Biological invasions are a population-level rather than a species-level phenomenon.

Biological invasions pose a rapidly expanding threat to the persistence, functioning and service provisioning of ecosystems globally, and to socio-economic interests. The stages of successful invasions are driven by the same mechanism that underlies adaptive changes across species in general-via natural selection on intraspecific variation in traits that influence survival and reproductive performance (i.e., fitness). Surprisingly, however, the rapid progress in the field of invasion science has resulted in a predominance of species-level approaches (such as deny lists), often irrespective of natural selection theory, local adaptation and other population-level processes that govern successful invasions. To address these issues, we analyse non-native species dynamics at the population level by employing a database of European freshwater macroinvertebrate time series, to investigate spreading speed, abundance dynamics and impact assessments among populations. Our findings reveal substantial variability in spreading speed and abundance trends within and between macroinvertebrate species across biogeographic regions, indicating that levels of invasiveness and impact differ markedly. Discrepancies and inconsistencies among species-level risk screenings and real population-level data were also identified, highlighting the inherent challenges in accurately assessing population-level effects through species-level assessments. In recognition of the importance of population-level assessments, we urge a shift in invasive species management frameworks, which should account for the dynamics of different populations and their environmental context. Adopting an adaptive, region-specific and population-focused approach is imperative, considering the diverse ecological contexts and varying degrees of susceptibility. Such an approach could improve and refine risk assessments while promoting mechanistic understandings of risks and impacts, thereby enabling the development of more effective conservation and management strategies.

Economic costs of non-native species in Türkiye: A first national synthesis.

Biological invasions are increasingly recognised as a major global change that erodes ecosystems, societal well-being, and economies. However, comprehensive analyses of their economic ramifications are missing for most national economies, despite rapidly escalating costs globally. Türkiye is highly vulnerable to biological invasions owing to its extensive transport network and trade connections as well as its unique transcontinental position at the interface of Europe and Asia. This study presents the first analysis of the reported economic costs caused by biological invasions in Türkiye. The InvaCost database which compiles invasive non-native species' monetary costs was used, complemented with cost searches specific to Türkiye, to describe the spatial and taxonomic attributes of costly invasive non-native species, the types of costs, and their temporal trends. The total economic cost attributed to invasive non-native species in Türkiye (from 202 cost reporting documents) amounted to US$ 4.1 billion from 1960 to 2022. However, cost data were only available for 87 out of 872 (10%) non-native species known for Türkiye. Costs were biased towards a few hyper-costly non-native taxa, such as jellyfish, stink bugs, and locusts. Among impacted sectors, agriculture bore the highest total cost, reaching US$ 2.85 billion, followed by the fishery sector with a total cost of US$ 1.20 billion. Management (i.e., control and eradication) costs were, against expectations, substantially higher than reported damage costs (US$ 2.89 billion vs. US$ 28.4 million). Yearly costs incurred by non-native species rose exponentially over time, reaching US$ 504 million per year in 2020-2022 and are predicted to increase further in the next 10 years. A large deficit of cost records compared to other countries was also shown, suggesting a larger monetary underestimate than is typically observed. These findings underscore the need for improved cost recording as well as preventative management strategies to reduce future post-invasion management costs and help inform decisions to manage the economic burdens posed by invasive non-native species. These insights further emphasise the crucial role of standardised data in accurately estimating the costs associated with invasive non-native species for prioritisation and communication purposes.

Recent advances in availability and synthesis of the economic costs of biological invasions.

Biological invasions are a global challenge that has received insufficient attention. Recently available cost syntheses have provided policy- and decision makers with reliable and up-to-date information on the economic impacts of biological invasions, aiming to motivate effective management. The resultant InvaCost database is now publicly and freely accessible and enables rapid extraction of monetary cost information. This has facilitated knowledge sharing, developed a more integrated and multidisciplinary network of researchers, and forged multidisciplinary collaborations among diverse organizations and stakeholders. Over 50 scientific publications so far have used the database and have provided detailed assessments of invasion costs across geographic, taxonomic, and spatiotemporal scales. These studies have provided important information that can guide future policy and legislative decisions on the management of biological invasions while simultaneously attracting public and media attention. We provide an overview of the improved availability, reliability, standardization, and defragmentation of monetary costs; discuss how this has enhanced invasion science as a discipline; and outline directions for future development.

Invasion impacts and dynamics of a European-wide introduced species.

Globalization has led to the introduction of thousands of alien species worldwide. With growing impacts by invasive species, understanding the invasion process remains critical for predicting adverse effects and informing efficient management. Theoretically, invasion dynamics have been assumed to follow an "invasion curve" (S-shaped curve of available area invaded over time), but this dynamic has lacked empirical testing using large-scale data and neglects to consider invader abundances. We propose an "impact curve" describing the impacts generated by invasive species over time based on cumulative abundances. To test this curve's large-scale applicability, we used the data-rich New Zealand mud snail Potamopyrgus antipodarum, one of the most damaging freshwater invaders that has invaded almost all of Europe. Using long-term (1979-2020) abundance and environmental data collected across 306 European sites, we observed that P. antipodarum abundance generally increased through time, with slower population growth at higher latitudes and with lower runoff depth. Fifty-nine percent of these populations followed the impact curve, characterized by first occurrence, exponential growth, then long-term saturation. This behaviour is consistent with boom-bust dynamics, as saturation occurs due to a rapid decline in abundance over time. Across sites, we estimated that impact peaked approximately two decades after first detection, but the rate of progression along the invasion process was influenced by local abiotic conditions. The S-shaped impact curve may be common among many invasive species that undergo complex invasion dynamics. This provides a potentially unifying approach to advance understanding of large-scale invasion dynamics and could inform timely management actions to mitigate impacts on ecosystems and economies.

The economic costs, management and regulation of biological invasions in the Nordic countries.

A collective understanding of economic impacts and in particular of monetary costs of biological invasions is lacking for the Nordic region. This paper synthesizes findings from the literature on costs of invasions in the Nordic countries together with expert elicitation. The analysis of cost data has been made possible through the InvaCost database, a globally open repository of monetary costs that allows for the use of temporal, spatial, and taxonomic descriptors facilitating a better understanding of how costs are distributed. The total reported costs of invasive species across the Nordic countries were estimated at $8.35 billion (in 2017 US$ values) with damage costs significantly outweighing management costs. Norway incurred the highest costs ($3.23 billion), followed by Denmark ($2.20 billion), Sweden ($1.45 billion), Finland ($1.11 billion) and Iceland ($25.45 million). Costs from invasions in the Nordics appear to be largely underestimated. We conclude by highlighting such knowledge gaps, including gaps in policies and regulation stemming from expert judgment as well as avenues for an improved understanding of invasion costs and needs for future research.

Two centuries for an almost complete community turnover from native to non-native species in a riverine ecosystem.

Non-native species introductions affect freshwater communities by changing community compositions, functional roles, trait occurrences and ecological niche spaces. Reconstructing such changes over long periods is difficult due to limited data availability. We collected information spanning 215 years on fish and selected macroinvertebrate groups (Mollusca and Crustacea) in the inner-Florentine stretch of the Arno River (Italy) and associated water grid, to investigate temporal changes. We identified an almost complete turnover from native to non-native fish (1800: 92% native; 2015: 94% non-native species) and macroinvertebrate species (1800: 100% native; 2015: 70% non-native species). Non-native fish species were observed ~50 years earlier compared to macroinvertebrate species, indicating phased invasion processes. In contrast, α-diversity of both communities increased significantly following a linear pattern. Separate analyses of changes in α-diversities for native and non-native species of both fish and macroinvertebrates were nonlinear. Functional richness and divergence of fish and macroinvertebrate communities decreased non-significantly, as the loss of native species was compensated by non-native species. Introductions of non-native fish and macroinvertebrate species occurred outside the niche space of native species. Native and non-native fish species exhibited greater overlap in niche space over time (62%-68%) and non-native species eventually replaced native species. Native and non-native macroinvertebrate niches overlapped to a lesser extent (15%-30%), with non-natives occupying mostly unoccupied niche space. These temporal changes in niche spaces of both biotic groups are a direct response to the observed changes in α-diversity and species turnover. These changes are potentially driven by deteriorations in hydromorphology as indicated by alterations in trait modalities. Additionally, we identified that angling played a considerable role for fish introductions. Our results support previous findings that the community turnover from native to non-native species can be facilitated by, for example, deteriorating environmental conditions and that variations in communities are multifaceted requiring more indicators than single metrics.

Invasion costs, impacts, and human agency: response to Sagoff 2020.

Article impact statement: In an era of profound biodiversity crisis, invasion costs, invader impacts, and human agency should not be dismissed.

Overlapping niches between two co-occurring invasive fish: the topmouth gudgeon Pseudorasbora parva and the common bleak Alburnus alburnus.

Invasive fish species impact aquatic ecosystems and modify native communities, often leading to a decline in local species. These ecological impacts include the transmission of pathogens, predation, competition as well as hybridization. Two invasive fish species, the common bleak Alburnus alburnus and the topmouth gudgeon Pseudorabora parva, have both been recently found co-occurring in several regions of southern Europe, such as the Italian Arno River. Nonetheless, the trophic relationships among invasive fish species, especially cyprinids, remain poorly understood, and no studies have reported the trophic interaction between these two species. This study compared length-weight relationship and used stomach content and stable isotope analysis of two co-occurring populations in the Arno River to characterize the growth and overlap of potential trophic niches. It also found similar allometric growth in both species, a wider generalist trophic niche for P. parva and a more specialized niche for A. alburnus. A considerable niche overlap was found, suggesting that feeding competition can occur if resources were to be limited. Moreover, the niche of P. parva was more likely to overlap with that of A. alburnus than vice versa, suggesting that P. parva can be considered as a potential over competitor. Nonetheless, the authors found in the overlapping populations no evidence of realized competition, probably avoided through a combination of fine-scale mechanisms. They also highlighted that these two invasive species can co-exist and share resources, at least in an open ecosystem like a river, thus potentially doubling up their trophic impact on local communities.

Assessing the potential phytosanitary threat of the house cricket Acheta domesticus.

Phytosanitary threats can pose substantial risks to global agriculture and ecological systems, affecting biodiversity, human well-being, and food security. Meanwhile, global warming is projected to exacerbate these threats in the future. One in Europe already widely distributed potential phytosanitary threat that may benefit from global warming is the house cricket Acheta domesticus. This study explored the potential of A. domesticus as a relevant non-native phytosanitary threat under changing climatic conditions by conducting a series of functional response experiments across a temperature gradient (20, 25, and 30 °C). Acheta domesticus exhibited comparable patterns of seed consumption and functional responses. Seed type (millet seeds, wheat grains) and temperature increase influenced the damage inflicted on seeds, with softer and smaller seeds being more susceptible to damage, further amplified by warmer temperatures. The study's outcomes underline the phytosanitary threat that A. domesticus may pose. Considering the species' established presence and adaptable nature in urban environments exacerbates the potential for A. domesticus to transition to rural and agricultural areas. Its increasing production as a food item, paired with the here-identified potential to damage seeds, emphasizes the need for proactive and science-based strategies to address emerging phytosanitary threats driven by non-native species under changing climatic conditions. As global temperatures continue to rise, the assessment and management of potential pest species like A. domesticus will be crucial for safeguarding agriculture productivity and ecological balance.

Economic impact disharmony in global biological invasions.

A dominant syndrome of the Anthropocene is the rapid worldwide spread of invasive species with devastating environmental and socio-economic impacts. However, the dynamics underlying the impacts of biological invasions remain contested. A hypothesis posits that the richness of impactful invasive species increases proportionally with the richness of non-native species more generally. A competing hypothesis suggests that certain species features disproportionately enhance the chances of non-native species becoming impactful, causing invasive species to arise disproportionately relative to the numbers of non-native species. We test whether invasive species with reported monetary costs reflect global numbers of established non-native species among phyla, classes, and families. Our results reveal that numbers of invasive species with economic costs largely reflect non-native species richness among taxa (i.e., in 96 % of families). However, a few costly taxa were over- and under-represented, and their composition differed among environments and regions. Chordates, nematodes, and pathogenic groups tended to be the most over-represented phyla with reported monetary costs, with mammals, insects, fungi, roundworms, and medically-important microorganisms being over-represented classes. Numbers of costly invasive species increased significantly with non-native richness per taxon, while monetary cost magnitudes at the family level were also significantly related to costly invasive species richness. Costs were biased towards a few 'hyper-costly' taxa (such as termites, mosquitoes, cats, weevils, rodents, ants, and asters). Ordination analysis revealed significant dissimilarity between non-native and costly invasive taxon assemblages. These results highlight taxonomic groups which harbour disproportionately high numbers of costly invasive species and monetary cost magnitudes. Collectively, our findings support prevention of arrival and containment of spread of non-native species as a whole through effective strategies for mitigation of the rapidly amplifying impacts of invasive species. Yet, the hyper- costly taxa identified here should receive greater focus from managers to reduce impacts of current invasive species.

The Plasticene era: Current uncertainties in estimates of the hazards posed by tiny plastic particles on soils and terrestrial invertebrates.

Plastics are ubiquitous in our daily life. Large quantities of plastics leak in the environment where they weather and fragment into micro- and nanoparticles. This potentially releases additives, but rarely leads to a complete mineralization, thus constitutes an environmental hazard. Plastic pollution in agricultural soils currently represents a major challenge: quantitative data of nanoplastics in soils as well as their effects on biodiversity and ecosystem functions need more attention. Plastic accumulation interferes with soil functions, including water dynamics, aeration, microbial activities, and nutrient cycling processes, thus impairing agricultural crop yield. Plastic debris directly affects living organisms but also acts as contaminant vectors in the soils, increasing the effects and the threats on biodiversity. Finally, the effects of plastics on terrestrial invertebrates, representing major taxa in abundance and diversity in the soil compartment, need urgently more investigation from the infra-individual to the ecosystem scales.

Exploring invasiveness and versatility of used microhabitats of the globally invasive Gambusia holbrooki.

Non-native species can lead to severe impacts on invaded ecosystems, including the decline of ecosystem function through deleterious impacts on species diversity. The successful establishment of non-native species in new environments is the first barrier a species must overcome, ultimately depending on its ability to either cope with or adapt to local site-specific conditions. Despite the widespread distribution and ecological consequences of many freshwater invaders, site-specific and climatic preferences are often unknown. This is also the case of the Eastern mosquitofish Gambusia holbrooki, a global invader considered as a pervasive threat to endemic species. Here, we determined the ecological features and preferred site-specific conditions of G. holbrooki in Türkiye, which spans a wide range of diverse biogeographically distinct ecosystems by surveying populations from 130 localities in 2016 and 2017. Gambusia holbrooki were detected by hand-net in 48 of these sites (19 lotic, 29 lentic). It showed a preference for shallow waters with medium sized rocks, and abundances differed spatially across a latitudinal gradient and was influenced predominantly by variations in pH. The only other factors predicting its presence were low current velocities and gravel substrate, highlighting its ecological versatility in utilising a wide range of microhabitats. Bioclimatic models suggest that G. holbrooki is found in areas with a wide average annual temperature ranging from 10 to 20 °C, but with temperature not being a limiting factor to its invasion. Gambusia holbrooki shows a preference for xeric freshwater ecosystems and endorheic basins, as well as temperate coastal rivers, temperate upland rivers, temperate floodplain rivers and wetlands, and tropical and subtropical coastal rivers. These results, particularly the wide occurrence with only few limiting factors, emphasise the invasion potential of mosquitofish and should substantiate the need for localised invasive species management and conservation efforts, particularly in smaller or insular areas where mosquitofish and endemic fish species co-exist.

Assessing the role of non-native species and artificial water bodies on the trophic and functional niche of Mediterranean freshwater fish communities.

Habitat alterations and the introduction of non-native species have many ecological impacts, including the loss of biodiversity and a deterioration of ecosystem functioning. The effects of these combined stressors on the community trophic web and functional niche are, however, not completely clear. Here, we investigated how artificial ecosystems (i.e. reservoirs) and non-native species may influence the trophic and functional niche space of freshwater fish communities. To do so, we used carbon and nitrogen stable isotope and abundance data to compute a set of isotopic, trait, and functional metrics for 13 fish communities sampled from 12 distinct ecosystems in Türkiye. We show that in reservoirs, fish were more similar in their trophic niche compared to lakes, where the trophic niche was more variable, due to higher habitat complexity. However, there were no differences in the trait and functional metrics between the two ecosystem types, suggesting a higher prey diversity than assumed in reservoirs. We also found that the number of non-native species did not affect the trophic niche space, nor the trait or functional space occupied by the fish community. This indicates that non-native species tended to overlap their trophic niche with native species, while occupying empty functional niches in the recipient community functional space. Similarly, the proportion of non-native species did not affect any trophic, trait, or functional metric, suggesting that changes in community composition were not reflected in changes in the community niche space. Moreover, we found that trait richness, but not functional richness, was positively related to the isotopic niche width and diversity, indicating that a wider occupied trait niche space corresponded with a wider occupied trophic niche and lesser interspecific similarity. Our findings underscore the complexity of ecological relationships within freshwater ecosystems and highlight the need for comprehensive management strategies to mitigate the impacts of human activities and biological invasions.

The wild cost of invasive feral animals worldwide.

Invasive non-native species are a growing burden to economies worldwide. While domesticated animals (i.e. livestock, beasts of burden or pets) have enabled our ways of life and provide sustenance for countless individuals, they may cause substantial impacts when they escape or are released (i.e. become feral) and then become invasive with impacts. We used the InvaCost database to evaluate monetary impacts from species in the Domestic Animal Diversity Information System database. We found a total cost of $141.95 billion from only 18 invasive feral species. Invasive feral livestock incurred the highest costs at $90.03 billion, with pets contributing $50.93 billion and beasts of burden having much lower costs at $0.98 billion. Agriculture was the most affected sector at $80.79 billion, followed by the Environment ($43.44 billion), and Authorities-Stakeholders sectors ($5.52 billion). Damage costs comprised the majority ($124.94 billion), with management and mixed damage-management costs making up the rest ($9.62 and $7.38 billion, respectively). These economic impacts were observed globally, where Oceania, North America and Europe were the most impacted regions. Islands recorded a higher economic burden than continental areas, with livestock species dominating costs more on islands than mainlands compared to other feral species. The costs of invasive feral animals were on average twice higher than those of wild species. The management of invasive feral populations requires higher investment, updated regulations, and comprehensive risk assessments. These are especially complex when considering the potential conflicts arising from interventions with species that have close ties to humans. Effective communication to raise public awareness of the impacts of feral populations and appropriate legislation to prevent or control such invasive feral populations will substantially contribute to minimizing their socioeconomic and environmental impacts.

Multidecadal data indicate increase of aquatic insects in Central European streams.

In recent years, declining insect biodiversity has sparked interest among scientists and drawn the attention of society and politicians. However, our understanding of the extent of this decline is incomplete, particularly for freshwater insects that provide a key trophic link between aquatic and terrestrial ecosystems, but that are also especially vulnerable to climate change. To investigate the response of freshwater insects to climate change, we quantified shifts in insect abundance and diversity across 7264 samples covering Central Europe during 1990-2018 and related these changes to annual data on temperature and precipitation. We observed both increases in richness (10.6 %) and abundance (9.5 %) of freshwater insects over the past three decades. These changes were related to increases in summer temperature and summer precipitation, which had negative effects on species richness, and to increases in winter temperature and precipitation, which had positive effects. Further we found that increased temperature was generally related to increased abundance, whereas increased precipitation was associated with declines, thus highlighting the particularly varying impacts on differing insect orders. Given that freshwater insects have been more severely affected by global change than marine and terrestrial species, the observed increases are a positive sign, but the overall situation of freshwater invertebrates is still critical.

Long-term trends and drivers of biological invasion in Central European streams.

Rates of biological invasion continue to accelerate and threaten the structure and function of ecosystems worldwide. High habitat connectivity, multiple pathways, and inadequate monitoring have rendered aquatic ecosystems vulnerable to species introductions. Past riverine invasion dynamics were largely restricted to large rivers, leaving out smaller rivers that commonly harbour high freshwater biodiversity. Moreover, biodiversity time series have rarely been used to investigate invasions across larger spatial-temporal scales, limiting our understanding of aquatic invasion dynamics. Here, we used 6067 benthic invertebrate samples from streams and small rivers from the EU Water Framework Directive monitoring program collected across Central Europe between 2000 and 2018 to assess temporal changes to benthic invertebrate communities as well as non-native species. We assessed invasion rates according to temperature, precipitation, elevation, latitude, longitude, and stream type. Overall, average daily temperatures significantly increased by 0.02 °C per annum (0.34 °C in total) while annual precipitation significantly decreased by 0.01 mm per annum (-67.8 mm over the study period), paralleled with significant increases in overall species richness (12.3 %) and abundance (14.9 %); water quality was relatively stable. Non-native species richness increased 5-fold and abundance 40-fold, indicating an ongoing community shift from native to non-native species. The observed increase in invasions was stronger in low mountain rivers compared to low mountain streams, with the share of non-native species abundance and richness declining with increasing elevation and latitude but increasing with temperature. We found thermophilic non-native species invasion success was greatest in larger sized streams, at lower latitudes, lower elevations and higher temperatures. These results indicate that widespread environmental characteristics (i.e., temperature) could heighten invasion success and confer refuge effects (i.e., elevation and latitude) in higher sites. High altitude and latitude environments should be prioritised for prevention efforts, while biosecurity and management should be improved in lowland areas subject to greater anthropogenic pressure, where non-native introductions are more likely.

Towards effective management of the marine-origin Prymnesium parvum (Haptophyta): A growing concern in freshwater reservoirs?

Freshwater ecosystems are highly susceptible to harmful algal blooms (HABs), which are often caused by monospecific dense blooms. Effective preventive management strategies are urgently needed to avoid wide-ranging and severe impacts often resulting in costly damage to resources and unsustainable management options. In this study, we utilized SDM techniques focused on Prymnesium parvum, one of the most notorious HABs species worldwide. We first compare the climatic space occupied by P. parvum in North America, Europe and Australia. Additionally, we use MaxEnt algorithm to infer, for the first time, the potentially suitable freshwater environments in the aforementioned ranges. We also discuss the risks of invasion in reservoirs - prone habitats to persistent blooms of pests and invasive phytoplanktonic species. Our results show populations with distinctive niches suggesting ecophysiological tolerances, perhaps reflecting different strains. Our model projections revealed that the potential extent for P. parvum invasions is much broader than its current geographic distribution. The spatial configuration of reservoirs, if not sustaining dense blooms due to non-optimal conditions, favors colonization of multiple basins and ecoregions not yet occupied by P. parvum. Our models can provide valuable insights to decision-makers and monitoring programs while reducing the resources required to control the spread of P. parvum in disturbed habitats. Lastly, as impact magnitude is influenced by toxicity which in turn varies between different strains, we suggest future studies to incorporate intraspecific genetic information and fine-scale environmental variables to estimate potential distribution of P. parvum.

Sigmoidal curves reflect impacts and dynamics of aquatic invasive species.

Identifying general patterns and trends underlying the impacts and dynamics of biological invasions has proven elusive for scientists. Recently, the impact curve was proposed as a means to predict temporal impacts of invasive alien species, characterised by a sigmoidal growth pattern with an initial exponential increase, followed by a subsequent rate of decline and approaching a saturation level in the long-term where impact is maximised. While the impact curve has been empirically demonstrated with monitoring data of a single invasive alien species (the New Zealand mud snail, Potamopyrgus antipodarum), broadscale applicability remains to be tested for other taxa. Here, we examined whether the impact curve can adequately describe the invasion dynamics of 13 other aquatic species (within Amphipoda, Bivalvia, Gastropoda, Hirudinea, Isopoda, Mysida, and Platyhelminthes) at the European level, employing multi-decadal time series of macroinvertebrate cumulative abundances from regular benthic monitoring efforts. For all except one tested species (the killer shrimp, Dikerogammarus villosus), the sigmoidal impact curve was strongly supported (R2 > 0.95) on a sufficiently long time-scale. For D. villosus, the impact had not yet reached saturation, likely reflecting the ongoing European invasion. The impact curve facilitated estimation of introduction years and lag phases, as well as parameterisation of growth rates and carrying capacities, providing strong support for the boom-bust dynamics typically observed in several invader populations. These findings suggest that impact can grow rapidly before saturating at a high level, with timely monitoring often lacking for the detection of invasive alien species post-introduction. We further confirm the applicability of the impact curve to determine trends in invasion stages, population dynamics, and impacts of pertinent invaders, ultimately helping inform the timing of management interventions. We hence call for improved monitoring and reporting of invasive alien species over broad spatio-temporal scales to permit further testing of large-scale impact consistencies across various habitat types.

Strong temporal variation of consumer δ13C value in an oligotrophic reservoir is related to water level fluctuation.

Using stable carbon and nitrogen isotope analysis (δ13C and δ15N) to assess trophic interactions in freshwater ecosystems is a well established method, providing insight into ecosystem functioning. However, the spatial and temporal variability of isotope values, driven by environmental fluctuation is poorly understood and can complicate interpretations. We investigated how the temporal variation of stable isotopes in consumers (fish, crayfish and macrozoobenthos) of a canyon-shaped oligotrophic reservoir is associated with environmental factors such as water temperature, transparency, flooded area, and water quality measures. Consumers and their putative food sources were sampled and analyzed for carbon and nitrogen stable isotopes annually, and environmental parameters were measured monthly from 2014 to 2016. Results revealed significant differences in δ13C and δ15N values in each consumer among studied years. Over the years, fish and crayfish expressed differences in δ13C between 3 and 5‰, whereas in zoobenthos differences were 12‰. Variability in δ15N was similar across all consumers (2-4‰). Moreover, results suggest that the flooded area of the reservoir was a major driver of δ13C stable isotope values variation in consumers, while variation in δ15N was not linked to any of the studied environmental factors. Bayesian mixing models further showed significant changes in the origin of detritivorous zoobenthos carbon sources (reversal shift from terrestrial detritus to algae origin) between years with low water level to years with the standard water level. Other species showed only slight differences in food source utilization among years. Our study highlights the importance of environmental factors as sources of variation in consumer's stable isotope values which should be considered especially when studied ecosystem strongly fluctuate in some environmental factor.

Alien species and climate change drive shifts in a riverine fish community and trait compositions over 35 years.

Alien fish substantially impact aquatic communities. However, their effects on trait composition remain poorly understood, especially at large spatiotemporal scales. Here, we used long-term biomonitoring data (1984-2018) from 31 fish communities of the Rhine river in Germany to investigate compositional and functional changes over time. Average total community richness increased by 49 %: it was stable until 2004, then declined until 2010, before increasing until 2018. Average abundance decreased by 9 %. Starting from 198 individuals/m2 in 1984 abundance largely declined to 23 individuals/m2 in 2010 (-88 %), and then consequently increased by 678 % up to 180 individuals/m2 until 2018. Increases in abundance and richness starting around 2010 were mainly driven by the establishment of alien species: while alien species represented 5 % of all species and 0.1 % of total individuals in 1993, it increased to 30 % (7 species) and 32 % of individuals in 2018. Concomitant to the increase in alien species, average native species richness and abundance declined by 26 % and 50 % respectively. We identified increases in temperature, precipitation, abundance and richness of alien fish driving compositional changes after 2010. To get more insights on the impacts of alien species on fish communities, we used 12 biological and 13 ecological traits to compute four trait metrics each. Ecological trait dispersion increased before 2010, probably due to diminishing ecologically similar native species. No changes in trait metrics were measured after 2010, albeit relative shares of expressed trait modalities significantly changing. The observed shift in trait modalities suggested the introduction of new species carrying similar and novel trait modalities. Our results revealed significant changes in taxonomic and trait compositions following alien fish introductions and climatic change. To conclude, our analyses show taxonomic and functional changes in the Rhine river over 35 years, likely indicative of future changes in ecosystem services.