Invasive species are a global problem with large ecological and economic costs. A better understanding of how invasive species populations change over time, how these species become integrated into ecosystems, and how their population demographics vary across different environments could help inform management priorities and shape control strategies. For 20 years (2002–2022), we have monitored round goby (Neogobius melanostomus) in Hamilton Harbour, Canada, an industrial harbour and an Area of Concern with high levels of contaminants. We sampled round goby across six sites that vary in contamination levels. We first quantified changes in round goby population demographics and morphology over a twenty-year period and second, we compared how abundance and other life history trajectories differ between sites of high and low contamination. Round goby abundance and body length both decreased over the study period. In contrast, body condition, gonadosomatic index (GSI), and the proportion of guarding parental males in the population increased over time. Over the many years of monitoring, there was no clear difference in round goby abundance between sites of high and low contamination, but individuals from sites of high contamination were smaller, had larger gonad investment, and higher hepatosomatic index compared to round goby from sites of low contamination. We also found there were fewer guarding parental males at sites of high contamination. Our results are valuable because they provide insights into how invasive species interact with different invaded habitats over the long-term. This information can help researchers and managers understand the effects of invasive species and develop strategies to predict, prevent, and manage them.

The jumping worm Amynthas tokioensis is invasive in North America, and has been expanding its range northward in recent years. Because low temperatures typically restrict the geographic poleward distribution of organisms, our goal was to characterize the cold tolerance physiology of adult jumping worms from a site in New Brunswick, Canada (c. 45°N), with the intent of better understanding their geographic range limits. Most of our experiments supported the conclusion that these worms are chill-susceptible: they die during or after exposure to relatively mild low temperatures. When gradually cooled, adult worms lost neuromuscular coordination at approximately 0 °C and froze at a mean temperature of − 4.5 °C. They did not survive freezing and showed poor survival following 1 h exposures to 0 °C and subzero temperatures. At higher mild temperatures (5 °C), the worms could survive short (up to 6 h) but not long (e.g., 48 h) chilling durations. We attempted to induce improved cold tolerance via a five-week gradual acclimation to fall-like temperatures, but fall-acclimated worms showed poor survival during and after this acclimation. Acclimation also did not induce accumulation of glucose, a typical cryoprotectant in earthworms. We suggest that A. tokioensis can likely persist wherever the growing season is sufficiently warm and long enough for the adults to mature, reproduce, and lay cocoons prior to the chilling temperatures associated with early fall. Future work examining the cold tolerance of the overwintering cocoons will be important for fully understanding the northern range limits of these jumping worms.

Wetlands around the world face unprecedented threats, including from invasive species. In North America, the invasive cattail hybrid Typha x glauca dominates wetlands around the Laurentian Great Lakes; more recently it was found in high abundance across the central and eastern Prairie Pothole Region, an area that includes one of the world’s largest wetland complexes. Surveys of the Typha hybrid zone have so far been largely conducted in areas where hybrids are well established, and it therefore remains unclear whether the range expansion of this invasive hybrid occurs after the establishment of its maternal species, T. angustifolia. We surveyed 50 wetlands in the western PPR and found that while 75% of plants were native T. latifolia, the second most common group was F1 hybrids, which had greater abundance and occupancy than T. angustifolia despite the fact that T. angustifolia produces relatively few hybrid seeds; our findings therefore highlight the importance of long-distance dispersal for this hybrid range expansion. The distribution of hybrids combined with the paucity of non-F1 hybrids suggest that the western PPR represents a leading edge of the range expansion by invasive T. × glauca. Our results show that T. × glauca has the capacity for continued range expansion that does not rely on the presence of T. angustifolia, and the impacts of this range expansion should be monitored because of its potential to impede ecosystem services and reduce local biodiversity.

There are 230 UNESCO World Heritage Sites that were designated based on their important natural features. These represent some of the most iconic and important natural places on Earth, with immense value for biodiversity conservation, ecosystem service supply and cultural well-being. Many of these sites are degraded by anthropogenic drivers, including socioeconomic factors (population growth, poverty and tourism), extractive activities (logging, mining, and hunting), external threats (fire, climate change, land-use change, pollution), and biological threats (disease and invasive species). Of these, invasive species remain one of the most problematic for management, and once introduced, populations can grow exponentially and spread to other locations even when actively managed. Given the economic and environmental threats imposed by invasive species, we asked how they compare to the other anthropogenic threats. We reviewed the primary literature for each World Heritage Site and scored the prevalence of 12 different anthropogenic drivers. We found that invasive species rank as one of the most frequently identified threats and pose the greatest degree of concern compared to all other threats, on par with the threat from pollution and greater than logging, land-use change and climate change. We compared our scoring, based on review of the literature, with that of the UNESCO and IUCN monitoring reports. Although there was general agreement between these two assessments, the literature provided information on 55 sites not included in the monitoring reports. We further examine the invasive species examined in the articles and the degree of their perceived impacts on biodiversity, habitat, rare species, ecosystem function, tourism, and economic impacts. It is important that invasive species are well monitored and managed in WHSs to reduce their impacts and meet policy mandated targets and conservation goals.

Invasive earthworms create widespread ecological changes after they are introduced. Non-native earthworms are transported mainly through anthropogenically-mediated activities, including fishing, agriculture, horticulture, and development. Here, we review the ways in which non-native earthworms are transported to new environments. Our conceptual framework involves invasion filters (human activities filter and climate & edaphic filter) that constrain which non-native earthworm species are transported within specific contexts. Differences in earthworms’ ecological behaviors, life cycle, and physiological tolerance of environmental conditions influence which species are transported and which regions can successfully be invaded. Within the human activities filter, we utilize the six invasion pathways that follow a continuum of human intention as laid out by Hulme et al. (J Appl Ecol 45(2):403–414, 2008). Five of these pathways are associated with human activity. Of these, the release, escape, and contaminant pathways are associated with commodities, and the stowaway plus corridor pathways with transportation infrastructure. Major human activities that transport invasive earthworms include the discarding of fishing bait, agriculture, composting and horticulture, and development (e.g., the construction of roads, trails, houses, or campgrounds), but the magnitudes that specific activities transport earthworms are vastly understudied. We conclude that more research needs to be conducted to understand the methods that transport non-native earthworms in order to slow their spread.

Effective management of invasive species often relies upon early detection and rapid implementation of management actions. Unfortunately, early detection can be hampered in coastal regions where sites may be challenging to access and survey or resources are limited. Conventionally considered a freshwater plant with a near-global invaded range, yellow flag iris (Iris pseudacorus) is sufficiently tolerant of salinity to disperse through seawater and successfully invade coastal habitats. Clarifying the characteristics that influence coastal habitat suitability for I. pseudacorus and identifying locations at risk of invasion is critical for monitoring and control efforts. We developed a Maxent species distribution model to inform the risk of coastal invasion by I. pseudacorus in southern British Columbia (Canada). Among the variables assessed, precipitation during the driest quarter was the most important contributor in predicting I. pseudacorus’s coastal habitat suitability (40.6% variable contribution [VC]). Urban areas (24.9% VC), south-facing aspects (6.2% VC) and locations at elevations (7.9% VC) at or above the mean high tide extent were also considered suitable for establishment. Though proximity to perennial freshwater sources (7% VC) was also predicted to provide suitable habitat, close distance to these features was not found to be essential. Regionally, invasion hotspots were predicted in coastal habitats around Tofino/Ucluelet (western Vancouver Island), the Fraser River delta (metro Vancouver), and among the southern Gulf Islands and southern Vancouver Island (capital region). Land managers can use these variables of interest and geographic locations to inform early detection monitoring efforts that help attenuate the emerging threat of I. pseudacorus in coastal ecosystems.

AbstractConservation translocations are widely used to recover threatened species, but can pose risks to recipient ecosystems, particularly in the case of conservation introductions. Because of limited data and uncertainty, risk assessments for such projects often rely on extrapolated evidence and expert opinion, further complicating decision making. The Environmental Impact Classification for Alien Taxa (EICAT) serves to classify the realised impacts of invasive species. We developed a protocol combining EICAT and formal expert elicitation to predict these impacts. We applied our protocol to the extinct-in-the-wild sihek (Guam kingfisher; Todiramphus cinnamominus), for which introduction outside the known historical range is being considered. We elicited from multiple experts probability estimates of impact levels across four impact mechanisms and five candidate release sites. We aggregated estimates using simulation-based and Bayesian approaches, with and without accounting for expert confidence. Experts generally agreed that sihek introduction might impact the recipient ecosystem through predation, competition, and disease, although they disagreed about the likely impact levels. Releases to Palmyra Atoll were considered to pose the lowest risk across candidate sites, so this site was selected for further detailed ecological assessments and risk mitigation efforts. EICAT, with its standardized impact mechanisms and definitions, helped reduce the linguistic uncertainty and subjectivity common to expert-based assessments. Expressing judgments as probabilities allowed us to evaluate uncertainty transparently and to assess the weight of expert confidence on the overall risk estimation. Formal quantitative elicitation and aggregation then allowed a transparent evaluation of results, facilitating communication with stakeholders and decision-makers.

Invasive species represent a threat to many ecosystems but are nearly absent from arctic and subarctic Canada. The treeline town of Churchill, Manitoba, has an unusually high number of invasive species due to the presence of a grain port and railway, but few of these species have spread to the surrounding tundra; for instance, common dandelion (Taraxacum officinale) currently is restricted primarily to town, in contrast with its native congener, Taraxacum lacerum. To investigate whether the distribution of T. officinale is affected by the tendency of buildings to create warm, sheltered microenvironments, we surveyed the abundance of both dandelion species relative to existing structures in town. We established transects extending up to 5 m from each side of 23 buildings, and recorded the number of plants in 50 cm × 50 cm quadrats along each transect. We found that T. officinale performed better on the south sides of buildings, that flowering decreased away from buildings, and that these effects were stronger for T. officinale than the native T. lacerum. Soil temperatures were warmer on the south rather than on the north-facing sides, providing a possible explanation. Our results show that the distribution of T. officinale currently is more influenced by human-created microclimate compared to the abundance of its native relative. However, T. officinale may spread beyond these refuges as the climate continues to warm.

Understanding landscape scale connectivity is an essential component in the management of invasive species since connectivity facilitates their invasion potential. Invasive wild pigs (Sus scrofa) are among the most prolific invaders on the planet, causing billions of dollars in agricultural and environmental damage annually. Newly introduced to Canada in the 1980s, we examined wild pig invasion potential across the northern prairies from western Canada into the currently wild pig-free northern U.S. states. We used GPS collar data collected in the Canadian prairies to quantify resource selection and incorporated results into an electric circuit theory framework to evaluate potential regional landscape connectivity. While available landcover types in this region were dominated by crops and grasslands, wild pigs were predominately located in deciduous forest, crops, and wetlands. Resource selection modelling indicated wild pigs selected deciduous forest and wetlands over other landcover types. These selection tendencies resulted in areas at greater risk of occupation in an intermixture dominated by crops interspersed with waterbodies and deciduous forest fragments, which facilitated movement. Given the pervasiveness of this intermixture across the northern prairies, there was a high potential for invasive wild pigs to move throughout much of the region with areas in southern Saskatchewan and Manitoba, northeastern Montana, North and South Dakota, and western portions of Minnesota being particularly vulnerable. Our work highlights a need for monitoring and science-based response strategies for likely southward spread of this invasive species to prevent or reduce potential crop damage, risks to native species, and disease transmission to humans, pets, livestock, and wildlife.

Many invasive fish conduct seasonal migrations, which create opportunities for large-scale removal. However, labor costs, cumbersome logistics, and environmental conditions (water depth, current, etc.) often make such efforts not feasible. We tested a semi-autonomous system for removing invasive common carp during spawning migrations in a natural stream (20 m wide, 1.5 m deep) over two migration seasons. A low-voltage, vertical deterrence and guidance system (DGS) was used to block the migrating carp and direct them into a large enclosure near shore. Additional electrodes placed in the enclosure created a sweeping electric field to push the carp towards one end and aggregate them over partially submerged conveyors that removed the carp from the water. Passive integrated transponder (PIT) tags and antennas were used to monitor carp behavior and removal efficacy. Each year, spawning migration lasted approximately two months and removal efforts occurred on 19 (year 1) and 21 (year 2) days. The DGS blocked over 90% of the carp and directed them into our trap. In year 1, 56% of the migrating carp were removed, and 68% were removed in year 2 (23,500 carp removed overall). In the final iteration of the system, a crew of three was able to conduct the removal, primarily by operating control systems on shore. Similar systems could be used for other invasive fish in larger and deeper environments where direct human labor is problematic. Such systems could also be used for native species to help them navigate passageways or to deflect them from entrainment areas.

Ants are some of the most widespread, successful, and damaging invasive species worldwide. The impacts of temperate ant introductions are increasing, as species that were introduced decades ago are only now expanding and becoming invasive. Post-introduction genetic changes that allow for larger, denser colonies often help introduced ants to become invasive. Here, we sought to characterize the population genetic structure of the invasive European fire ant, Myrmica rubra in an urban-parkland system in Toronto ON Canada. We genotyped 294 workers from 105 nests at eight microsatellite markers. Ants clustered into two unequally sized genetic groups. The larger group contained 89% of workers from 96 nests across the entire study region and were often collected from large, high-density nests. The smaller group comprised the remaining workers from 14 small, isolated, urban nests. The small group also had higher genetic diversity with 1.5 times the number of unique alleles. The two genetic groups likely represent a post-introduction separation; with larger, denser, genetically similar nests in high-quality parkland habitats, characteristic of a more aggressively invasive population. Furthermore, COI sequencing of 18 workers from both groups suggests Western or Northern Europe as a single origin for Toronto M. rubra. Incorporating these sequences with those previously published found that distinct regions of M. rubra’s North American range result from separate introductions from different European source regions (with fewer within-introduced range transfers). Thus, we suggest post-introduction genetic changes, not separate introductions, are responsible for a link between habitat quality, population density, and genetic diversity of M. rubra in Toronto. Our work with M. rubra suggests that introduction pathways of recently invasive temperate ants are complex and include more introductions compared to tropical ants with a longer record of invasiveness.

Bythotrephes cederströemi are a predatory cladoceran zooplankter that have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like Cisco (Coregonus artedi). While changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations (Hg), this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes. Here, we compare temporal changes in Cisco bioaccumulation slopes (i.e., slopes of relationships between Cisco Hg and trophic position) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that bioaccumulation slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco bioaccumulation slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age from a second complimentary study were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg bioaccumulation are likely due to increased conversion efficiency among invaded populations. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations.

Predictive information on the trophic impacts of an invasive consumer can be gained by experimentally measuring its functional response—the rate of prey consumption in relation to available prey density. However, a common gap in such information is how functional response varies with the consumer’s reproductive state and thermal environment. Here, we tested the effect of reproductive state and temperature on the functional response of the marbled crayfish (Procambarus virginalis), a parthenogenetic species that is popular in the pet trade and considered to be an invasion threat to freshwater habitats globally, including the North American Great Lakes. We applied two thermal treatments: 18 °C and 26 °C, representing current and projected mean summer surface water temperatures, respectively, for nearshore areas of the lower Great Lakes. We found that gravid individuals readily fed on benthic invertebrate prey (chironomid larvae), but their rate of consumption was substantially reduced compared with non-gravid individuals at both temperatures. When tested at 26 °C, the functional response of gravid individuals did not change, whereas non-gravid individuals exhibited an increase in attack rate and a reduction in maximum feeding rate. A generalized linear model of bootstrapped attack rates and handling times showed that these parameters were affected by temperature and reproductive state. In addition, warming altered reproductive cycles by increasing the frequency and duration of gravid states. Our findings suggest that invasive species risk assessments would be better informed by quantifying variation in invader per capita effects across reproductive cycles and temperatures.

AbstractNon-native earthworms found in Eastern Canada substantially affect soil properties and plant diversity, but less is known about their impacts on higher faunal species. We investigated the effects of non-native earthworms on populations of Plethodon cinereus, a common woodland salamander. We hypothesized that earthworms could adversely affect P. cinereus by consuming the forest floor, thereby decreasing soil moisture and the abundance of native preys. Conversely, earthworms could positively affect P. cinereus by providing refuge in their abandoned burrows and by being a novel prey. We installed 25 coverboards in 38 mature sugar maple (Acer saccharum) forests, 24 of which were earthworm-free. Over the next two years, we monitored earthworm and salamander populations using hot mustard extractions and visible implant elastomers, respectively. At a subset of four sites, two with and two without earthworms, we determined salamander diets in the spring (May–June), summer (July–August) and fall (September–October) seasons, using gastric lavage techniques. Forest floor depth decreased, whereas population density, body size and total prey volume of P. cinereus increased, with earthworm abundance. Earthworms, which are soft-bodied and nutritious prey, composed most of the salamander diet at sites with earthworms, volumetrically accounting for > 50% of total prey volume. Despite this, we found fewer prey items in the stomach of salamanders at earthworm-invaded sites, indicating that salamanders are getting a higher caloric intake per feeding while expending less energy. We conclude that non-native earthworms have a net beneficial effect on P. cinereus populations in Eastern Canada, mainly by improving diet quality.

Invasive species management practices often include genetic analyses to better inform decision-making and resource allocation. Yet, past management actions may artificially bias recovered patterns of genetic variation; for example a population bottleneck caused by contemporary culling may mirror some patterns associated with historical isolation. Here, we aimed to disentangle the impacts of past management activities from natural processes for Sitka black-tailed deer (Odocoileus hemionus sitkensis), an invasive species that has been repeatedly culled on two islands, SGang Gwaay and Reef, within the Haida Gwaii archipelago (Canada). We applied a recently developed Genotyping-in-Thousands by sequencing panel to contemporary (e.g., blood, serum, tissue, muscle, feces) and archived deer samples, the latter collected prior to management activity within the system (c. 1997–1998), which allowed us to contextualize conflicting patterns of isolation and connectivity previously observed on SGang Gwaay and Reef. Successful genotyping (92.6%) and population genetic analysis of 292 individuals at 236 SNPs revealed signals of historical isolation on SGang Gwaay and Reef, provided evidence of a founder effect during initial colonization, and indicated an absence of ongoing gene flow. Furthermore, our spatiotemporal analyses consistently supported a priori predictions associated with bottlenecks within post-cull populations, such as within-island loss of genetic variation, elevated within-island kinship, and increased levels of among-island genetic differentiation. These findings are promising for future management of deer on SGang Gwaay and Reef, suggesting that eradications on these islands may be durable. More broadly, our work highlights the importance of understanding management history when interpreting contemporary population genetic data.

AbstractBighead Carp currently threatens to invade the Laurentian Great Lakes from the Mississippi River, but the novel climatic conditions it will encounter by expanding northwards could affect its population performance. Bighead Carp in colder climates exhibits slower growth and matures later, with later maturation typically leading to larger adult size and increased fecundity and survival. Accordingly, the life-history strategies of Bighead Carp at its northern range limits could differ from those observed in its current invaded range. To explore how population performance could differ across changing environmental conditions, we used a stage- and age-based matrix population model parameterized with values reported for Bighead Carp populations around the world. The model was used to evaluate how different ages of maturity and their resulting impacts to body size, survival, and fecundity could impact rates of population growth and establishment. Age of maturity had a non-linear effect on population growth, with maturation at intermediate ages (4–6 years) resulting in better performance. However, performance differed less between maturation ages when fecundity was allowed to increase disproportionately with body size. Greater population growth at younger ages of maturity suggest that invasion at lower latitudes could enable establishment in fewer years due to faster rates of development in warmer temperatures. Across all maturation schedules, population growth was most sensitive to the recruitment of age-1 individuals and least sensitive to adult survival, and vital rates overall varied more in their contribution to population growth at younger ages of maturity. Thus, understanding the factors that control age-1 recruitment would inform projections of population performance for Bighead Carp in the Laurentian Great Lakes.

AbstractThe threat of invasive species to biodiversity and ecosystem structure is exacerbated by the increasingly concerning outlook of predicted climate change and other human influences. Developing preventative management strategies for invasive plant species before they establish is crucial for effective management. To examine how climate change may impact habitat suitability, we modeled the current and future habitat suitability of two terrestrial species, Geranium lucidum and Pilosella officinarum, and two aquatic species, Butomus umbellatus and Pontederia crassipes, that are relatively new invasive plant species regionally, and are currently spreading in the Pacific Northwest (PNW, North America), an area of unique natural areas, vibrant economic activity, and increasing human population. Using North American presence records, downscaled climate variables, and human influence data, we developed an ensemble model of six algorithms to predict the potential habitat suitability under current conditions and projected climate scenarios RCP 4.5, 7.0, and 8.5 for 2050 and 2080. One terrestrial species (P. officinarum) showed declining habitat suitability in future climate scenarios (contracted distribution), while the other terrestrial species (G. lucidum) showed increased suitability over much of the region (expanded distribution overall). The two aquatic species were predicted to have only moderately increased suitability, suggesting aquatic plant species may be less impacted by climate change. Our research provides a template for regional-scale modelling of invasive species of concern, thus assisting local land managers and practitioners to inform current and future management strategies and to prioritize limited available resources for species with expanding ranges.

Dryland native plant communities are threatened by increasing aridity and the progressive expansion of invasive plant species regionally. Red brome (Bromus madritensis ssp. rubens; “B. rubens” hereafter) is one of the most rapidly spreading invasive plants in California. Here, we experimentally tested the independent and joint effects of B. rubens competition and water availability on the establishment, survival, and per capita biomass of three native forb species common to California drylands. In a controlled greenhouse setting, five watering regimes ranging from extreme dry to extreme wet were introduced through controlled watering every 10 days, and B. rubens competition was introduced by growing native plants either in the presence or absence of B. rubens. Bromus rubens did not impact the establishment or survival of any of the three native forbs but did negatively influence the per capita biomass of two of the three native species. Competitive effects did not vary with water availability, suggesting that the negative effects of B. rubens on native competitors may depend more upon species identity than soil moisture conditions in California drylands. We suggest that the native forb Phacelia tancetifolia may be useful in restoration plantings [in areas affected by B. rubens] because this native species maintained consistent biomass across the range of watering levels tested and regardless of B. rubens presence.

AbstractUpon arrival to a new area, alien species have to overcome a series of biotic and abiotic barriers to survive, reproduce, and spread and thus, succeed along the invasion continuum. Failing to understand the role of the different sets of barriers and factors operating across the stages of the invasion continuum limit our ability to predict invasion dynamics, leading to misinformed management. Here, we explore how the European plant Cytisus scoparius overcomes the survival and reproductive barriers to establish along the roadsides of Nahuel Huapi National Park (Argentina). We evaluate the direct and indirect influence of climatic and landscape factors, species traits and their interaction with patch cover, plant height, and pollinator visitation rates as proxies of population persistence, plant growth, and reproduction, respectively. Cytisus scoparius cover was positively associated with the length of water shores and urban cover, factors that may relate to the arrival of propagules, new introduction events, and high levels of disturbance. Plant height was positively associated with annual precipitation and mean temperature. Visitation rates positively related to shrubland cover and to a lesser extent to slope, two factors that may influence pollinator availability and long-distance detectability, respectively. However, factors positively affecting survival had no effect (in the case of height) or negative effect (in the case of cover) on visitation rates, probably due to the saturation of the pollinator pool within large flowering patches. Despite the strong environmental gradients, climatic variables did not seem to influence the cover nor the visitation rates of C. scoparius. The microhabitat provided by roadsides seemed to buffer the climatic variability acting at larger spatial scales. This study shows how the relevance of different barriers across the invasion process can vary due to the characteristics of the species and of the spatial context. Actions directed towards the most limiting barriers and limiting factors could be an efficient way to manage invasions and reduce their impacts. In our system, actions aimed at reducing propagule arrival, soil disturbance and availability of pollinators could limit the survival and reproductive success of C. scoparius.

Dinocampus coccinellae (Hymenoptera: Braconidae) can parasitize over 50 species of ladybeetles (Coleoptera: Coccinellidae), including the invasive Harmonia axyridis. The biological invasion success of H. axyridis has been in part attributed to weak control by natural enemies (Enemy Release Hypothesis). The main aim of this study was to examine (i) the intraspecific variation in field parasitism of D. coccinellae attacking H. axyridis populations in its native (Asia) and invasive ranges (North and South America and Europe), and (ii) the interspecific variation in field parasitism of D. coccinellae attacking the invasive H. axyridis and native hosts in the US (Coleomegilla maculata), Chile (Eriopis chilensis), and Italy (Coccinella septempunctata). We also characterized the genetic differentiation of D. coccinellae populations (from different host species and geographical areas) to infer its putative origin. Parasitism of D. coccinellae on invasive H. axyridis and native hosts were remarkably similar at 4–7%, suggesting a low level of resistance, not supporting the enemy release hypothesis. However, parasitism of H. axyridis was much lower in its native than in the invaded areas studied, indicating that under certain field conditions, it may contribute to the control of invasive populations of H. axyridis. Genetic relationships showed widespread D. coccinellae haplotypes with no host-associated genetic structure, consistent with a generalist parasitoid. Genetic relationships of the haplotypes also suggest a complex introduction scenario, with multiple introductions into America and an inferred European origin for D. coccinellae.

AbstractInvasion of nonindigenous species is considered one of the most urgent problems affecting native ecosystems and agricultural systems. Mechanistic models that account for short-term population dynamics can improve prediction because they incorporate differing demographic processes that link the environmental conditions of a spatial location explicitly with the invasion process. Yet short-term population dynamics are rarely accounted for in spatial models of invasive species spread.Accounting for transient (short-term) population dynamics that arise from the interaction of age structure and vital rates, we predict the stochastic population growth rate and establishment probability of wild pigs following introduction into any location in North America. Established ecological theory suggests that the rate of spatial spread is proportional to population growth rate. Using observed geographic distribution data for wild pigs we calculated geographic spread rates (watersheds/year) from 1982 to 2021. We investigated if observed spread rates increased in watersheds with higher stochastic population growth rates. Stochastic population growth rate and establishment probability of wild pigs increased with increasing initial population (propagule) size and length of establishment time. Areas along the Mississippi, Ohio, and lower portions of the Missouri river drainages had the highest probability of wild pig establishment with many regions having probabilities close to 1. Spread rates demonstrated strong spatial heterogeneity with the greatest rates of spread (5.8 watersheds/year) occurring from 2008 to 2013 prior to the establishment of a National wild pig control program in 2013. Spread rates declined 82% (0.57 watersheds/year) in the period from 2013 to 2021 compared to the period from 1982 to 2013. We found significant positive associations among stochastic population growth rate and observed geographic rates of spread. Stochastic population growth rate explained a large amount of variation (79.3–92.1%) in annual rate of watershed spread of wild pigs. Our predicted probabilities of establishment and population growth can be used to inform surveillance and control efforts to reduce the potential for establishment and spread of wild pigs.