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Bottlenecks in population size can reduce fitness and evolutionary potential, yet introduced species often become invasive. This poses a dilemma referred to as the genetic paradox of invasion. Three characteristics must hold true for an introduced population to be considered paradoxical in this sense. First, it must pass through a bottleneck that reduces genetic variation. Second, despite the bottleneck, the introduced population must not succumb to the many problems associated with low genetic variation. Third, it must adapt to the novel environment. Some introduced populations are not paradoxical as they do not combine these conditions. In some cases, an apparent paradox is spurious, as seen in introduced populations with low diversity in neutral markers that maintain high genetic variation in ecologically relevant traits. Even when the genetic paradox is genuine, unique aspects of a species' biology can allow a population to thrive. We propose research directions into remaining paradoxical aspects of invasion genetics.

The distribution of marine life has been alarmingly reshaped lately and the number of non-indigenous species and their impacts are rapidly escalating globally. Timely and accurate information about the occurrence of non-indigenous species are of major importance for the mitigation of the issue. However, still large gaps in knowledge about marine bioinvasion exist. Mediterranean Sea is among the most impacted ecoregions globally. In this work we present a comprehensive overview of the project “Is is Alien to you? Share it!!!” which monitors non-indigenous species in Greece and Cyprus with the help of citizen scientists. The goal of this work is to present this project as a case study in order to demonstrate how citizen science can substantially contribute to the monitoring of biological invasions. We compared the projects database with the databased of ELNAIS and EASIN, for discuss weaknesses and advantages and future steps for advancing the effort. In total 691 records of marine alien and cryptogenic species were collected in these 2 years from Greece and Cyprus, with the density of records reaching 20 observations per km2 in some locations. The project has contributed significantly in the assessment of descriptor D2 “Exotic Species” of the Marine Strategy Framework Directive, with 3 new species for Greece. Future steps should focus on training citizens to report less reported taxa and raising the awareness of all relevant stakeholders.

Abstract Ceratium is a planktonic dinoflagellate commonly found in temperate continental waters. In recent decades, Ceratium species have been reported in tropical water bodies, where they are considered invasive. These organisms exhibit high dispersal capacity, adaptability, and rapid colonization of new environments. This study reports the first occurrence of Ceratium furcoides in a natural environment in the state of Santa Catarina, Brazil, specifically in Peri Lake, a coastal freshwater lake in Florianópolis used for public water supply, recreation, and tourism. For species identification, several Ceratium specimens were analyzed focusing on the arrangement of apical plates, a key morphological trait distinguishing C. furcoides from C. hirundinella. The presence of C. furcoides was recorded during monthly zooplankton monitoring of Peri Lake. Ceratium furcoides is often associated with significant ecological impacts, causing alterations in aquatic communities and contributing to water quality degradation. Its uncontrolled proliferation can affect trophic structure, nutrient availability, and compromise local biodiversity. Continuous monitoring and studies correlating the dinoflagellate's occurrence with limnological variables are necessary to elucidate the main factors driving C. furcoides proliferation and to inform management strategies aimed at mitigating potential ecological, social, and economic issues.

Ballast water-mediated bioinvasion is a major threat to the health of aquatic ecosystems. Port environments are hotspots for marine bioinvasion due to ballast water uptake and discharge activities. As the first of its kind pilot study in a tropical ecosystem, we investigated short-term changes in the bacterial community at a port during ballast water discharge (BWD). Total bacterial count and total viable bacteria varied significantly in the port, which can be attributed to BWD. While Vibrio alginolyticus numbers increased, V. parahaemolyticus decreased during BWD, whereas total coliforms were reduced over time. Before BWD, Actinobacteria, β- and γ-proteobacteria were dominant at the port. However, Bacteroidetes and γ-proteobacteria were abundant after BWD, which may have been inoculated in the port through ballast water. Such a shift in bacterial diversity can alter the functional characteristics of the port and is a cause for concern. Investigating the long-term changes due to repeated inoculations will help in developing appropriate environmental management practices.

A significant challenge in comparing and contrasting regional reviews of non-native marine species diversity is that evaluation methods vary widely, resulting in highly inconsistent taxonomic, habitat and historical coverage even in ostensibly well-studied regions. It is thus difficult to interpret whether strikingly different numbers of non-native species in different regions reflect differential invasion patterns or different assessment criteria and capabilities. We provide a comprehensive guide to the methods and techniques to assess the diversity and timing history of non-native and cryptogenic marine species. We emphasize the need to broaden taxonomic and habitat breadth when documenting invasions, to use a broader and deeper search term menu (including using older terms), to thoroughly access global systematic and invasion literature for local, regional records, and to delve deeper into invasion timing to avoid the use of dates-of-publication to assess invasion tempo and rates. Fundamental in all invasions work is the reassessment of the status of ostensibly native species which in fact may have been introduced decades or centuries earlier. We expand to 14 categories the criteria for the recognition of non-native species. Without thorough and vetted modern and historical assessments of the scale of invasions across temperate, subtropical, and tropical marine ecosystems, our ability to look deep into marine community ecology, evolution, and biogeography is strikingly compromised, as is our ability to frame robust invasion policy and management plans.

This article examines the use of economic analysis to inform bioinvasion management, with particular focus on forest resources. Economics is key for understanding invasion processes, impacts, and decision-making. Biological invasions are driven by and affect economic activities at multiple scales and stages of an invasion. Bioeconomic modeling seeks to inform how resources can be optimally allocated across invasion management activities—including prevention, surveillance programs for early detection and management, and controlling invasion populations and spread—to minimize the long-term costs and damages. Economic analysis facilitates understanding of decisions by public and private decision-makers, gaps between these, and the design of policies to achieve socially desirable outcomes. Private decision-makers may undercontrol invasions relative to socially optimal levels, because they generally account for their own costs and benefits of control but less often for broader ecosystem impacts or future spread across the landscape. Economic analysis considers approaches for increasing private invasion management and evaluates feedbacks between ecological and economic systems that can affect policy outcomes. Future research should continue evaluation and design of control strategies across the biosecurity continuum and across species to enhance cost-effectiveness, better incorporate uncertainty into policy design, increase focus on incentives and behavioral tools to influence private behaviors that affect invasion spread, and incorporate invasive species consideration within broader systems-focused science. In addition, challenges in valuing biodiversity and ecosystem service impacts and the costs and effectiveness of control measures are key data gaps. Greater collaboration between decision-makers and researchers will facilitate development and communication of usable economic research.

Although range expansions have occurred recurrently in the history of most species, their genetic consequences have been little investigated. Theoretical studies show that range expansions are quite different from pure demographic expansions and that the extent of recent gene flow conditions expected patterns of molecular diversity within and between populations. Spatially explicit simulation studies have led to unexpected and fascinating results about genetic patterns emerging after a range expansion. For instance, spatial expansions can generate allele frequency gradients, promote the surfing of rare variants into newly occupied territories, induce the structuring of newly colonized areas into distinct sectors of low genetic diversity, or lead to massive introgression of local genes into the genome of an invading species. Interestingly, most of these patterns had been previously attributed to distinct selective processes, showing that taking into account the dynamic nature of a species range can lead to a paradigm shift in our perception of evolutionary processes.

Recreational boats and their marinas are important components of the hub-and-spoke model of invasion in coastal marine systems. Like most vectors, however, species transfers by boats are under-sampled and the extent of invasion among coastal bays is patchily sampled, with a bias towards larger urbanized bays with commercial port systems. We assessed both the hubs (bays with marinas) and the spokes (recreational boat movements and their vector biota) that form the basis of a non-shipping hub-and-spoke system for the coast of California. Non-native invertebrates and algae have been reported from most bays in California (26 of 27), with only five of these having commercial shipping ports. Recreational boats offer frequent opportunities for transfers of species among bays; 292,000 recreational boats were registered in coastal California, and more than half (54%) of surveyed recreational boat owners made voyages outside their home bays. Destinations for recreational boaters often included bays without commercial ports and areas of conservation value inside National Marine Sanctuaries. We recorded 158 taxa from the hulls and niche areas of 49 transient recreational boats upon arrival at Californian marinas. These included several non-native species that are already well-established at arrival locations and four that are not known from the state or sampling location. Biofouling abundance on vessels ranged from zero to remarkably high levels (estimated at 106 organisms) for vessels that traverse open coastal systems. Management opportunities and models exist for the recreational boat vector but, absent operational incentives, these are challenging to implement broadly. An initial management focus on areas recognised for their distinct marine habitats and biodiversity may be an effective implementation approach.

Construction of water diversions is a common response to the increasing demands for freshwater, often resulting in benefits to communities but with the risk of multiple environmental, economic, and social impacts. Water-diversion projects can favor massive introductions and accelerate biotic homogenization. This study provides empirical evidence on the consequences of a proposed law intended to divert water from two large and historically isolated river basins in Brazil: Tocantins to São Francisco. Compositional similarity (CS) and β-diversity were quantified encompassing aquatic organisms: mollusks, zooplankton, crustaceans, insects, fishes, amphibians, reptiles, mammals, and plants. For CS we (i) considered only native species, and (ii) simulated the introduction of non-natives and assumed the extinction of threatened species due to this water-diversion project. We highlight the environmental risks of such large-scale projects, which are expected to cause impacts on biodiversity linked to bioinvasion and homogenization, and we recommend alternatives in order to solve water-demand conflicts.

The increasing spread of marine non‐indigenous species (NIS) due to the growth in global shipping traffic is causing widespread concern for the ecological and economic impacts of marine bioinvasions. Risk management authorities need tools to identify pathways and source regions of priority concern to better target efforts for preventing NIS introduction. The probability of a successful NIS introduction is affected by the likelihood that a marine species entrained in a transport vector will survive the voyage between origin and destination locations and establish an independently reproducing population at the destination. Three important risk factors are voyage duration, range of environmental conditions encountered during transit and environmental similarity between origin and destination. In this study, we aimed for a globally comprehensive approach to assembling quantifications of source‐destination risk factors from every potential origin to every potential destination. To derive estimates of voyage‐related marine biosecurity risk, we used computer‐simulated vessel paths between pairs of ecoprovinces in the Marine Ecoregions Of the World biogeographic classification system. We used the physical length of each path to calculate voyage duration risk and the cross‐latitudinal extent of the path to calculate voyage path risk. Environmental similarity risk was based on comparing annual average sea surface temperature and salinity within each ecoprovince to those of other ecoprovinces. We derived three separate sets of risk quantifications, one each for voyage duration, voyage path and environmental similarity. Our quantifications can be applied to studies that require source‐destination risk estimates. They can be used separately or combined, depending on the importance of the types of source‐destination risks that might be relevant to particular scientific or risk management questions or applications. The probability of a successful marine bioinvasion is affected by the likelihood that a potentially invasive marine species entrained in a transport vector (e.g., cargo ships) will survive the voyage between origin and destination locations, and establish an independently reproducing population at the destination. Three important risk factors are voyage duration, range of environmental conditions encountered during transit and environmental similarity between origin and destination. In this study, we aimed for a globally comprehensive approach to assembling quantifications of source‐destination risk factors from every potential origin to every potential destination.