Exotic Range Research Articles

Population growth of two strains of the larger grain borer, Prostephanus truncatus on three maize hybrids grown in Europe

The larger grain borer, Prostephanus truncatus (Horn) (Coleoptera: Bostrychidae), continues to threaten maize worldwide, with an endemic range in Mexico, Central and Northern South America, and an exotic range in Sub-Saharan Africa. There was a targeted effort beginning in the 1990s to create maize hybrids in the hope of controlling P. truncatus. Through those efforts, several resistant maize hybrids were developed, which were able to reduce progeny production by P. truncatus. However, where P. truncatus has yet to fully establish, including parts of Europe such as Italy and Greece, existing maize hybrids developed for production in Europe have not been investigated for resistance against P. truncatus. In this study, we sought to compare the fitness of two different strains of P. truncatus, one from the native range in Mexico and the other from the invaded range in Ghana, by investigating the progeny production, number of damaged kernels, and frass production of both strains on three maize hybrids (Pico 1, Hamilton 2, and AGN 672) developed for use in Greece and wider Europe to evaluate susceptibility and/or tolerance to P. truncatus. The Mexico strain of P. truncatus produced more progeny on the Pico 1 than the AGN 672 hybrid, while the Ghana strain did not significantly differ in progeny production among hybrids. All hybrids were susceptible to both strains of P. truncatus. There were no real differences in damaged kernels or frass production between the two strains and across the three maize hybrids. Overall, this study provides a foundation for exploring maize resistance in European cultivars to P. truncatus as a possible new tool.

Plant-soil interactions during the native and exotic range expansion of an annual plant.

Range expansions, whether they are biological invasions or climate change-mediated range shifts, may have profound ecological and evolutionary consequences for plant-soil interactions. Range-expanding plants encounter soil biota with which they have a limited coevolutionary history, especially when introduced to a new continent. Past studies have found mixed results on whether plants experience positive or negative soil feedback interactions in their novel range, and these effects often change over time. One important theoretical explanation is that plants locally adapt to the soil pathogens and mutualists in their novel range. We tested this hypothesis in Dittrichia graveolens, an annual plant that is both expanding its European native range, initially coinciding with climate warming, and rapidly invading in California after human introduction. In parallel greenhouse experiments on both continents, we used plant genotypes and soils from five locations at the core and edge of each range to compare plant growth in soil inhabited by D. graveolens and nearby control microsites as a measure of plant-soil feedback. Plant-soil interactions were highly idiosyncratic across each range. On average, plant-soil feedbacks were more positive in the native range than in the exotic range. In line with the strongly heterogeneous pattern of soil responses along our biogeographic gradients, we found no evidence for evolutionary differentiation between plant genotypes from the core to edge of either range. Our results suggest that the evolution of plant-soil interactions during range expansion may be more strongly driven by local evolutionary dynamics varying across the range than by large-scale biogeographic shifts.

Home-and-away comparisons of life history traits indicate enemy release and founder effects of the solitary bee, Megachile sculpturalis

Occurrences of introduced and invasive pollinators are increasing worldwide. To predict the potential impacts of exotic bees on native ecosystems we need to understand their ecological interactions. Life history traits are fundamental for understanding ecological interactions and often help to explain the spread of exotic species. We conducted home-and-away comparisons of life history traits of the first invasive bee in Europe, the sculptured resin bee Megachile sculpturalis Smith 1853. We compiled information on nest architecture, offspring, natural enemies, body size and phenology using published literature, museum specimen, data from citizen science initiatives, field observations and reared specimen. Megachile sculpturalis uses a broad variety of nesting materials for brood cell construction, including even plastic at the exotic range. Body size at warm temperate climate was similar in the native and exotic ranges, but phenology shifted forward by about one month (28.9 ± 3.3 SE days) in the exotic ranges. The abundance of natural enemies was similar between native and exotic ranges but specialist enemies were missing in the exotic ranges. These trait shifts may be explained by founder effects or ecological filtering. The comparison of life history traits in native and exotic ranges sheds light on the ecological-evolutionary process of this quickly spreading species and provides a better understanding of invasion processes in solitary bees.

Spatial Population Structuring and Genetic Analysis of Exotic Grevillea robusta in Northwestern India

Abstract The introduction of exotics is common in forestry, and majorly important species, like eucalypts, poplar, and Casuarina sp., occupy vast areas in the Indian subcontinent. Importantly, with the latest cost-effective sequencing techniques, genetic diversity research through molecular marker approaches on major exotics provides novel information for genetic improvement in economic traits with climatic adaptation. The study was carried out on Grevillea robusta to assess genetic relatedness and diversity among 228 genotypes belonging to five northwestern states in India. These genotypes were assayed using twelve simple sequence repeat (SSR) markers. A population structure analysis using structure software identified four major gene pool subgroups with clear-cut differences from each other. Principal coordinate analysis also supports the clustering patterns of the unweighted pair group method with arithmetic mean analysis. In the case of genetic diversity assessment, a total of seventy-three alleles were detected from twelve SSRs, with an average of 6.08 ± 0.71 per locus. Polymorphism information content ranged from 0.17 to 0.67 with a mean of 0.44 ± 0.045, indicating high levels of polymorphism across the genotypes. All the evaluated SSRs demonstrated moderate genetic diversity (observed heterozygosity = 0.31 ± 0.03; expected heterozygosity = 0.32 ± 0.03; and genetic differentiation = 0.295) among the sampled genotypes. These findings indicate significant genetic variability in the germplasm to warrant selection and have potential for a long-term tree improvement program of G. robusta in India. Study Implications: The study on population genetics of Grevillea robusta in exotic environments assessed probable gene pools, provenances, and genetic diversity in geographical distribution range of the species in the Indian scenario. The basic population genetic measures revealed the high diversity areas and probable seed zones of G. robusta in its exotic range. Importantly, the study will be helpful in the selection of candidate plus trees of G. robusta to further strengthen the genotype × environment interaction study for future breeding programs. Further, studying the genetic makeup for moderately diverse species offers valuable insights into evolutionary history, population dynamics, and ecological interactions, which may lead to conservation efforts and guidance for the management of trees such as G. robusta in their native and introduced regions.

The physical, mechanical, and chemical properties of black cherry tree wood (Prunus serotina Ehrh.) and its susceptibility to fungal decomposition in areas where it is secondary and invasive: a case study in the Kampinos National Park (Poland)

The properties of black cherry mature wood (Prunus serotina Ehrh.) and its susceptibility to fungal decay were studied in the invaded range of the species on six trees aged between 39 and 47 years old growing in poor, acidic soils with varying levels of moisture and organic carbon and nitrogen content. Wood from trees that grew in wetter and richer soil had better physical properties. Of the 95 parameters analyzed, 80 showed significant differences in favor of this wood. These differences included wider rings that averaged 3.25 mm, a higher density of 662.71 kg/m3 at 12% humidity, and 1.5 times higher content of extractives. Gas chromatography with mass spectrometry revealed the presence of 44 extractives. Out of these, six had antifungal properties and were found in the wood of trees grown in richer soil, corresponding to 62.93% of the peaks area of all identified substances. Only three were found in trees grown in poorer soil, corresponding to 8.68% of the peaks area respectively. The wood of trees grown in more fertile soil was also less prone to fungal decay, which was generally low. Only Trametes versicolor caused a mass loss of more than 10% of the wood in both sites out of the four basidiomycete species tested. The results indicate that even slight variations in soil fertility and moisture can benefit black cherry, leading to differences in wood features and properties in its exotic range.

Mycodiversity in a micro-habitat: twelve Cladosporium species, including four new taxa, isolated from uredinia of coffee leaf rust, Hemileia vastatrix

During surveys in the centres of origin of the coffee leaf rust (CLR), Hemileia vastatrix in Africa, as well asin its exotic range in Brazil, 23 isolates of the genus Cladosporium were obtained from uredinial pustules. Using a phylogenetic analysis of all isolates involving a combination of partial sequences of the internal transcribed spacer region of rDNA (ITS) and two gene regions: actin (act) and translation elongation factor-1α (tef1), 12 species weredelimited; including four new species – Cladosporium chlamydosporiformans, C. hemileiicola, C. mucilaginosum and C. setoides. GCPSR criteria were employed for species recognition, supported by morphological and cultural characters. The potential of these purported mycoparasites is discussed in the context of biological control of CLR in Latin America.

Population genomics unveils the century-old invasion of the Seagrass Halophila stipulacea in the Mediterranean Sea

The tropical seagrass Halophila stipulacea invaded the Eastern Mediterranean Sea in the late nineteenth century and progressively spread throughout the basin ever since. Its spread is expected to continue north-westward as the Mediterranean Sea becomes warmer, potentially changing the seagrass biogeography of the basin. Given the power of genomics to assess invasion dynamics in non-model species, we report the first ddRAD-seq study of H. stipulacea and small-scale population genomic analysis addressing its century-old Mediterranean invasion. Based on 868 SNPs and 35 genotyped native (Red Sea) and exotic (from Cyprus, Greece, and Italy) samples, results suggest that genetic structure was high, especially between major geographic discontinuities, and that exotic populations maintain comparably lower genetic diversity than native populations, despite 130 years of invasion. The evidence of high heterozygosity excess, coupled with previously reported male-dominated and rare flowering records in the exotic range, suggests that clonal propagation likely played a pivotal role in the successful colonization and spread of H. stipulacea in the Mediterranean. This shift in reproductive strategy, particularly evident in the Italian populations located closest to the western boundary and representing more recent establishments, underscores the importance of this cost-effective mode of reproduction, especially during the initial stages of invasion, raising questions about the species future expansion trajectory. Our findings serve as a catalyst for future research into the species’ invasion dynamics, including deciphering the intricate roles of acclimatization and rapid adaptation, important for a comprehensive assessment of invasion risks and improving management strategies aimed at conserving seagrass ecosystems.

Leaf, root, and soil microbiomes of an invasive plant, Ardisia crenata, differ between its native and exotic ranges

IntroductionEcological underpinnings of the invasion success of exotic plants may be found in their interactions with microbes, either through the enemy release hypothesis and the enhanced mutualism hypothesis. Whereas recent high-throughput sequencing techniques have significantly expanded our understanding of plant-associated microbiomes and their functional guilds, few studies to date have used these techniques to compare the microbiome associated with invasive plants between their native and exotic ranges.MethodsWe extracted fungal and bacterial DNA within leaf endosphere, root endosphere and soil of an invasive plant, Ardisia crenata, sampled from their native range Japan and exotic range Florida, USA. Using Illumina sequencing data, we compared microbial community compositions and diversity between the native and exotic ranges, and tested whether abundance of pathogenic or mutualistic microbes differ between the native or exotic ranges in accordance to the enemy release hypothesis or the enhanced mutualism hypothesis.ResultsFungal and bacterial community compositions differed among leaves, roots and soil, and between the native and exotic ranges. Despite a higher microbial diversity in the soil in the exotic range than in the native range, the microbial diversity within leaf and root was lower in the exotic range compared to the native range. In addition, leaves in the native range harbored a greater number of plant pathogenic fungi compared to those in the exotic range.DiscussionThese patterns suggest plant controls over what microbes become associated with leaves and roots. The higher abundance of leaf pathogenic fungi, including the pathogen which is known to cause specific disease in A. crenata in the exotic range than in the native range, support the enemy release hypothesis and highlighted potential importance of examining microbial communities both above- and below-ground.

Tracing the origins and tracking the movements of invasive rubber vines (Cryptostegia spp., Apocynaceae)

Cryptostegia grandiflora and C. madagascariensis (Apocynaceae) are the only two species of this Madagascan plant genus. Both have been transported around the world as ornamentals due to their attractive flowers and based on a perceived potential as sources of rubber – hence, the common name rubber vine – because of their copious latex, which also contains toxic cardiac glycosides. As a result of their vigorous growth and ability to climb over and smother vegetation, both species have become invasive, posing an actual or potential threat to native ecosystems in many tropical and sub-tropical countries, as well as to human and animal health. Classical biological control (CBC), or the introduction of co-evolved natural enemies to control an invasive alien species in its exotic range, has successfully been used to tackle C. grandiflora in northern Queensland, Australia. This strategy is currently being evaluated for its suitability to manage C. madagascariensis in north-eastern Brazil using the same Madagascan rust fungus, Maravalia cryptostegiae, released as a CBC agent in Australia. For CBC to be successful, it is critical to understand the taxonomy of the invader as well as the origin(s) of its weedy biotype(s) in order to select the best-matched co-evolved natural enemies. Based on an exhaustive search in published and unpublished sources, we summarise the taxonomy and uses of these rubber vines, follow their historical movements and track their earliest records and current weed status in more than 80 countries and territories around the world.

From Alien Species to Alien Communities: Host- and Habitat-Associated Microbiomes in an Alien Amphibian

Alien species can host diverse microbial communities. These associated microbiomes may be important in the invasion process and their analysis requires a holistic community-based approach. We analysed the skin and gut microbiome of Eleutherodactylus johnstonei from native range populations in St Lucia and exotic range populations in Guadeloupe, Colombia, and European greenhouses along with their respective environmental microbial reservoir through a 16S metabarcoding approach. We show that amphibian-associated and environmental microbial communities can be considered as meta-communities that interact in the assembly process. High proportions of bacteria can disperse between frogs and environment, while respective abundances are rather determined by niche effects driven by the microbial community source and spatial environmental properties. Environmental transmissions appeared to have higher relevance for skin than for gut microbiome composition and variation. We encourage further experimental studies to assess the implications of turnover in amphibian-associated microbial communities and potentially invasive microbiota in the context of invasion success and impacts. Within this novel framework of “nested invasions,” (meta-)community ecology thinking can complement and widen the traditional perspective on biological invasions.

Exotic tree species have consistently lower herbivore load in a cross-Atlantic tree biodiversity experiment.

It is commonly expected that exotic plants experience reduced herbivory, but experimental evidence for such enemy release is still controversial. One reason for conflicting results might be that community context has rarely been accounted for, although the surrounding plant diversity may moderate enemy release. Here, we tested the effects of focal tree origin and surrounding tree diversity on herbivore abundance and leaf damage in a cross-Atlantic tree-diversity experiment in Canada and Germany. We evaluated six European tree species paired with six North American congeners in both their native and exotic range, expecting lower herbivory for the exotic tree species in each pair at each site. Such reciprocal experiments have long been called for, but have not been realized thus far. In addition to a thorough evaluation of overall enemy release effects, we tested whether enemy release effects changed with the surrounding tree diversity. Herbivore abundance was indeed consistently lower on exotics across all six tree genera (12 comparisons). This effect of exotic status was independent of the continent, phylogenetic relatedness, and surrounding tree diversity. In contrast, leaf damage associated with generalist leaf chewers was consistently higher on North American tree species. Interestingly, several species of European weevils were the most abundant leaf chewers on both continents and the dominant herbivores at the Canadian site. Thus, most observed leaf damage is likely to reflect the effect of generalist herbivores that feed heavily on plant species with which they have not evolved. At the German site, sap suckers were the dominant herbivores and showed a pattern consistent with enemy release. Taken together, the consistently lower herbivory on exotics on both continents is not purely a pattern of enemy release in the strictest sense, but to some degree additionally reflects the susceptibility of native plants to invasive herbivores. In conclusion, our cross-Atlantic study is consistent with the idea that nonnative trees have generally reduced herbivory, regardless of tree community diversity and species identity, but for different reasons depending on the dominant herbivore guild.

Allometric plasticity and the evolution of environment-by-environment (E×E) interactions during a rapid range expansion of a dung beetle.

Plastic responses to environmental conditions may themselves depend on other environmental conditions, but how such environment-by-environment (E×E) interactions may impact evolution remains unclear. We investigate how temperature shapes the nutritional polyphenism in horn length in a beetle and test whether "allometric plasticity" (a form of E×E) predicts latitudinal differentiation during a rapid range expansion. Rearing populations under common garden conditions demonstrates that increased temperatures reduce the body size threshold separating two male morphs in all populations but also that the magnitude of temperature-dependent changes in allometry diverged across recently established populations. Furthermore, we found a latitudinal increase in the threshold in the species' exotic range at one of the temperatures, suggesting that allometric plasticity in response to temperature may predict evolved clinal differences. Our findings demonstrate that E×E interactions can be similar in magnitude to G×E interactions and that allometric plasticity and its evolution may impact population's responses to environmental changes.

Co-invading ectomycorrhizal fungal succession in pine-invaded mountain grasslands

Ectomycorrhizal (EM) fungal communities that associate with invading pines ( Pinus spp.) are expected to be poor in species diversity. However, long-term successional trajectories and the persistence of dispersal limitations of EM fungi in the exotic range are not well understood. We sampled the roots and surrounding soil of Pinus elliottii and P. taeda trees invading mountain grasslands of Argentina. We also sampled the EM fungal spore bank in grassland soil near (∼150 m) and far (∼850 m) from the original pine plantations. We found 86 different co-invasive EM fungal OTUs. Differential dispersal capacities among EM fungi were detected in the spore bank of grassland soil, but not under mature pines. After thirty years of invasion, the age, but not the degree of spatial isolation of pine individuals affected the EM fungal composition. We showed how EM fungal succession occurs during pine invasions, which may have clear consequences for ecosystem functioning of co-invaded sites. • Pines often rely on co-introduced ectomycorrhizal (EM) fungi to invade native ecosystems. • EM fungal succession and persistence of dispersal limitations are poorly understood. • We found that invading pines can host a highly diverse EM fungal community. • The age but not the spatial isolation of pines affected EM fungal composition. • Dispersal limitations of EM fungi are only relevant during early pine invasion.

First report of ectomycorrhizae in Prunus serotina in the exotic range

First report of ectomycorrhizae in Prunus serotina in the exotic range

Invasive plant species that experience lower herbivory pressure may evolve lower diversities of chemical defense compounds in the exotic range.

Invasive plant species often escape from specialist herbivores and are more likely to be attacked by generalist herbivores in the exotic range. Consequently, the shifting defense hypothesis predicts that invasive plants will produce higher concentrations of qualitative defense compounds to deter dominant generalist herbivores in the exotic range. Here, I additionally propose a reduced chemical diversity hypothesis (RCDH), which predicts that reduced herbivory pressure will select for invasive plant genotypes that produce lower diversities of chemical defense compounds in the exotic range. I tested whether (1) invasive Brassica nigra populations express a lower diversity and an overall higher concentration of glucosinolate compounds than native-range B. nigra; (2) Brassica nigra individuals that express high diversities and concentrations of glucosinolate compounds are more attractive to specialist and deterrent to generalist herbivores; and (3) tissues of invasive B. nigra are less palatable than tissues of native-range B. nigra to the generalist herbivores Theba pisana and Helix aspersa. Invasive B. nigra populations produced a significantly lower diversity of glucosinolate compounds, a marginally higher concentration of total glucosinolates,and a significantly higher concentration of sinigrin (the dominant glucosinolate). Leaf tissues of invasive B. nigra were significantly less palatable to T. pisana and marginally less so to H. aspersa. Brassica nigra individuals that expressed high concentrations of total glucosinolate compounds were visited by a low diversity of generalist herbivore species in the field. In line with the RCDH, the lower diversity of glucosinolate compounds produced by invasive B. nigra populations likely resulted from selection imposed by reduced herbivory pressure in the exotic range.

Climate and human influence shape the interactive role of the honeybee in pollination networks beyond its native distributional range

Empirical evidence has shown that introduced honeybees, Apis mellifera L., can change the structural organization of ecological networks involving pollinators and flowering plants. In this case, studies have shown that A. mellifera is highly connected within networks (i.e., high interactive role) mainly due to its high abundances, long colony lifetime, generalist diet, and great capacity of spread and adaptation to new environments. However, there is little knowledge on how the interactive role of A. mellifera in these flower-visitor networks changes spatially and the potential correlates of such variation across ecological and geographical gradients worldwide. In this study, we evaluated how native and exotic distribution ranges, local climate, human influence (i.e., anthropogenic disturbance), and latitude shape the interactive role of the honeybee in plant-floral visitor networks globally. To achieve that, we compiled 95 weighted plant-floral visitor networks encompassing ca. 87 degrees of latitude and 227 degrees of longitude across the globe. We found that the interactive role of A. mellifera is higher in its exotic range than in the native distributional range and positively associated with mean annual temperature and precipitation. Further, the interactive role of A. mellifera was high at intermediate values of global human influence and increased with latitude. Our findings help us to understand how different drivers related to rapid global change could affect the capacity of A. mellifera to colonize and increase its impact on other pollinator species in different environments.

How well do species distribution models predict occurrences in exotic ranges?

AbstractAimSpecies distribution models (SDMs) are widely used predictive tools to forecast potential biological invasions. However, the reliability of SDMs extrapolated to exotic ranges remains understudied, with most analyses restricted to few species and equivocal results. We examine the spatial transferability of SDMs for 647 non‐indigenous species extrapolated across 1,867 invaded ranges, and identify what factors may help differentiate predictive success from failure.LocationGlobal.Time periodCurrent.Major taxa studiedSix hundred and forty‐seven terrestrial species; eight taxonomic classes.MethodsWe performed a large‐scale assessment of the transferability of SDMs using two modelling approaches: generalized additive models (GAMs) and MaxEnt. We fitted SDMs on the native ranges of species and extrapolated them to exotic ranges. We examined the influence of general factors and factors related to biological invasions on spatial transferability.ResultsDespite both modelling approaches performing well in the range of the species used for fitting, we observed moderate to low spatial transferability on average (mean area under the receiver operating characteristic curve [AUC] ~ .7) when extrapolating to their invaded ranges. Transferability differed between taxonomic classes and invaded continents and was positively influenced by the performance of the model and environmental generalism in the native range, and the year of first record. Models performed worse with greater environmental coverage in the exotic range, gross domestic product and number of occurrences in the native range, geographic distance between ranges and when extrapolating to islands.Main conclusionsAfter controlling for sampling bias, half of SDMs were only weakly predictive, which should affect how SDM‐based forecasts are interpreted. Performance differed based on characteristics of the data, species, and ranges, and can suggest when SDMs may be reliable and when we should be most cautious. These considerations touch directly upon the potential use of SDMs for management of biological invasions. We discuss possible mechanisms of these findings.

Warming Threatens to Propel the Expansion of the Exotic Seagrass Halophila stipulacea

The spread of exotic species to new areas can be magnified when favored by future climatic conditions. Forecasting future ranges using species distribution models (SDMs) could be improved by considering physiological thresholds, because models solely based on occurrence data cannot account for plasticity due to acclimation of individuals to local conditions over their life-time or to adaptation due to selection within local populations. This is particularly relevant for the exotic seagrassHalophila stipulacea, which colonized the Mediterranean Sea a century ago and shifted its thermal niche, coping with a colder regime. Here, we used two hybrid models combining correlative SDMs with the thermal limits for growth of native and exoticH. stipulaceapopulations to predict the distribution of the species in its native (Indian Ocean and Red Sea) and exotic ranges (Mediterranean Sea and Caribbean Sea) under two scenarios forecasting limited (RCP 2.6) and severe (RCP 8.5) future climate changes by 2050 and 2100. Then, we assessed the differences between hybrid models based on native Red Sea thermal limits (niche conservatism: 17–36°C) and on exotic Mediterranean thermal limits (local adaptation: 14–36°C). At the Mediterranean exotic range, the local adaptation hybrid model accurately agreed with the present distribution of the species while the niche conservatism-based hybrid model failed to predict 87% of the current occurrences of the species. By contrast, both hybrid models predicted similar species distributions for the native range and exotic Caribbean range at present and projected thatH. stipulaceawill maintain its current worldwide under all future greenhouse gas emission scenarios. The hybrid model based on Mediterranean thermal limits projected the expansion ofH. stipulaceathrough the western Mediterranean basin (except the gulf of Leon) under the most severe scenario (RCP 8.5) by 2100, increasing its distribution by 50% in the Mediterranean. The future expansion ofH. stipulaceais related to its capacity to cope with warm waters and it may become a relevant species in the future, particularly under the projected decline of native Mediterranean seagrasses, resulting in important shifts in seagrass communities and overall ecosystem functions.

Prey naïveté rather than enemy release dominates the relation of an invasive spider toward a native predator.

Ecosystems may suffer from the impact of invasive species. Thus, understanding the mechanisms contributing to successful invasions is fundamental for limiting the effects of invasive species. Most intuitive, the enemy release hypothesis predicts that invasive species might be more successful in the exotic range than resident sympatric species owing to the absence of coevolution with native enemies. Here, we test the enemy release hypothesis for the invasion of Europe by the North American spider Mermessus trilobatus. We compare the susceptibility of invasive Mermessus trilobatus and a native species with similar life history to a shared predator with which both species commonly co‐occur in Europe. Contrary to our expectations, invasive Mermessus trilobatus were consumed three times more frequently by native predators than their native counterparts. Our study shows that invasive Mermessus trilobatus is more sensitive to a dominant native predator than local sympatric species. This suggests that the relation between the invasive spider and its native predator is dominated by prey naïveté rather than enemy release. Further studies investigating evolutionary and ecological processes behind the invasion success of Mermessus trilobatus, including testing natural parasites and rapid reproduction, are needed to explain its invasion success in Europe.

An upper limit for the growth of inner planets?

ABSTRACT The exotic range of known planetary systems has provoked an equally exotic range of physical explanations for their diverse architectures. However, constraining formation processes requires mapping the observed exoplanet population to that which initially formed in the protoplanetary disc. Numerous results suggest that (internal or external) dynamical perturbation alters the architectures of some exoplanetary systems. Isolating planets that have evolved without any perturbation can help constrain formation processes. We consider the Kepler multiples, which have low mutual inclinations and are unlikely to have been dynamically perturbed. We apply an adaption of previous modelling efforts, accounting for the two-dimensionality of the radius ($R_\mathrm{pl} =0.3\!-\!20\, R_\oplus$) and period (Porb = 0.5–730 d) distribution. We find that an upper limit in planet mass of the form $M_\mathrm{lim} \propto a_\mathrm{pl}^{\beta } \exp (-a_\mathrm{in}/a_\mathrm{pl})$, for semimajor axis apl and a broad range of ain and β, can reproduce a distribution of Porb, Rpl that is indistinguishable from the observed distribution by our comparison metric. The index is consistent with β = 1.5, expected if growth is limited by accretion within the Hill radius. This model is favoured over models assuming a separable PDF in Porb, Rpl. The limit, extrapolated to longer periods, is coincident with the orbits of RV-discovered planets (apl > 0.2 au, $M_\mathrm{pl}\gt 1\, M_\mathrm{J}$) around recently identified low density host stars, hinting at isolation mass limited growth. We discuss the necessary circumstances for a coincidental age-related bias as the origin of this result; such a bias is possible but unlikely. We conclude that, in light of the evidence suggesting that some planetary systems have been dynamically perturbed, simple models for planet growth during the formation stage are worth revisiting.