Spontaneous hybrids between native and exotic Rubus in the Western United States produce offspring both by apomixis and by sexual recombination

Facultative asexual reproduction is a trait commonly found in invasive species. With a combination of sexual and asexual reproductive modes, such species may adapt to new environments via sexual recombination during range expansion, while at the same time having the benefits of asexuality such as the maintenance of fitness effects that depend upon heterozygosity. In the Western United States, native species of Rubus (Rosaceae) reproduce sexually whereas exotic naturalized Rubus species reproduce by pseudogamous apomixis. We hypothesized that new asexual lineages of Rubus could arise from hybridization in this range. To detect hybridization between native and exotic Rubus, we genotyped 579 individuals collected across California, Oregon and Washington with eight nuclear microsatellites and two chloroplast markers. Principal Coordinate Analysis and Bayesian clustering revealed a limited amount of hybridization of the native R. ursinus with the exotic R. armeniacus and R. pensilvanicus, as well as cultivated varieties. Genetic distances between these hybrids and their offspring indicated that both R. ursinus × R. armeniacus and R. ursinus × R. pensilvanicus produced a mix of apomictic and sexual seeds, with sexual seeds being more viable. Although neither of these hybrid types is currently considered invasive, they model the early stages of evolution of new invasive lineages, given the potential for fixed heterosis and the generation of novel genotypes. The hybrids also retain the ability to increase their fitness via sexual recombination and natural selection. Mixed reproductive systems such as those described here may be an important step in the evolution of asexual invasive species.     

Contrasting patterns of genetic diversity and spatial structure in an invasive symbiont-host association

Do host invaders and their associated symbiont co-invaders have different genetic responses to the same invasion process? To answer this question, we compared genetic patterns of native and exotic populations of an invasive symbiont-host association. This is an approach applied by very few studies, of which most are based on parasites with complex life cycles. We used the mitochondrial genetic marker cytochrome oxidase subunit I (COI) to investigate a non-parasitic freshwater ectosymbiont with direct life-cycle, low host specificity and well-documented invasion history. The study system was the crayfish Procambarus clarkii and its commensal ostracod Ankylocythere sinuosa, sampled in native (N American) and exotic (European) ranges. Results of analyses indicated: (1) higher genetic diversity in the symbiont than its host; (2) genetic diversity loss in the exotic range for both species, but less pronounced in the symbiont; (3) native populations genetically structured in space, with stronger patterns in the symbiont and (4) loss of spatial genetic structure in the exotic range in both species. The combination of historical, demographic and genetic data supports a higher genetic diversity of source populations and a higher propagule size that allowed the symbiont to overcome founder effects better than its host co-invader. Thus, the symbiont might be endowed with a higher adaptive potential to new hosts or off-host environmental pressures expected in the invasive range. We highlight the usefulness of this relatively unexplored kind of symbiont-host systems in the invasion context to test important ecological and evolutionary questions.     

No consistent association between changes in genetic diversity and adaptive responses of Australian acacias in novel ranges

Common garden studies comparing trait differences of exotic species between native and introduced ranges rarely incorporate an analysis of genetic variation, but simply infer that trait shifts between ranges are genetically determined. We compared four growth-related traits (total biomass, relative growth rate RGR, specific leaf area SLA, and root to shoot ratio R:S) of five invasive Fabaceae species (Acacia cyclops, A. longifolia, A. melanoxylon, A. saligna, Paraserianthes lophantha), grown in a common garden experiment using seeds from introduced and native ranges across Australia. Chloroplast microsatellite loci were used to compare genetic diversity of native and introduced populations to determine standing genetic diversity and infer introduction history. We asked whether shifts in traits associated with faster growth due to enemy release in the introduced range were associated with levels of genetic diversity associated with introduction history. We found differences in traits between ranges, although these traits varied among the species. Compared to native-range populations, introduced-range Acacia longifolia had greater biomass and larger SLA; A. cyclops had greater RGR; and A. melanoxylon displayed lower R:S. Genetic diversity in the introduced range was lower for one of those species, A. longifolia, and two others that did not show differences in traits, A. saligna and P. lophantha. Diversity was higher in the introduced range for A. melanoxylon and did not differ among ranges for A. cyclops. These patterns of genetic diversity suggest that a genetic bottleneck may have occurred following the introduction of A. longifolia, A. saligna and P. lophantha. In contrast greater or comparable genetic diversity in the introduced range for A. melanoxylon and A. cyclops suggests introductions from multiple sources. This study has shown that a reduction in genetic diversity in the introduced range is not necessarily associated with a reduced capacity for adaptive responses or invasion potential in the novel range.     

Testing the role of genetic factors across multiple independent invasions of the shrub Scotch broom (Cytisus scoparius)

Knowledge of the introduction history of invasive plants informs on theories of invasiveness and assists in the invasives management. For the highly successful invasive shrub Scotch broom, Cytisus scoparius, we analysed a combination of nuclear and chloroplast microsatellites for eight native source regions and eight independent invasion events in four countries across three continents. We found that two exotic Australian populations came from different sources, one of which was derived from multiple native populations, as was an invasive sample from California. An invasive population from New Zealand appeared to be predominantly sourced from a single population, either from the native or exotic ranges. Four invasive populations from Chile were genetically differentiated from the native range samples analysed here and so their source of introduction could not be confirmed, but high levels of differentiation between the Chilean populations suggested a combination of different sources. This extensive global data set of replicated introductions also enabled tests of key theories of invasiveness in relation to genetic diversity. We conclude that invasive populations have similar levels of high genetic diversity to native ranges; levels of admixture may vary across invasive populations so admixture does not appear to have been an essential requirement for invasion; invasive and native populations exhibit similar level of genetic structure indicating similar gene flow dynamics for both types of populations. High levels of diversity and multiple source populations for invasive populations observed here discount founder effects or drift as likely explanations for previously observed seed size differences between ranges. The high levels of genetic diversity, differential and source admixture identified for most exotic populations are likely to limit the ability to source biocontrol agents from the native region of origin of invasive populations.     

Invasive and Non-Invasive Congeners Show Similar Trait Shifts between Their Same Native and Non-Native Ranges

Differences in morphological or ecological traits expressed by exotic species between their native and non-native ranges are often interpreted as evidence for adaptation to new conditions in the non-native ranges. In turn this adaptation is often hypothesized to contribute to the successful invasion of these species. There is good evidence for rapid evolution by many exotic invasives, but the extent to which these evolutionary changes actually drive invasiveness is unclear. One approach to resolving the relationship between adaptive responses and successful invasion is to compare traits between populations from the native and non-native ranges for both exotic invaders and congeners that are exotic but not invasive. We compared a suite of morphological traits that are commonly tested in the literature in the context of invasion for three very closely related species of Centaurea, all of which are sympatric in the same native and non-native ranges in Europe and North America. Of these, C. solstitialis is highly invasive whereas C. calcitrapa and C. sulphurea are not. For all three species, plants from non-native populations showed similar shifts in key traits that have been identified in other studies as important putative adaptive responses to post-introduction invasion. For example, for all three species plants from populations in non-native ranges were (i) larger and (ii) produced seeds that germinated at higher rates. In fact, the non-invasive C. calcitrapa showed the strongest trait shift between ranges. Centaurea solstitialis was the only species for which plants from the non-native range increased allocation to defensive spines, and allocated proportionally less resources to reproduction, patterns contrary to what would be predicted by theory and other empirical studies to enhance invasion. Our results suggest caution when interpreting the commonly observed increase in size and reproductive capacity as factors that cause exotics to become invaders.     

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.     

Inferring the introduction history of the invasive apomictic grass Cortaderia jubata using microsatellite markers

Aim Reconstructing the introduction history of exotic species is critical to understanding ecological and evolutionary processes that underlie invasive spread and to designing strategies that prevent or manage invasions. The aims of this study were to infer the introduction history of the invasive apomictic bunchgrass Cortaderia jubata and to determine if molecular data support the postulated horticultural origin of invasive populations. Location Invaded areas in the USA (California, Maui) and New Zealand; native areas in Bolivia, Ecuador and Peru. Methods We used nuclear microsatellite markers to genotype 281 plants from invaded areas in California, Maui and New Zealand, and 77 herbarium specimens from native South America, and compared the genotypic and clonal variation of C. jubata from the invaded and native ranges. Clonal diversity was determined from genotypic diversity using two analytical methods. Results Invasive C. jubata from invaded regions in California, Maui and New Zealand consisted of the same single clone that probably originated from a single introduced genotype. In contrast, 14 clones were detected in herbarium specimens from the native areas of Bolivia, Ecuador and Peru. The invasive clone matched the most common clone identified in herbarium specimens from southern Ecuador where horticultural stock is presumed to have originated. Main conclusions The lack of clonal and genotypic diversity in invasive plants, but moderately high diversity detected in native plants, indicates a significant reduction in genetic variation associated with the introduction of C. jubata outside of its native range. Based on historical accounts of the horticultural introduction of C. jubata and the results of this study, a severe founder effect probably occurred during deliberate introduction of C. jubata into cultivation. Our results are consistent with the postulated horticultural origin of invasive C. jubata and point to southern Ecuador as the geographical source of invasive populations.     

Admixture between native and invasive populations may increase invasiveness of Mimulus guttatus

Self-fertilization and admixture of genotypes from different populations can have major fitness consequences in native species. However, few studies have addressed their potential roles in invasive species. Here, we used plants of Mimulus guttatus from seven native North American, three invasive Scottish and four invasive New Zealand populations to address this. We created seeds from self-fertilization, within-population outcrossing, between-population outcrossing within the same range, and outcrossing between the native and invasive ranges. A greenhouse experiment showed that native and invasive plants of M. guttatus suffered to similar degrees from inbreeding depression, in terms of asexual reproduction and biomass production. After outcrossing with plants from other populations, M. guttatus benefited from heterosis, in terms of asexual and sexual reproduction, and biomass production, particularly when plants from native and invasive populations were crossed. This suggests that, when novel genotypes of M. guttatus from the native North American range will be introduced to the invasive ranges, subsequent outcrossing with M. guttatus plants that are already there might further boost invasiveness of this species.     

Inferring the complex origins of horticultural invasives: French broom in California

Investigating the origins of invasive populations provides insight into the evolutionary and anthropogenic factors underlying invasions, and can inform management decisions. Invasive species introduced for horticultural purposes often have complex origins typified by multiple introductions of species, cultivars, and genotypes, and interspecific and intraspecific hybridizations in introduced ranges. Such complex introduction histories may result in complex genetic signatures in the invaded range, making inferences about origins difficult, particularly when all putative sources cannot be sampled. In this study, we inferred the origins of the invasive French broom complex in California using 12 nuclear microsatellite markers. We characterized the genetic diversity and population structure of invasive and horticultural brooms in their invaded range in California and of Genista monspessulana in its native Mediterranean range. Overall, no significant differences in allelic richness, observed heterozygosity, inbreeding, or genetic structure were observed between the invaded and native ranges, but differences existed among populations within ranges. Bayesian STRUCTURE analysis revealed three genetic clusters in the French broom complex. Nearly all native G. monspessulana assigned highly to a single cluster. Many invasives assigned to a second cluster that contained Genista canariensis, Genista stenopetala, and ornamental sweet broom, and the remaining invasives assigned to a third cluster that also contained some G. monspessulana individuals from Sardinia and Corsica. Admixture between the second and third clusters was detected. Approximate Bayesian Computation analysis of six alternative scenarios supported the hypothesis that some invasive French broom is derived from an unsampled population branching from ornamental sweet broom. A combination of factors, including multiple introductions, escapes from cultivation, and inter-taxon hybridization, likely contribute to the invasive success of French broom in California and may have important implications for management, in particular biological control.     

A fast-track for invasion: invasive plants promote the performance of an invasive herbivore

With the greater frequency of biological invasions worldwide there is an increased likelihood that exotic species will interact with each other, and such interactions could enhance one another??s invasion potential. Although direct and indirect interactions between exotic species have been well documented for plant-herbivore interactions, the majority of studies have focused on a single interaction and on plant rather than herbivore performance. In this study we investigated whether invasive exotic plants could contribute to the invasion of California by an exotic generalist herbivore (Epiphyas postvittana). We tested this expectation in the greenhouse by monitoring the performance of larval and pupal stages of E. postvittana on six pairs of congeneric invasive and native plants. Larval survivorship and pupal weight of E. postvittana were both greater on the invasive species, and larval development time was shorter on the invasive plant species for two of the plant genera. Our results suggest that prior invasion of exotic plants could function as a catalyst for the subsequent invasion of an exotic insect herbivore, at least in the case where they have shared some history, thereby accelerating the invasion process and expansion of its novel geographic range.     

Multiple introductions boosted genetic diversity in the invasive range of black cherry (Prunus serotina; Rosaceae)

Background and Aims

Black cherry (Prunus serotina) is a North American tree that is rapidly invading European forests. This species was introduced first as an ornamental plant then it was massively planted by foresters in many countries but its origins and the process of invasion remain poorly documented. Based on a genetic survey of both native and invasive ranges, the invasion history of black cherry was investigated by identifying putative source populations and then assessing the importance of multiple introductions on the maintenance of gene diversity.

Methods

Genetic variability and structure of 23 populations from the invasive range and 22 populations from the native range were analysed using eight nuclear microsatellite loci and five chloroplast DNA regions.

Key Results

Chloroplast DNA diversity suggests there were multiple introductions from a single geographic region (the north-eastern United States). A low reduction of genetic diversity was observed in the invasive range for both nuclear and plastid genomes. High propagule pressure including both the size and number of introductions shaped the genetic structure in Europe and boosted genetic diversity. Populations from Denmark, The Netherlands, Belgium and Germany showed high genetic diversity and low differentiation among populations, supporting the hypothesis that numerous introduction events, including multiple individuals and exchanges between sites, have taken place during two centuries of plantation.

Conclusions

This study postulates that the invasive black cherry has originated from east of the Appalachian Mountains (mainly the Allegheny plateau) and its invasiveness in north-western Europe is mainly due to multiple introductions containing high numbers of individuals.     

Abundance and dispersion of the invasive Mediterranean annual, Centaurea melitensis in its native and non-native ranges

In general, invasive plants are assumed to behave more aggressively in their invasive ranges than in their native range, and studies of the mechanisms of invasion often assume these differences. However, comparisons of abundances between native and invasive ranges are rarely carried out. We compared density and dispersion of the invasive plant, Centaurea melitensis (Asteraceae) in its native range and two invasive ranges of similar mediterranean-climate type. The objective was to quantify the differences in its abundance among three distant regions. We surveyed six sites in the native range (Spain) and in each of two invaded ranges (California and central Chile) for population density, relative dominance and spatial distribution of Centaurea. Centaurea occurred at higher densities in invasive sites than in native ones, with a median of 100 plants per m² and 70 plants per m² in California and Chile, respectively, compared to only 4 plants per m² in Spain. Centaurea was more dominant in both invasive ranges than in the native range. Centaurea density and relative dominance were highly variable within regions. Plants in Spain were randomly dispersed, while those in both invasive ranges were more aggregated. Annual precipitation and mean annual temperature were the best predictors of Centaurea density. In California sites, density was negatively correlated with soil nutrients. The presence of at least one high-density population with near total dominance in Spain suggests that there might be ecological mechanisms for invasiveness in Centuarea that are not unique to invaded ranges.     

Niche dynamics of two cryptic <Emphasis Type="Italic">Prosopis</Emphasis> invading South American drylands

Whether or not species track native climatic conditions during invasions (i.e., climate match hypothesis) is fundamental to understand and prevent potential impacts of invasive species. Recent empirical work suggests that climatic mismatches between native and invasive ranges are pervasive. Whether these differences are due to adaptation to new climatic spaces in the invasive range or due to partial filling of the potential climatic space are still subject to debate. Here, we analyze climatic niche dynamics associated with the invasion of the two most common invasive plants in Brazilian semi-arid areas, Prosopis juliflora and Prosopis pallida. These species have been simultaneously introduced in the region, which creates a unique opportunity to compare their niche dynamics during invasion. Given that P. juliflora have a much wider native range size, we expect these species would present different dispersal potentials, which might translate into different unfilling levels. Using an ordination method with kernel smoother and null models, we contrasted climate spaces occupied by each species in both native and invasive ranges. We further used ecological niche models (ENMs) to compare reciprocal predictions of potentially suitable areas. Against our expectation based on differences in native range sizes, climatic niches of P. juliflora and P. pallida overlapped greatly, both in their native and invasive ranges. Our results support niche conservatism during the invasion process. Climatic mismatches among native and invaded ranges were exclusively attributed to unfilling of native climates in the invasive range. Both species showed similar unfilling levels. Likewise, ENMs predicted regions not yet occupied in the invasive range, revealing a potential for further expansion. We discuss colonization time lag and founder effect as potential mechanisms that may have prevented these species to fully occupy their native niches in the invasive range.     

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

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

Asexual propagations of introduced exotic macrophytes <Emphasis Type="Italic">Elodea nuttallii</Emphasis>, <Emphasis Type="Italic">Myriophyllum aquaticum</Emphasis>, and <Emphasis Type="Italic">M. propinquum</Emphasis> are improved by nutrient-rich sediments in China

An increasing number of recent studies indicate that multiple interacting factors can affect the invasion of plants. However, few studies have focused on asexual propagation and the interaction of propagation with environmental factors that regulate the invasive potential of introduced exotic species in aquatic habitats. This study was designed to investigate the differences in asexual propagation between introduced exotic and non-invasive native aquatic macrophytes in nutrient-poor and nutrient-rich sediments and to test the hypothesis that differences in asexual propagation (stem fragment production) and propagule establishment between introduced exotic and non-invasive native macrophytes are driven by sediment nutrient levels. Three exotic aquatic macrophytes (Elodea nuttallii, Myriophyllum aquaticum, and M. propinquum) recently introduced to China and their non-invasive native counterparts (Hydrilla verticillata, M. oguraense, and M. ussuriense) were used for comparison in nutrient-poor (TN 0.59 and TP 0.03 mg g⁻¹) and nutrient-rich (TN 2.35 and TP 0.10 mg g⁻¹) sediments. After 8 weeks of growth, the exotic species tended to produce more total biomass, branch biomass and apical shoots and have higher relative growth rate (RGR) than their native counterparts in nutrient-rich sediment. Rooting efficiency and root growth of exotic fragments were higher than that of native counterparts in nutrient-rich sediment, although the survival rates of fragments did not differ between native and exotic species. In addition, superior traits (rooting efficiency and root growth) of exotic species were also observed in nutrient-poor sediment, but to a lesser degree than in nutrient-rich sediment. These results suggest that asexual propagation of these three introduced exotic macrophytes is more effective in nutrient-rich sediment than in nutrient-poor sediment in China.     

ORIGINS OF GENOTYPIC VARIATION IN NORTH AMERICAN DANDELIONS INFERRED FROM RIBOSOMAL DNA AND CHLOROPLAST DNA RESTRICTION ENZYME ANALYSIS

Restriction enzyme analysis of ribosomal DNA (rDNA) and chloroplast DNA (cpDNA) is used to assess the relative contribution of hybridization and mutation as sources of genotypic variation in weedy asexual dandelions, with focus on the dandelion flora of North America. Of 318 North American dandelions surveyed, 145 rDNA-cpDNA clones are detected. The combined rDNA-cpDNA genotypes show that most of the polymorphic rDNA and cpDNA restriction sites or lengths in these plants are also present in weedy asexual dandelions collected from natural populations in Europe and in asexual and diploid taxa (microspecies) chosen to represent diverse Eurasian members of the genus. However, of 222 combined rDNA-cpDNA genotypes found in 427 asexual plants surveyed, only 9 genotypes are found in both North American and Eurasian dandelions. Two rDNA and three cpDNA characters are unique to individual plants in North America and are consistent with mutational origins of genotypic variation in asexual lineages. But the array of genotypic diversity, characterized by different combinations of the rDNA and cpDNA characters, show that multiple hybridization events are a more important source of genotypic variation than mutation in the asexual polyploids. The rDNA and cpDNA data also indicate polyphyletic origin of several asexual Taraxacum taxa.     

The host specificity ofHartigia albomaculatus (Hym.: Cephidae] and its potential effectiveness in the biological control of European blackberry

The host specificity of the cephid stem-borerHartigia albomaculatus (Stein), was studied to determine its suitability for the biological control of European blackberry (Rubus fruticosus L. agg.) in Australia. Field observations indicated that the insect was specific toR. fruticosus, however laboratory tests showed that it can attack some cultivatedRubus and garden rose (Rosa spp.) varieties. Raspberry (R. idaeus L.) the most importantRubus crop in Australia, was not attacked, nor were 4Rubus spp. endemic to Australia. Observations in the Montpellier area, France, indicate thatH. albomaculatus has no significant detrimental effect on the vegetative reproduction of its host.     

Present and future distribution of three aquatic plants taxa across the world: decrease in native and increase in invasive ranges

Inland aquatic ecosystems are vulnerable to both climate change and biological invasion at broad spatial scales. The aim of this study was to establish the current and future potential distribution of three invasive plant taxa, Egeria densa, Myriophyllum aquaticum and Ludwigia spp., in their native and exotic ranges. We used species distribution models (SDMs), with nine different algorithms and three global circulation models, and we restricted the suitability maps to cells containing aquatic ecosystems. The current bioclimatic range of the taxa was predicted to represent 6.6–12.3% of their suitable habitats at global scale, with a lot of variations between continents. In Europe and North America, their invasive ranges are predicted to increase up to two fold by 2070 with the highest gas emission scenario. Suitable new areas will mainly be located to the north of their current range. In other continents where they are exotic and in their native range (South America), the surface areas of suitable locations are predicted to decrease with climate change, especially for Ludwigia spp. in South America (down to ?55% by 2070 with RCP 8.5 scenario). This study allows to identify areas vulnerable to ongoing invasions by aquatic plant species and thus could help the prioritisation of monitoring and management, as well as contribute to the public awareness regarding biological invasions.     

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.     

Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

Identifying impacts of exotic species on native populations is central to ecology and conservation. Although the effects of exotic predators on native prey have received much attention, the role of exotic prey on native predators is poorly understood. Determining if native predators actively prefer invasive prey over native prey has implications for interpreting invasion impacts, identifying the presence of evolutionary traps, and predator persistence. One of the world’s most invasive species, Pomacea maculata, has recently established in portions of the endangered Everglade snail kite’s (Rostrhamus sociabilis plumbeus) geographic range. Although these exotic snails could provide additional prey resources, they are typically much larger than the native snail, which can lead to lower foraging success and the potential for diminished energetic benefits in comparison to native snails. Nonetheless, snail kites frequently forage on exotic snails. We used choice experiments to evaluate snail kite foraging preference in relation to exotic species and snail size. We found that snail kites do not show a preference for native or exotic snails. Rather, snail kites generally showed a preference for medium-sized snails, the sizes reflective of large native snails. These results suggest that while snail kites frequently forage on exotic snails in the wild, this behavior is likely driven simply by the abundance of exotic snails rather than snail kites preferring exotics. This lack of preference offers insights to hypotheses regarding effects of exotic species, guidance regarding habitat and invasive species management, and illustrates how native-exotic relationships can be misleading in the absence of experimental tests of such interactions.