Continued Range Expansion Research Articles

Shift from income breeding to capital breeding with latitude in the invasive Asian shore crab Hemigrapsus sanguineus

Organisms vary in the timing of energy acquisition and use for reproduction. Thus, breeding strategies exist on a continuum, from capital breeding to income breeding. Capital breeders acquire and store energy for breeding before the start of the reproductive season, while income breeders finance reproduction using energy acquired during the reproductive season. Latitude and its associated environmental drivers are expected to heavily influence breeding strategy, potentially leading to latitudinal variation in breeding strategies within a single species. We examined the breeding strategy of the Asian shore crab Hemigrapsus sanguineus at five sites spanning nearly 10° of latitude across its invaded United States range. We hypothesized that the primary breeding strategy of this species would shift from income breeding to capital breeding as latitude increases. We found that though this species’ breeding strategy is dominated by capital breeding throughout much of the range, income breeding increases in importance at lower latitudes. This latitudinal pattern is likely heavily influenced by the duration of the foraging and breeding seasons, which also vary with latitude. We also found that reproductive characteristics at the northern and southern edges of the invaded range were consistent with continued range expansion. We suggest that the reproductive flexibility of the Asian shore crab is a key facilitator of its continued invasion success. Our results highlight the influence of latitude on the breeding strategy of a species and emphasize the need for further research regarding the ecological importance and implications of flexibility in breeding strategies within species.

Spatial patterns of reproduction suggest marginal habitat limits continued range expansion of black bears at a forest-desert ecotone.

Investigating spatial patterns of animal occupancy and reproduction in peripheral populations can provide insight into factors that form species range boundaries. Following historical extirpation, American black bears (Ursus americanus) recolonized the western Great Basin in Nevada from the Sierra Nevada during the late 1900s. This range expansion, however, has not continued further into the Great Basin despite the presence of additional habitat. We aimed to quantify whether reduced reproduction toward the range edge contributes to this range boundary. We analyzed black bear detections from 100 camera traps deployed across black bear distribution in western Nevada using a multistate occupancy model that quantified the probability of occupancy and reproduction (i.e., female bears with cubs occupancy) in relation to changes in habitat type and habitat amount toward the range boundary. We detected a strong effect of habitat amount and habitat type on the probability of black bear occupancy and reproduction. At similar levels of landscape-scale habitat amount (e.g., 50%), estimated probability of occupancy for adult bears in piñon-juniper woodlands near the range boundary was 0.39, compared to ~1.0 in Sierra Nevada mixed-conifer forest (i.e., core habitat). Furthermore, estimated probability of cub occupancy, conditional on adult bear occupancy, in landscapes with 50% habitat was 0.32 in Great Basin piñon-juniper woodlands, compared to 0.92 in Sierra Nevada mixed-conifer forest. Black bear range in the western Great Basin conforms to the center-periphery hypothesis, with piñon-juniper woodland at the range edge supporting ecologically marginal habitat for the species compared to habitat in the Sierra Nevada. Further geographic expansion of black bears in the Great Basin may be limited by lower occupancy of reproducing females in piñon-juniper woodland. Center-periphery range dynamics may be common in large carnivore species, as their dispersal ability allows them to colonize low-quality habitat near range edges.

Phylogeography of Populus koreana reveals an unexpected glacial refugium in Northeast Asia.

The genetic structure of temperate plants in the northern hemisphere was significantly influenced by the Quaternary climate oscillations. A species' biological characteristics and ecological niche are significant elements that can affect its phylogeographic history. We adopted the cold-tolerant, anemophilous and anemochorous tree, Populus koreana, as a model species to examine the impact of historical climate changes and biological characteristics on the evolutionary history of vegetation in Northeast Asia throughout the Quaternary period. The results showed that there is moderate genetic differentiation and a lack of phylogeographic structure among populations of P. koreana based on nuclear microsatellite and plastid markers. Demographic analyses and ecological niche modeling suggested that P. koreana is likely to have experienced a bottleneck around the last glacial maximum (LGM), followed by a rapid and continued range expansion coupled with a northward migration from the LGM to the mid-holocene (MH), present, and 2050. Notably, there were several separate refugia present throughout the range of P. koreana in Northeast Asia during the LGM. These include two widely recognized refugia located in the Changbai Mountains and the southern Korean Peninsula. We also unexpectedly found a previously unknown one in the northern Greater Khingan Mountains. Our study contributes to the understanding of the phylogeographic history of plant species in Northeast Asia, providing novel insights into the Greater Khingan Mountains as glacial refugia for a cold-tolerant tree species. These findings provide valuable insights into the Quaternary historical patterns of temperate forests in East Asia.

Environmental Drivers of Gulf Coast Tick (Acari: Ixodidae) Range Expansion in the United States.

In the United States, the Gulf Coast tick (Amblyomma maculatum Koch) is a species of growing medical and veterinary significance, serving as the primary vector of the pathogenic bacterium, Rickettsia parkeri (Rickettsiales: Rickettsiaceae), in humans and the apicomplexan parasite, Hepatozoon americanum, in canines. Ongoing reports of A. maculatum from locations outside its historically reported distribution in the southeastern United States suggest the possibility of current and continuing range expansion. Using an ecological niche modeling approach, we combined new occurrence records with high-resolution climate and land cover data to investigate environmental drivers of the current distribution of A. maculatum in the United States. We found that environmental suitability for A. maculatum varied regionally and was primarily driven by climatic factors such as annual temperature variation and seasonality of precipitation. We also found that presence of A. maculatum was associated with open habitat with minimal canopy cover. Our model predicts large areas beyond the current distribution of A. maculatum to be environmentally suitable, suggesting the possibility of future northward and westward range expansion. These predictions of environmental suitability may be used to identify areas at potential risk for establishment and to guide future surveillance of A. maculatum in the United States.

Natural Regulation of the Baltic Population of the Great Cormorant Phalacrocorax carbo sinensis: The Interplay between Winter Severity and Density Dependence

After a period of continuous increase and range expansion, the Baltic Great Cormorant population has stabilised in large parts of its range in recent years. Ringing recoveries reveal that considerable proportions of the population winter in areas that can be affected by prolonged frost periods. There is evidence that winter severity is an important density-dependent regulation factor: if the population is large, ice cover of coastal and inland water surfaces during harsh winters affects the population by reducing the availability of food resources. As long as the population remained small, however, it was not affected even by very cold winters, since the remaining accessible food resources were presumably still sufficient. The analysis presented here uses the average winter temperature in Germany as a proxy for winter severity in the frost-affected parts of the wintering areas of Baltic Cormorants. The Baltic Cormorant population in 1980–2016 is estimated from annual counts in Denmark, Schleswig-Holstein, Mecklenburg-Western Pomerania, Estonia, Finland and Gotland, which account for about 50% of the total population. The interplay between winter severity and density dependence is analysed using a linear and a non-linear regression model approach. The non-linear model gives a better description of the relationship between the size of the Baltic breeding population during the year (n), the winter temperature Tn, and the population size during the previous year (n–1). According to the model, a population of less than 41,400 breeding pairs would not suffer declines during even the coldest winters recorded since 1882. In 1989, the Baltic Cormorant population exceeded for the first time the threshold value for density-dependent regulation caused by severe winters. The winter 1995/96 was then the first one cold enough to cause a population decline. According to the model, during the years 2002/2003, 2005/06, 2008/09, 2009/10 and 2010/11 the winters have been cold enough to reduce population numbers. Furthermore, the model shows that the regulative winter effect is restricted to the low temperature range.

The role of hydraulic and geomorphic complexity in predicting invasive carp spawning potential: St. Croix River, Minnesota and Wisconsin, United States.

Since they were first introduced to the United States more than 50 years ago, invasive carp have rapidly colonized rivers of the Mississippi River Basin, with detrimental effects on native aquatic species. Their continued range expansion, and potential for subsequent invasion of the Great Lakes, has led to increased concern for the susceptibility of as-yet uncompromised lotic and lentic systems in the central United States. Because invasive carp eggs and larvae must drift in the river current for the first several days following spawning, numerical drift modeling has emerged as a useful technique for determining whether certain river systems and reaches have the potential to support suspension-to-hatching survival of invasive carp eggs, a critical first step in recruitment. Here we use one such numerical modeling approach, the Fluvial Egg Drift Simulator (FluEgg), to estimate bighead carp (Hypophthalmichthys nobilis) egg hatching success and larval retention in a 47.8-kilometer (km) reach of the multi-thread St. Croix River, Minnesota and Wisconsin, United States. We explore three approaches for obtaining the hydraulic data required by FluEgg, parameterizing the model with either (a) field hydraulic data collected within the main channel during a high-flow event, or hydraulic data output from a one-dimensional hydrodynamic model with both (b) steady, and (c) unsteady flows. We find that the three approaches, along with the range of water temperatures and discharge used in simulations, produce vastly different predictions of streamwise transport and in-river egg hatching probability (0% for field data, 0 to 96% for steady-state hydraulic modeling, and 1.8 to 65% for unsteady modeling). However, all FluEgg simulations, regardless of the source of hydraulic data, predicted that no larvae reach the gas bladder inflation stage within the study reach where nursery habitat is abundant. Overall, these results indicate that the lower St. Croix River is suitable for invasive carp spawning and egg suspension until hatching for a range of discharge and water temperatures. These results highlight the role of complex channel hydraulics and morphology, particularly multi-thread reaches, and their inclusion in ecohydraulic-suitability modeling to determine susceptibility of river systems for invasive carp reproduction. Our work also emphasizes the scientific value of multi-dimensional hydrodynamic models that can capture the spatial heterogeneity of flow fields in geomorphically complex rivers. This work may help to guide management efforts based on the targeted monitoring and control and improve invasive carp egg and larvae sampling efficiency.

Invandringsförloppet hos vitstjärnig blåhake Luscinia svecica cyanecula på svenska västkusten

White-spotted Bluethroat Luscinia svecica cyanecula is the latest addition to the Swedish bird fauna, with the first breeding in Scania in 2015. There were only three records on the Swedish West Coast north of Scania through 2016 and the first breeding was reported in 2018. Censuses performed in 2020 and 2021 revealed 12 and 17 singing males, respectively. Out of 39 singing males 2017–2021, 36 were found in Halland. Most birds were found in coastal wetlands with reedbeds, to which they arrived mainly in April. Nine males represented breeding pairs and second clutches were later observed for four breeding pairs. The Swedish expansion should be seen in the context of Danish immigration in combination with a large-scale advance via the Netherlands and Germany. The White-spotted Bluethroat belongs to those range-expanding taxa that are expected to increase due to warmer climate. As there is plenty of appropriate habitats, not least rapeseed fields, the conditions favour a continued range expansion in southern Sweden.

Predicting range shifts of pikas (Mammalia, Ochotonidae) in China under scenarios incorporating land use change, climate change and dispersal limitations

AbstractAimTwo of the most important forces affecting biodiversity are land use change (LUC) and global climate change (GCC). Previous studies have modelled their impacts on species separately, and together, but few have done so for multiple species with dispersal limitations incorporated into the models.LocationQinghai–Tibet plateau region.MethodsWe integrate species distribution models plus a dispersal model to predict LUC and GCC impacts on the ranges of five species of pikas in the Qinghai–Tibet plateau region of China. Pikas are sensitive to land use and climate change and have limited dispersal abilities.ResultsThe predicted impacts of LUC and GCC on pikas vary between species as well as between LUC and GCC projections. Incorporation of dispersal limitations appreciably restricts the amount of colonized habitat. For all five species, the amount of habitat abandoned or colonized when LUC and GCC are modelled together is less than the sum of LUC and GCC modelled separately. Three of the five species experience a net increase in occupied habitat by 2080 relative to their current ranges under all modelled projections. However, relative to a “Dispersal Only” baseline scenario that assumes no environmental change but continued range expansion into suitable, unoccupied habitat, all five species suffer a net loss of occupied habitat by 2080 under some or all projections.Main conclusionsPredictions of future distributions of species based solely on LUC or GCC, as well as predictions assuming additive impacts, can be misleading. Inclusion of dispersal limitations in models markedly alters predicted future distributions of species. The use of a “Dispersal Only” scenario provides a different and perhaps more accurate way to gauge net impacts to species. Future work should consider incorporating all these parameters to better predict the impacts of LUC and GCC on biodiversity.

CONTINUED EASTWARD RANGE EXPANSION OF BLACK PHOEBES (SAYORNIS NIGRICANS) IN SOUTHERNMOST TEXAS

We observed black phoebe (Sayornis nigricans) nests, breeding pairs, and juveniles in Cameron County, Texas, in 2017. Our first confirmed nesting records in Cameron County document a continued eastward range expansion by black phoebes.

Breeding population increase and range expansion of the Great Crested Grebe Podiceps cristatus in southeastern Norway

The Great Crested Grebe Podiceps cristatus is listed as near-threatened on the Norwegian red list due to small population size, estimated in 2015 at 220–380 pairs. Population size is considered to be stable. Approximately one quarter of the population (50–100 pairs) is thought to occur in Oslo and Akershus, but this estimate is not based on detailed data, and the only previous systematic estimate was 90–100 pairs in 13 sites in 1982. In 2018, I conducted a comprehensive survey of all known and potential breeding sites to assess current population size in Oslo and Akershus. I recorded 233 pairs in 34 sites, suggesting a large increase in population size. To analyse the population increase in more detail, I collected all known records of Great Crested Grebes during the breeding season for the period 1995–2018. Analyses confirmed that there has been a strong increase, at a yearly rate of 4.2%. The increase was both due to increases in already established populations (69% of total increase), and establishment of new sites (31%). New sites were colonised in particular the last 10–15 years, and new sites were located gradually further away from the sites that were already used in 1982, indicating continuous range expansion. New sites were also located in smaller, less nutrient-rich lakes at higher elevations, perhaps indicating occupation of lower quality sites that could limit further population increase. Similar population increases have also occurred in other parts of the distribution range of Great Crested Grebes in Norway, and I present a new population size estimate for Norway at 531–634 pairs. Hence, the species no longer qualifies for red listing, and one may consider to downlist the species to least concern.
 
 Cover photo: A pair of Great Crested Grebes with chicks. Photo: Frode Falkenberg.

Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region

The recent trend of global warming has exerted a disproportionately strong influence on the Eurasian land surface, causing a steady decline in snow cover extent over the Himalayan-Tibetan Plateau region. Here we show that this loss of snow is undermining winter convective mixing and causing stratification of the upper layer of the Arabian Sea at a much faster rate than predicted by global climate models. Over the past four decades, the Arabian Sea has also experienced a profound loss of inorganic nitrate. In all probability, this is due to increased denitrification caused by the expansion of the permanent oxygen minimum zone and consequent changes in nutrient stoichiometries. These exceptional changes appear to be creating a niche particularly favorable to the mixotroph, Noctiluca scintillans which has recently replaced diatoms as the dominant winter, bloom forming organism. Although Noctiluca blooms are non-toxic, they can cause fish mortality by exacerbating oxygen deficiency and ammonification of seawater. As a consequence, their continued range expansion represents a significant and growing threat for regional fisheries and the welfare of coastal populations dependent on the Arabian Sea for sustenance.

Priority setting for invasive species management by the water industry

The expansion of Invasive Alien Species (IAS) is a growing concern to the UK water industry because of their diverse impacts on water quality, infrastructure and eradication costs. New regulations reinforcing the industry’s responsibilities beyond operational costs, coupled with continued range expansion and establishment of new IAS will increase damages. To tackle IAS effectively, the water industry requires reliable information about which species pose the greatest risk to operations and which areas are most vulnerable to invasion. Here we assess potential biosecurity threats for the 24 water companies in the UK using well-established modelling research techniques such as risk assessment and distribution modelling. Using a consensus approach with environmental managers and water companies, we identified 11 IAS of concern for the UK water industry, including five plants, three crustaceans, two molluscs and one fish. These invaders pose important hazards in terms of water quality, flood protection, human health, integrity of infrastructures, recreational and aesthetic values, amongst others. We used distribution models to predict their potential expansion under current and future 2050 climate scenarios within each of the 24 water companies in the UK. Water companies in the South East of England (Cambridge Water, Anglian Water, Affinity Water and Thames Water) are under the highest risk of invasional meltdown from multiple IAS, both now and under future scenarios. The quagga mussel poses the most serious risk of immediate spread and may exacerbate the impacts of the widespread zebra mussel for the water industry. The information generated in this study can support the prioritization of species and regions at risk, so that funds for prevention and eradication of invasions are well allocated. Ultimately, this study demonstrates that scientific risk assessments, usually restricted to the academic and public sectors, can be extremely useful to guide decision-making by the private sector.

The hidden faces of a biological invasion: parasite dynamics of invaders and natives

One of the primary drivers of emerging infectious diseases (EIDs) is human intervention via host or parasite translocations. A unique opportunity to study host and parasite dispersal during a bio-invasion currently exists in Ireland due to the introduction of the bank vole (Myodes glareolus) in the 1920s. The continuing range expansion of M. glareolus within Ireland presents a natural large-scale perturbation experiment. This study used the Irish M. glareolus model to conduct a spatiotemporal study analysing the parasite dynamics of native and invasive species throughout their range. Myodes glareolus and native Apodemus sylvaticus were trapped in woodlands across Ireland and surveyed for their helminth parasites. Myodes glareolus in Ireland were found to have lower parasite diversity in comparison to records of M. glareolus from across Europe and A. sylvaticus in Ireland. Increased density of M. glareolus resulted in a dilution effect, with significantly lower levels of parasitism overall in native hosts, where M. glareolus has been established longest. However, three helminth parasite species of A. sylvaticus increased in abundance in the presence of M. glareolus. Furthermore, M. glareolus at the expansion front were less parasitised (lower abundance and prevalence of certain parasites and lower parasite diversity) than M. glareolus from the core population. This “enemy release” is believed to be mediating the continued successful spread of the invader across Ireland. Our results identify two important variables, seasonality and the stage of the invasion, which should not be overlooked when investigating or managing the changing distribution of hosts and their parasites. Studies of bio-invasions and parasite transmission have primarily focused on the invasive host species or the native host species in cases where virulent pathogen spillover is observed. Our results demonstrate how the concurrent study of invasive and native hosts, and the careful identification of their parasite communities, allows the dynamic processes influencing the parasite component and intracommunity to be identified.

Cold Tolerance of Mountain Pine Beetle (Coleoptera: Curculionidae) Pupae.

Determining the cold tolerance of mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), is critical for assessing its long-term persistence and eruptive potential in its new habitat, as well as the risk of continued range expansion across Canada's boreal forest. We used supercooling points (SCPs) and mortality assessments with exposure to different temperatures to determine the cold tolerance of pupae. Mountain pine beetle pupae cold tolerance did not increase with chilling and there was little change in the lethal temperature regardless of treatment or sample time. SCPs were reflective of expected mortality due to freezing: the lethal temperature for 50% mortality was -19.3°C and the mean SCP was -18.7°C. However, significant mortality occurred over time at much warmer temperatures (0 and -9°C), indicating that this life stage suffers significant prefreeze mortality. On the basis of our results, it is unlikely that pupae would be able to successfully overwinter in most regions in Canada. This study is part of a larger project aimed at producing a comprehensive assessment of the cold tolerance of all life stages of the mountain pine beetle to feed population models, climatic suitability indices, and spread assessments.

Superior photosynthetic performance of the invasive kelp Undaria pinnatifida may contribute to continued range expansion in a wave-exposed kelp forest community

In New Zealand, the kelp Undaria pinnatifida has colonized nearly all major coastal ports and has spread to colonize diverse habitats over a range of hydrodynamic environments. We report one of the earliest surveys of the distribution of this species in Macrocystis pyrifera dominated kelp forest habitats in New Zealand alongside experiments comparing the photosynthetic physiology of U. pinnatifida to nine native co-occurring macroalgal species. Undaria pinnatifida was identified at four locations that had previously been reported to be uninvaded and was found throughout the depth range of the kelp forest. Highest densities were observed at shallow depths, reflecting the commonly accepted mode of invasion via sheltered rock pools. Photosynthesis versus Irradiance (P vs. E) curves were constructed for 10 dominant brown macroalgal species. Undaria pinnatifida exhibited greater photosynthetic ability than native macroalgae with maximal rates of photosynthesis at least 1.6-times higher than the habitat forming native kelp species Macrocystis pyrifera and Ecklonia radiata and more than 2.5 times higher than eight other native species. The photosynthetic efficiency of U. pinnatifida under low light was at least as high as macroalgae common on deeper sections of surveyed reefs (e.g. M. pyrifera and E. radiata). Collectively, this information shows that U. pinnatifida has superior photosynthetic performance across all ecologically relevant light intensities. Our data show that the photosynthetic physiology of U. pinnatifida is not likely to limit invasion into deeper sections of reef. This trend could continue, with the potential to fundamentally alter the structure of these wave-exposed kelp communities.

Populationsutveckling hos svarthakad buskskvätta Saxicola rubicola under expansionens inledningsskede i Västsverige

With its 25 confirmed breeding records between 2000 and 2015, the European Stonechat Saxicola rubicola is historically a rare bird in Sweden. The first breeding in the West Coast was found in 2014. Censuses performed in 2017 and 2018 revealed 28 and 30 breedings respectively. Out of 83 investigated territories, most were found in coastal heathlands in Halland. The arrival occurred in the turn of the months March/April and the majority of the males consisted of one year old birds (2CY). Fledgling date for 68 clutches were distributed from May to August. Second clutches were observed for ten out of 32 investigated breedings. The Swedish expansion should be seen in the context of Danish immigration in combination with a large-scale advance via the German Schleswig-Holstein area. The European Stonechat belongs to those range-expanding species that are expected to increase according to predictions of the future bird fauna. Due to warmer climate, plenty of appropriate habitats and high probability of reproduction, the conditions are good for a continued range expansion in southern Sweden.

Transplant experiments predict potential future spread of alien succulents along an elevation gradient

Determining whether an alien species is at environmental equilibrium is fundamental to understanding the risk of its continued range expansion, but this is rarely quantitatively assessed. We used experimental transplants within and beyond the current elevation range (~ 0–300 m a.s.l.) of two naturalized succulent plant species, Aeonium arboreum and Aeonium haworthii, to investigate whether they have reached their climatic limits in New Zealand. The two species have similar invasion histories but differ in climatic tolerances in their native ranges, therefore we expected them to differ in their potential to spread within New Zealand. We transplanted individuals of both species along an elevation and precipitation gradient and measured plant growth, mortality and seedling establishment over 2 years. Both species responded strongly and in similar ways to climatic variation along the gradient. Annual frost days was a strong predictor of performance for both species; plants grown at sites with more frost days had lower growth, survival and reproduction. We used our results to predict plant growth, survival and reproduction beyond their current naturalised range. We found that neither species is likely to survive at elevations greater than 300 m a.s.l. and that, while A. haworthii appears to be at or near climatic equilibrium, A. arboreum has the potential for spread further up in elevation. We provide the first evidence of the climatic tolerances of Aeonium sp. in their introduced range, and the risk of their continued spread in New Zealand.

Expansion in geographical and morphological space drives continued lineage diversification in a global passerine radiation.

Why diversification rates vary so extensively across the tree of life remains an important yet unresolved issue in biology. Two prominent and potentially independent factors proposed to explain these trends reflect the capacity of lineages to expand into new areas of (i) geographical or (ii) ecological space. Here, we present the first global assessment of how diversification rates vary as a consequence of geographical and ecological expansion, studying these trends among 15 speciose passerine families (together approximately 750 species) using phylogenetic path analysis. We find that relative slowdowns in diversification rates characterize families that have accumulated large numbers of co-occurring species (at the 1° scale) within restricted geographical areas. Conversely, more constant diversification through time is prevalent among families in which species show limited range overlap. Relative co-occurrence is itself also a strong predictor of ecological divergence (here approximated by morphological divergence among species); however, once the relationship between co-occurrence and diversification rates have been accounted for, increased ecological divergence is an additional explanatory factor accounting for why some lineages continue to diversify towards the present. We conclude that opportunities for prolonged diversification are predominantly determined by continued geographical range expansion and to a lesser degree by ecological divergence among lineages.

Shigella sonnei Does Not Use Amoebae as Protective Hosts.

ABSTRACTShigella flexneri and Shigella sonnei bacteria cause the majority of all shigellosis cases worldwide. However, their distributions differ, with S. sonnei predominating in middle- and high-income countries and S. flexneri predominating in low-income countries. One proposed explanation for the continued range expansion of S. sonnei is that it can survive in amoebae, which could provide a protective environment for the bacteria. In this study, we demonstrate that while both S. sonnei and S. flexneri can survive coculture with the free-living amoebae Acanthamoebae castellanii, bacterial growth is predominantly extracellular. All isolates of Shigella were degraded following phagocytosis by A. castellanii, unlike those of Legionella pneumophila, which can replicate intracellularly. Our data suggest that S. sonnei is not able to use amoebae as a protective host to enhance environmental survival. Therefore, alternative explanations for S. sonnei emergence need to be considered.IMPORTANCE The distribution of Shigella species closely mirrors a country's socioeconomic conditions. With the transition of many populous nations from low- to middle-income countries, S. sonnei infections have emerged as a major public health issue. Understanding why S. sonnei infections are resistant to improvements in living conditions is key to developing methods to reduce exposure to this pathogen. We show that free-living amoebae are not likely to be environmental hosts of S. sonnei, as all Shigella strains tested were phagocytosed and degraded by amoebae. Therefore, alternative scenarios are required to explain the emergence and persistence of S. sonnei infections.

Molecular population structure for feral swine in the United States

ABSTRACTFeral swine (Sus scrofa) have invaded most of the United States and continue to expand throughout North America. Given the ecological and economic threats posed by increasing feral swine abundance, it is imperative to develop an understanding of their patterns of natural range expansion and human‐mediated introductions. Towards this goal, we used molecular markers to elucidate the genetic structure of feral swine populations throughout the United States and evaluated the association between historical introductions and contemporary patterns of genetic organization. We used STRUCTURE and discriminant analysis of principal components (DAPC) to delineate genetic clusters for 959 individuals genotyped at 88 single nucleotide polymorphism loci. We identified 10 and 12 genetic clusters for the 2 clustering approaches, respectively. We observed strong agreement in clusters across approaches, with both describing clusters having strong geographic association at regional levels reflecting past introduction and range expansion patterns. In addition, we evaluated patterns of isolation by distance to test for and estimate spatial scaling of population structure within western, central, and eastern regions of North America. We found contrasting spatial patterns of genetic relatedness among regions, suggesting differences in the invasion process, likely as a result of regional variation in landscape heterogeneity and the influence of human‐mediated introductions. Our results indicate that molecular analyses of population genetic structure can provide reliable insights into the invasion processes of feral swine, thus providing a useful basis for management focused on minimizing continued range expansion by this problematic species. © 2018 The Wildlife Society.