace its banned invasive counterpart I. glandulifera in trade. The increase in popularity in trade and cultivation would inevitably lead to an increase in propagule pressure, which could be crucial for increasing the rate of invasion of I. balfourii on a large scale in Europe.

Postintroduction evolution contributes to the successful invasion of Chromolaena odorata.

The evolution of increased competitive ability (EICA) hypothesis states that, when introduced in a novel habitat, invasive species may reallocate resources from costly quantitative defense mechanisms against enemies to dispersal and reproduction; meanwhile, the refinement of EICA suggests that concentrations of toxins used for qualitative defense against generalist herbivores may increase. Previous studies considered that only few genotypes were introduced to the new range, whereas most studies to test the EICA (or the refinement of EICA) hypotheses did not consider founder effects.In this study, genetic and phenotypic data of Chromolaena odorata populations sampled across native and introduced ranges were combined to investigate the role of postintroduction evolution in the successful invasion of C. odorata.Compared with native populations, the introduced populations exhibited lower levels of genetic diversity. Moreover, different founder effects events were interpreted as the main cause of the genetic structure observed in introduced ranges. Three Florida, two Trinidad, and two Puerto Rico populations may have been the sources of the invasive C. odorata in Asia.When in free of competition conditions, C. odorata plants from introduced ranges perform better than those from native ranges at high nutrient supply but not at low nutrient level. The differences in performance due to competition were significantly greater for C. odorata plants from the native range than those from the introduced range at both nutrient levels. Moreover, the differences in performance by competition were significantly greater for putative source populations than for invasive populations.Quantities of three types of secondary compounds in leaves of invasive C. odorata populations were significantly higher than those in putative source populations. These results provide more accurate evidence that the competitive ability of the introduced C. odorata is increased with postintroduction evolution.

A test of the evolution of increased competitive ability in two invaded regions

Non-native plant species invasions can have significant ecological and economic impacts. Finding patterns that predict and explain the success of non-native species has thus been an important focus in invasion ecology. The evolution of increased competitive ability (EICA) hypothesis has been a frequently used framework to understand invasion success. Evolution of increased competitive ability predicts that (1) non-native populations will escape from coevolved specialist herbivores that were present within the native range and this release from specialist herbivores should result in relaxed selection pressure on specialist-related defense traits, (2) there will be a trade-off between allocation of resources for resistance against specialist herbivores and allocation to traits related to competitive ability, and (3) this shift will allow more allocation to competitive ability traits. We tested the predictions of EICA in the model plant Mimulus guttatus, a native of western North America (WNA). We compared how well the predictions of EICA fit patterns in two non-native regions, the United Kingdom (UK), an older more successful invasion, and eastern North America (ENA), a younger less successful invasion. We completed extensive herbivore surveys and grew plants derived from multiple populations in each region in a common greenhouse environment to test adherence to the predictions of EICA. We found evidence of specialist herbivore escape in the UK, but not the ENA plants. Compared to native plants the UK plants had lower levels of resistance traits, were taller, and produced larger and more flowers, while the ENA plants had mostly equivalent traits to the WNA plants. Plants from the UK conformed to the predictions of EICA more closely than those from ENA. The UK invasion is an older, more successful invasion, suggesting that support for EICA predictions may be highest in more successful invasions.

Evolution of Increased Photosynthetic Capacity and Its Underlying Traits in Invasive Jacobaea vulgaris.

The evolution of increased competitive ability (EICA) hypothesis and the shifting defense hypothesis (SDH) predict that evolutionary changes occur in a suite of traits related to defense and growth in invasive plant species as result of the absence of specialist herbivores. We tested how this suite of traits changed due to the absence of specialist herbivores in multiple invasive regions that differ in climatic conditions with native and invasive Jacobaea vulgaris in a controlled environment. We hypothesized that invasive J. vulgaris in all invasive regions have i) a higher plant growth and underlying traits, such as photosynthetic capacity, ii) lower regrowth-related traits, such as carbohydrate storage, and iii) an increased plant qualitative defense, such as pyrrolizidine alkaloids (PAs). Our results show that invasive J. vulgaris genotypes have evolved a higher photosynthetic rate and total PA concentration but a lower investment in root carbohydrates, which supports the SDH hypothesis. All the traits changed consistently and significantly in the same direction in all four invasive regions, indicative of a parallel evolution. Climatic and soil variables did differ between ranges but explained only a very small part of the variation in trait values. The latter suggests that climate and soil changes were not the main selective forces on these traits.

Evidence for evolution of increased competitive ability for invasive Centaurea solstitialis, but not for naturalized C. calcitrapa

Congeneric species with the same native and non-native ranges, but exhibiting different invasiveness, provide opportunities to assess the relative importance of factors contributing to successful invasions. For example, comparing the competitive ability of such congeners against other species from the native and non-native regions of invasive species can provide insight into the role of evolutionary experience with different competitors. We selected two congeneric Centaurea species with overlapping native and non-native ranges but with strikingly different invasive success, Centaurea solstitialis and C. calcitrapa, and conducted experiments with populations from each region using a suite of different native grass species from each region. When grown with Spanish grass species, competitive responses were 32% stronger for C. calcitrapa and 30% stronger for C. solstitialis, than when in competition with grasses native to California. Centaurea solstitialis from California had 66% higher competitive effects on grasses, all species considered together, than Spanish C. solstitialis; whereas the competitive effects of C. calcitrapa on grasses were not significantly different between C. calcitrapa ranges. Our results are consistent with the prediction that the evolution of increased competitive ability (EICA) might contribute to the devastating success of C. solstitialis in California, and that its absence might contribute to the modest naturalization of the non-invasive C. calcitrapa. Escape from the strong competitive effects and responses of grasses from Spain may also contribute to invasive success in California. Our results suggest that studies of EICA should be based on competitive tests, since biomass alone is not always a good predictor of competitive ability.

The evolution of chemical defenses along invasion routes: Harmonia axyridis Pallas (Coccinellidae: Coleoptera) as a case study.

The evolution of increased competitive ability (EICA) hypothesis (Blossey & Nötzold, 1995) postulates that escaping from coevolved enemies increases invaders fitness by energy reallocation from defenses and immunity to growth and reproduction. In this context, we evaluated the evidence of evolutionary change in invasive populations of Harmonia axyridis Pallas (Coccinellidae: Coleoptera). We measured egg defenses—cocktail of hydrocarbons on the egg's surface flagging egg toxicity and the concentration of the main alkaloid harmonine—in individuals from three populations along the invasion route (Japan: native, United States: introduced more than 30 years ago, South Africa: introduced in the early 2000s) in a common garden experiment. Our results support the EICA hypothesis: We found changes along the invasion route in the profiles of the hydrocarbons coating the eggs’ surface and a decrease in the concentration of harmonine in eggs from the most recent invasive South African population compared to the long established in the United States and the native Japanese ones.

Evolution of increased intraspecific competitive ability following introduction: The importance of relatedness among genotypes

Abstract A long‐standing explanation for invasion success is that invasive plants could evolve to be more competitive following introduction. This evolution of increased competitive ability (EICA) hypothesis, however, has seldom been tested with regard to intraspecific competition. Given that plants can display different responses to related and unrelated conspecifics, the evolution of intraspecific competitive ability might be specific to genotypes of different relatedness. Here, we grew five native (South American) and five introduced (North American) genotypes of the clonal herbaceous invasive plant Alternanthera philoxeroides alone, with above‐ground competition from kin (the same genotype) or from one of two types of strangers (another genotype from the same range or another genotype from the other range). When grown alone, introduced and native genotypes produced similar total biomass and storage‐root biomass. However, in response to intraspecific competition, introduced genotypes showed increases in total biomass and stem length, and a decrease in specific stem length, whereas native genotypes showed the opposite pattern. When grown with kin instead of strangers, introduced genotypes showed an increase in branch number, whereas native genotypes showed the opposite. Synthesis. Our study provided evidence for evolution of increased intraspecific competitive ability in an invasive plant. We also found, for the first time, that the interactions among kin were likely to shift from competition towards facilitation following introduction.

A meta-analysis of the evolution of increased competitive ability hypothesis: genetic-based trait variation and herbivory resistance trade-offs

Non-native organisms are an abundant component of almost all global ecosystems. A prominent framework to explain the success of non-native plants is the evolution of increased competitive ability (EICA) hypothesis. EICA predicts that plants escape from co-evolved herbivores after introduction into a non-native habitat. Assuming limited resources, a relaxation in selection pressures for resistance traits against the co-evolved specialist herbivores allows plants to allocate increased resources to traits related to fitness and/or competitive ability. Despite the prominence of the EICA hypothesis in the literature, empirical evidence has been mixed. We conducted a meta-analysis on 30 studies that focused on genetic-based trait variation and the trade-off between resistance traits and fitness to assess support for the EICA hypothesis. We found general support for EICA across studies. Performance of herbivores was higher on non-native plant populations than on native populations of the same species. Fitness trait values were higher in non-native populations, relative to native, and we found evidence for trade-offs between herbivore performance and plant fitness traits. Support for EICA was strongest when we focused on direct measurements of herbivore performance, and weakest when we assessed resistance traits, highlighting the complex and often unknown relationship between resistance traits and particular herbivores in many plant–herbivore systems.

Breakdown of a geographic cline explains high performance of introduced populations of a weedy invader

Abstract What drives the evolution of increased growth and fecundity in plants introduced to a novel range is not well understood. We investigate between‐range differences in performance for Verbascum thapsus, a weedy invader known to grow larger in its introduced than native range. Specifically, we question whether adaptation to herbivory or climate best explains increased performance of introduced populations. We grew 14 native and 22 introduced populations of V. thapsus in two common garden locations: near Prague, Czech Republic (native range) and in Colorado, USA (introduced range). By removing herbivores from half of the plants within each garden we tested the prediction of the evolution of increased competitive ability (EICA) hypothesis: increased performance is driven by an evolutionary shift of resources away from defence against herbivory towards growth and reproduction. We then investigated whether genetically based clines in performance are expressed along climate gradients within both the native and introduced ranges. On average, seeds produce larger rosettes when collected from the introduced vs. native range. While this evolution of increased growth in introduced populations in part matches the prediction of EICA, climate, not herbivory, best explains this between‐range difference. Specifically, seeds collected from the native range produce smaller rosettes as the climate of origin becomes cooler and drier, while there is no cline in performance in rosettes grown from seed collected from the introduced range, which are large regardless of climate of origin. Thus, a climate‐based cline within the native range best explains lower average performance of native compared to introduced populations. Synthesis. The breakdown in a potentially adaptive cline emphasizes the need to more closely investigate the evolutionary processes that shape geographic structuring (or its absence) within the introduced range. In addition, EICA is not universally applicable to all invasion scenarios, and our findings underscore the importance of testing the underlying assumptions alongside the predictions of this hypothesis.

In the presence of specialist root and shoot herbivory, invasive‐range Brassica nigra populations have stronger competitive effects than native‐range populations

SUMMARY The evolution of increased competitive ability (EICA) hypothesis predicts that release from specialist herbivores enables invasive plants to evolve increased growth. The most powerful tests of EICA hypothesis are provided by approaches that simultaneously assess the effects of specialist herbivory and competitive interactions. However, such approaches are extremely rare, and hence how simultaneous release from root and shoot herbivory influence competitive ability of invasive plants remains little understood. Here, we tested whether invasive‐range Brassica nigra plants have evolved increased competitive ability, and whether expression of competitive ability depends on separate and simultaneous effects of specialist root and shoot herbivory. To do this, we grew B. nigra plants from eight invasive‐range and eight native‐range populations in the presence vs. absence of competition with a community of native plant species, and in the absence vs. presence of separate and simultaneous damage by a specialist root herbivore (Delia radicum) and a specialist shoot herbivore (Plutella xylostella). Brassica nigra performance was assessed by measuring biomass production and flowering of individual B. nigra plants. In partial support of the EICA hypothesis, invasive‐range B. nigra had greater flowering than native‐range conspecifics in the absence of competition. However, contrary to a prediction of the EICA hypothesis, invasive‐range B. nigra produced less above‐ground biomass than native‐range B. nigra in the absence of shoot herbivory and competition. Moreover, with simultaneous root and shoot herbivory, invasive‐range B. nigra suppressed a competing community more strongly than native‐range B. nigra did. Synthesis. Our results suggest that invasiveness may be driven by mechanisms other than increased individual size. Simultaneous root and shoot herbivory in the invasive range may enhance suppressive effects of introduced plant species that have not completely escaped herbivore damage in the introduced range.

Contemporary variations of immune responsiveness during range expansion of two invasive rodents in Senegal

Biological invasions provide unique opportunities for studying life history trait changes over contemporary time scales. As spatial spread may be related to changes in parasite communities, several hypotheses (such as the evolution of increased competitive ability (EICA) or EICA‐refined hypotheses) suggest immune changes in invasive species along invasion gradients. Although native hosts may be subject to similar changes in parasite selection pressures, their immune responses have been rarely investigated in invasion contexts. In this study, we evaluated immune variations for invasive house mice Mus musculus domesticus, invasive black rats Rattus rattus and native rodents Mastomys erythroleucus and Mastomys natalensis along well‐characterised invasion gradients in Senegal. We focused on antibody‐mediated (natural antibodies and complement) and inflammatory (haptoglobin) responses. One invasion route was considered for each invasive species, and environmental conditions were recorded. Natural‐antibody mediated responses increased between sites of long‐established invasion and recently invaded sites only in house mice. Both invasive species exhibited higher inflammatory responses at the invasion front than in sites of long‐established invasion. The immune responses of native species did not change with the presence of invasive species. These patterns of immune variations do not support the EICA and EICA refined hypotheses, and they rather suggest a higher risk of exposure to parasites on the invasion front. Altogether, these results provide a first basis to further assess the role of immune changes in invasion success.

Reduced inflammation in expanding populations of a neotropical bird species.

The loss of regulating agents such as parasites is among the most important changes in biotic interactions experienced by populations established in newly colonized areas. Under a relaxed parasite pressure, individuals investing less in costly immune mechanisms might experience a selective advantage and become successful colonizers as they re‐allocate resources to other fitness‐related traits. Accordingly, a refinement of the evolution of increased competitive ability (EICA) hypothesis proposed that immunity of invasive populations has evolved toward a reduced investment in innate immunity, the most costly component of immunity, and an increased humoral immunity that is less costly. Biogeographical approaches comparing populations between native and expansion ranges are particularly relevant in exploring this issue, but remain very scarce. We conducted a biogeographical comparison between populations of Spectacled Thrush (Turdus nudigenis) from the native area (South America) and from the expansion range (Caribbean islands). First, we compared haemosporidian prevalence and circulating haptoglobin (an acute‐phase protein produced during inflammation). Second, we challenged captive birds from both ranges with Escherichia coli lipopolysaccharides (LPS) and measured postchallenge haptoglobin production and body mass change. Birds from the expansion range showed lower haemosporidian prevalence and lower levels of haptoglobin than birds from the native range. In addition, the inflammation elicited by LPS injection and its associated cost in terms of body mass loss were lower in birds from the expansion range than in birds from the native range. In accordance with the enemy release hypothesis, our results suggest that range expansion is associated with a reduced infection risk. Our study also supports the hypothesis that individuals from newly established populations have evolved mechanisms to dampen the inflammatory response and are in accordance with one prediction of the refined EICA hypothesis, proposed to understand biological invasions.

Eco-immunology and bioinvasion: revisiting the evolution of increased competitive ability hypotheses.

Immunity is at the core of major theories related to invasion biology. Among them, the evolution of increased competitive ability (EICA) and EICA‐refined hypotheses have been used as a reference work. They postulate that the release from pathogens often experienced during invasion should favour a reallocation of resources from (costly) immune defences to beneficial life‐history traits associated with invasive potential. We review studies documenting immune changes during animal invasions. We describe the designs and approaches that have been applied and discuss some reasons that prevent drawing generalized conclusions regarding EICA hypotheses. We detail why a better assessment of invasion history and immune costs, including immunopathologies and parasite communities, could improve our understanding of the relationships between immunity and invasion success. Finally, we propose new perspectives to revisit the EICA hypotheses. We first emphasize the neutral and adaptive mechanisms involved in immune changes, as well as timing of the later. Such investigation will help decipher whether immune changes are a consequence of pre‐adaptation, or the result of postintroduction adaptations to invasion front conditions. We next bring attention to new avenues of research that remain unexplored, namely age‐dependent immunity and gut microbiota, potential key factors underlying adaptation to invasion front environment and modulating invasion success.