Aquaculture enclosures relate to the establishment of feral populations of introduced species

Nonlinear Abundance-Area Relationship Underlying Processes of the American Bullfrog Invasion in the Zhoushan Archipelago, China

The small-island effect (SIE) has been used to quantify the increase of established non-native species richness with island area but has not yet been applied to explore the dynamics of non-native species abundance, which is important to develop timely mitigation strategies on established populations. Based on field surveys of established populations of the American bullfrog (Lithobates catesbeianus = Rana catesbeiana) across 92 permanent water bodies on 31 islands in the Zhoushan Archipelago, China, we explored the abundance-area relationship (AAR) of the invasive bullfrogs at the island and habitat (i.e., permanent still waters) scales, respectively. We did not detect the non-linear increase of bullfrog abundance with island area in the Zhoushan Archipelago, but found a piecewise trend of the bullfrog abundance with the area of invaded waters. Overall, bullfrogs were more abundant on larger islands, less isolated islands, and in waters with lower densities of native anurans. Our findings indicate that the invasive bullfrogs may have not reached the threshold of rapid increase of population abundance in the Zhoushan Archipelago and highlight the importance of continued close monitoring to prevent future population outbreaks.

Taming the terminological tempest in invasion science

Standardised terminology in science is important for clarity of interpretation and communication. In invasion science - a dynamic and rapidly evolving discipline - the proliferation of technical terminology has lacked a standardised framework for its development. The result is a convoluted and inconsistent usage of terminology, with various discrepancies in descriptions of damage and interventions. A standardised framework is therefore needed for a clear, universally applicable, and consistent terminology to promote more effective communication across researchers, stakeholders, and policymakers. Inconsistencies in terminology stem from the exponential increase in scientific publications on the patterns and processes of biological invasions authored by experts from various disciplines and countries since the 1990s, as well as publications by legislators and policymakers focusing on practical applications, regulations, and management of resources. Aligning and standardising terminology across stakeholders remains a challenge in invasion science. Here, we review and evaluate the multiple terms used in invasion science (e.g. 'non-native', 'alien', 'invasive' or 'invader', 'exotic', 'non-indigenous', 'naturalised', 'pest') to propose a more simplified and standardised terminology. The streamlined framework we propose and translate into 28 other languages is based on the terms (i) 'non-native', denoting species transported beyond their natural biogeographic range, (ii) 'established non-native', i.e. those non-native species that have established self-sustaining populations in their new location(s) in the wild, and (iii) 'invasive non-native' - populations of established non-native species that have recently spread or are spreading rapidly in their invaded range actively or passively with or without human mediation. We also highlight the importance of conceptualising 'spread' for classifying invasiveness and 'impact' for management. Finally, we propose a protocol for classifying populations based on (i) dispersal mechanism, (ii) species origin, (iii) population status, and (iv) impact. Collectively and without introducing new terminology, the framework that we present aims to facilitate effective communication and collaboration in invasion science and management of non-native species.

Variations in Genetic Diversity of Invasive Species Lithobates catesbeianus in China

The introduction and subsequent range expansion of the American bullfrog (Lithobates catesbeianus) is part of a rising trend of troublesome biological invasions happening in China. This detrimental amphibious invasive species has strong adaptability. After its introduction and spread, it established its own ecological niche in many provinces of China, and its range has continued to expand to more areas. Previous studies recorded the introduction time of bullfrogs and calculated the changes in their genetic diversity in China using mitochondria, but the specific introduction route in China is still unknown. Expanding upon previous research, we employed whole-genome scans (utilizing 2b-RAD genomic sequencing) to examine single nucleotide polymorphisms (SNPs) and microsatellites within Lithobates catesbeianus to screen the genomes of these invasive amphibian species from eight Chinese provinces and two U.S. states, including Kansas, where bullfrogs originate. A total of 1,336,475 single nucleotide polymorphic loci and 17 microsatellite loci were used to calculate the genetic diversity of bullfrogs and their migration pathways. Our results suggest that the population in Hunan was the first to be introduced and to spread, and there may have been multiple introductions of subpopulations. Additionally, the genetic diversity of both the SNP and microsatellite loci in the Chinese bullfrog population was lower than that of the US population due to bottleneck effects, but the bullfrogs can adapt and spread rapidly. This study will offer crucial insights for preventing and controlling future introductions into the natural habitats in China. Additionally, it will assist in devising more precise strategies to manage the existing populations and curtail their continued expansion, as well as aim to improve clarity and originality while mitigating plagiarism risk.

Pathogenic selection promotes adaptive immune variations against serious bottlenecks in early invasions of bullfrogs

Adaptive genetic variations are key for understanding evolutionary processes influencing invasions. However, we have limited knowledge on how adaptive genetic diversity in invasive species responds to new pathogenic environments. Here, we compared variations in immune major histocompatibility complex (MHC) class-II β gene and neutral loci in relation to pathogenic chytrid fungus (Batrachochytrium dendrobatidis, Bd) infection across invasive and native populations of American bullfrog between China and United States (US). Chinese invasive populations show a 60% reduction in neutral cytb variations relative to US native populations, and there were similar MHC variation and functional diversity between them. One MHC allele private to China was under recent positive selection and associated with decreased Bd infection, partly explaining the lower Bd prevalence for Chinese populations than for native US populations. These results suggest that pathogen-mediated selection favors adaptive MHC variations and functional diversity maintenance against serious bottlenecks during the early invasions (within 15 generations) of bullfrogs.

Genetic structure of American bullfrog populations in Brazil

Non-native species are a major problem affecting numerous biomes around the globe. Information on their population genetics is crucial for understanding their invasion history and dynamics. We evaluated the population structure of the non-native American bullfrog, Aquarana catesbeiana, in Brazil on the basis of 324 samples collected from feral and captive groups at 38 sites in seven of the nine states where feral populations occur. We genotyped all samples using previously developed, highly polymorphic microsatellite loci and performed a discriminant analysis of principal components together with Jost’s D index to quantify pairwise differentiation between populations. We then amplified 1,047 base pairs of the mitochondrial cytochrome b (cytb) gene from the most divergent samples from each genetic population and calculated their pairwise differences. Both the microsatellite and cytb data indicated that bullfrogs comprise two populations. Population grouping 1 is widespread and possesses two cytb haplotypes. Population grouping 2 is restricted to only one state and possesses only one of the haplotypes from Population grouping 1. We show that there were two imports of bullfrogs to Brazil and that there is low genetic exchange between population groupings. Also, we find that there is no genetic divergence among feral and captive populations suggesting continuous releases. The limited genetic variability present in the country is associated to the small number of introductions and founders. Feral bullfrogs are highly associated to leaks from farms, and control measures should focus on preventing escapes using other resources than genetics, as feral and captive populations do not differ.

The Species Composition and Distribution Patterns of Non-Native Fishes in the Main Rivers of South China

Non-native fish invasions are among the greatest threats to the sustainability of freshwater ecosystems worldwide. Tilapia and catfish are regularly cultured in South China which is similar to their climate in native areas and may also support their invasive potential. We systematically collected fish from eight main rivers of South China, from 2016 to 2018, to investigate and analyse species’ composition and the distribution of non-native fishes. The data reveal that non-native fishes are widespread and abundant in the sampled rivers: of the 98,887 fish collected, 11,832 individuals representing 20 species were not native, which were distributed in the 96% sampled sites. Of the non-native fish species, 17 are used in aquaculture and 19 are native to the tropics; 13 are omnivores while the other seven are predators. Based on dissimilarity of the non-native fish species distributions across the eight rivers, the different rivers could be divided into four assemblages. Geographical isolation and temperature were identified as affecting the distribution patterns of non-native fishes, thereby influencing fish species composition, species number, dominant species, and distribution variations in the South China rivers. Species composition of the non-native fishes in these rivers are related to their introduction vector, compatibility with their native habitat, and feeding strategies. Their distribution was mainly influenced by geographical location and temperature. To mitigate the impacts of non-native fish, a series of stricter management practices, systematic monitoring, and more research are needed.

Propagule pressure and hunting pressure jointly determine genetic evolution in insular populations of a global frog invader

Islands are often considered to be more susceptible to biological invasions and to suffer greater impacts from invaders than mainland areas, and this difference is generally attributed to differences in species introductions, ecological factors or human activities between islands and mainland areas. Genetic variation, as a good estimate of evolutionary potential, can influence the invasion process and impacts of alien species. However, few studies have compared the genetic diversity of alien species between islands and a corresponding mainland. Here, we examined the genetic variation and differentiation in feral populations (30 sampled individuals/population) of a globally invasive species (the American bullfrog, Lithobates catesbeianus) that was extensively farmed on 14 islands in the Zhoushan Archipelago of China and in three nearby regions on the mainland. We quantified the relative importance of propagule pressure and hunting pressures on the genetic variation of bullfrog populations and found that insular populations have greater genetic variation than their mainland counterparts. Although genetic differentiation between the populations was observed, no evidence of recent bottlenecks or population expansion in any of the tested population was found. Our results suggest that the propagule pressures of bullfrogs escaping from farms, multiple releases and hunting pressure influence the genetic variation among bullfrog populations. These results might have important implications for understanding the establishment and evolution of alien species on islands and for the management of invasive species.

The North American bullfrog draft genome provides insight into hormonal regulation of long noncoding RNA

Frogs play important ecological roles, and several species are important model organisms for scientific research. The globally distributed Ranidae (true frogs) are the largest frog family, and have substantial evolutionary distance from the model laboratory Xenopus frog species. Unfortunately, there are currently no genomic resources for the former, important group of amphibians. More widely applicable amphibian genomic data is urgently needed as more than two-thirds of known species are currently threatened or are undergoing population declines. We report a 5.8 Gbp (NG50 = 69 kbp) genome assembly of a representative North American bullfrog (Rana [Lithobates] catesbeiana). The genome contains over 22,000 predicted protein-coding genes and 6,223 candidate long noncoding RNAs (lncRNAs). RNA-Seq experiments show thyroid hormone causes widespread transcriptional change among protein-coding and putative lncRNA genes. This initial bullfrog draft genome will serve as a key resource with broad utility including amphibian research, developmental biology, and environmental research.

Can Protected Areas with Agricultural Edges Avoid Invasions? The Case of Bullfrogs in the Southern Atlantic Rainforest in Brazil

Bruno Madalozzo, Camila Both, and Sonia Cechin (2016) The American bullfrog, Lithobates catesbeianus, is one of the 100 most harmful invasive species of the planet. Climatic and topographic models predict that the Atlantic Forest regions of southern Brazil are favorable for the establishment of invasive bullfrog populations. The predicted increase of temperature and concentration of gases associated with the greenhouse effect will augment the vulnerability of protected areas of the Atlantic forest to bullfrog invasions in the coming years. In this study we investigated to what extent protected areas of the Atlantic Forest surrounded by anthropogenic landscapes are vulnerable to bullfrog invasions. We conducted surveys in 36 waterbodies located either in a protected area or in anthropogenically modified adjacent locations on a forest-edge-agriculture gradient. We collected data on abundance and breeding to identify the main descriptors (local and landscape variables) that explain the distribution of bullfrogs along this gradient. The variance partitioning analysis showed a strongest association of bullfrog abundance with local waterbody descriptors (area-depth-hydroperiod) and secondarily with a forest-edge-agriculture gradient, i.e., the landscape. The observed distribution pattern suggests that protected areas are likely to be invaded by bullfrogs. Therefore, management strategies should focus on both scales: landscape and waterbody. Supervising the construction of large (permanent or deep) waterbodies in edge habitats of the park and adjacent areas can be effective and agriculture and forest management could importantly complement the prevention of invasions.

Genetic reconstruction of a bullfrog invasion to elucidate vectors of introduction and secondary spread

Reconstructing historical colonization pathways of an invasive species is critical for uncovering factors that determine invasion success and for designing management strategies. The American bullfrog (Lithobates catesbeianus) is endemic to eastern North America, but now has a global distribution and is considered to be one of the worst invaders in the world. In Montana, several introduced populations have been reported, but little is known of their sources and vectors of introduction and secondary spread. We evaluated the genetic composition of introduced populations at local (Yellowstone River floodplain) and regional (Montana and Wyoming) scales in contrast to native range populations. Our objectives were to (1) estimate the number of introductions, (2) identify probable native sources, (3) evaluate genetic variation relative to sources, and (4) characterize properties of local‐ and regional‐scale spread. We sequenced 937 bp of the mitochondrial cytochrome b locus in 395 tadpoles collected along 100 km of the Yellowstone River, from three additional sites in MT and a proximate site in WY. Pairwise Φ_ST revealed high divergence among nonnative populations, suggesting at least four independent introductions into MT from diverse sources. Three cyt b haplotypes were identical to native haplotypes distributed across the Midwest and Great Lakes regions, and AMOVA confirmed the western native region as a likely source. While haplotype (H_d = 0.69) and nucleotide diversity (π = 0.005) were low in introduced bullfrogs, the levels of diversity did not differ significantly from source populations. In the Yellowstone, two identified haplotypes implied few introduction vectors and a significant relationship between genetic and river distance was found. Evidence for multiple invasions and lack of subsequent regional spread emphasizes the importance of enforcing legislation prohibiting bullfrog importation and the need for continuing public education to prevent transport of bullfrogs in MT. More broadly, this study demonstrates how genetic approaches can reveal key properties of a biological invasion to inform management strategies.

Filling a gap in the distribution of Batrachochytrium dendrobatidis: evidence in amphibians from northern China

Chytridiomycosis caused by Batrachochytrium dendrobatidis (Bd) has been recognized as a major driver of amphibian declines worldwide. Central and northern Asia remain as the greatest gap in the knowledge of the global distribution of Bd. In China, Bd has recently been recorded from south and central regions, but areas in the north remain poorly surveyed. In addition, a recent increase in amphibian farming and trade has put this region at high risk for Bd introduction. To investigate this, we collected a total of 1284 non-invasive skin swabs from wild and captive anurans and caudates, including free-ranging, farmed, ornamental, and museum-preserved amphibians. Bd was detected at low prevalence (1.1%, 12 of 1073) in live wild amphibians, representing the first report of Bd infecting anurans from remote areas of northwestern China. We were unable to obtain evidence of the historical presence of Bd from museum amphibians (n = 72). Alarmingly, Bd was not detected in wild amphibians from the provinces of northeastern China (>700 individuals tested), but was widely present (15.1%, 21 of 139) in amphibians traded in this region. We suggest that urgent implementation of measures is required to reduce the possibility of further spread or inadvertent introduction of Bd to China. It is unknown whether Bd in northern China belongs to endemic and/or exotic genotypes, and this should be the focus of future research.

Amplified recruitment pressure of biofouling organisms in commercial salmon farms: potential causes and implications for farm management

The development of biofouling on finfish aquaculture farms presents challenges for the industry, but the factors underlying nuisance growths are still not well understood. Artificial settlement surfaces were used to examine two possible explanations for high rates of biofouling in Norwegian salmon farms: (1) increased propagule release during net cleaning operations, resulting in elevated recruitment rates; and (2) potential reservoir effects of farm surfaces. The presence of salmon farms was associated with consistently and substantially (up to 49-fold) elevated recruitment rates. Temporal patterns of recruitment were not driven by net cleaning. Resident populations of biofouling organisms were encountered on all submerged farm surfaces. Calculations indicate that a resident population of the hydroid Ectopleura larynx, a major biofouling species, could release between 0.3 × 10(9) and 4.7 × 10(9) larvae per farm annually. Such resident populations could form propagule reservoirs and be one explanation for the elevated recruitment pressure at salmon farms.

An exotic species is the favorite prey of a native enemy

Although native enemies in an exotic species' new range are considered to affect its ability to invade, few studies have evaluated predation pressures from native enemies on exotic species in their new range. The exotic prey naiveté hypothesis (EPNH) states that exotic species may be at a disadvantage because of its naïveté towards native enemies and, therefore, may suffer higher predation pressures from the enemy than native prey species. Corollaries of this hypothesis include the native enemy preferring exotic species over native species and the diet of the enemy being influenced by the abundance of the exotic species. We comprehensively tested this hypothesis using introduced North American bullfrogs (Lithobates catesbeianus, referred to as bullfrog), a native red-banded snake (Dinodon rufozonatum, the enemy) and four native anuran species in permanent still water bodies as a model system in Daishan, China. We investigated reciprocal recognition between snakes and anuran species (bullfrogs and three common native species) and the diet preference of the snakes for bullfrogs and the three species in laboratory experiments, and the diet preference and bullfrog density in the wild. Bullfrogs are naive to the snakes, but the native anurans are not. However, the snakes can identify bullfrogs as prey, and in fact, prefer bullfrogs over the native anurans in manipulative experiments with and without a control for body size and in the wild, indicating that bullfrogs are subjected to higher predation pressures from the snakes than the native species. The proportion of bullfrogs in the snakes' diet is positively correlated with the abundance of bullfrogs in the wild. Our results provide strong evidence for the EPNH. The results highlight the biological resistance of native enemies to naïve exotic species.

Increasing potential risk of a global aquatic invader in Europe in contrast to other continents under future climate change

BACKGROUND

Anthropogenically-induced climate change can alter the current climatic habitat of non-native species and can have complex effects on potentially invasive species. Predictions of the potential distributions of invasive species under climate change will provide critical information for future conservation and management strategies. Aquatic ecosystems are particularly vulnerable to invasive species and climate change, but the effect of climate change on invasive species distributions has been rather neglected, especially for notorious global invaders.

METHODOLOGY/PRINCIPAL FINDINGS

We used ecological niche models (ENMs) to assess the risks and opportunities that climate change presents for the red swamp crayfish (Procambarus clarkii), which is a worldwide aquatic invasive species. Linking the factors of climate, topography, habitat and human influence, we developed predictive models incorporating both native and non-native distribution data of the crayfish to identify present areas of potential distribution and project the effects of future climate change based on a consensus-forecast approach combining the CCCMA and HADCM3 climate models under two emission scenarios (A2a and B2a) by 2050. The minimum temperature from the coldest month, the human footprint and precipitation of the driest quarter contributed most to the species distribution models. Under both the A2a and B2a scenarios, P. clarkii shifted to higher latitudes in continents of both the northern and southern hemispheres. However, the effect of climate change varied considerately among continents with an expanding potential in Europe and contracting changes in others.

CONCLUSIONS/SIGNIFICANCE

Our findings are the first to predict the impact of climate change on the future distribution of a globally invasive aquatic species. We confirmed the complexities of the likely effects of climate change on the potential distribution of globally invasive species, and it is extremely important to develop wide-ranging and effective control measures according to predicted geographical shifts and changes.

First evidence of Batrachochytrium dendrobatidis in China: discovery of chytridiomycosis in introduced American bullfrogs and native amphibians in the Yunnan Province, China

Although the chytrid fungus Batrachochytrium dendrobatidis (Bd), the etiological agent of amphibian chytridiomycosis, has been implicated in mass mortality and population declines on several continents around the world, there have been no reports on the presence of Bd infections in amphibians in China. We employed quantitative PCR and histological techniques to investigate the presence of Bd in introduced North American bullfrogs (Rana catesbeiana) (referred to hereafter as bullfrog) and native amphibians in bullfrog-invaded areas of the Yunnan Province, China. A total of 259 samples at five wild sites were collected between June and September in 2007 and 2008, including bullfrogs and four native amphibian species (Rana pleuraden, Rana chaochiaoensis, Odorrana andersonii, and Bombina maxima). In addition, 37 samples of adult bullfrogs were obtained from a food market. Bd infections were discovered in bullfrogs and three native amphibian species from all of the surveyed sites. Of the 39 Bd-positive samples, 35 were from wild-caught bullfrog tadpoles, postmetamorphic bullfrogs, R. pleuraden, R. chaochiaoensis, and O. andersonii, and four were from adult bullfrogs from the market. Our results provide the first evidence of the presence of Bd in Chinese amphibians, suggesting that native amphibian diversity in China is at risk from Bd. There is an urgent need to monitor the distribution of Bd in amphibians in China and understand the susceptibility of native amphibian species to chytridiomycosis. Strict regulations on the transportation of bullfrogs and the breeding of bullfrogs in markets and farms should be drafted in order to stop the spread of Bd by bullfrogs.