The role of connectivity in the interplay between climate change and the spread of alien fish in a large Mediterranean river

Aquatic connectivity: challenges and solutions in a changing climate

The challenge of managing aquatic connectivity in a changing climate is exacerbated in the presence of additional anthropogenic stressors, social factors, and economic drivers. Here we discuss these issues in the context of structural and functional connectivity for aquatic biodiversity, specifically fish, in both the freshwater and marine realms. We posit that adaptive management strategies that consider shifting baselines and the socio-ecological implications of climate change will be required to achieve management objectives. The role of renewable energy expansion, particularly hydropower, is critically examined for its impact on connectivity. We advocate for strategic spatial planning that incorporates nature-positive solutions, ensuring climate mitigation efforts are harmonized with biodiversity conservation. We underscore the urgency of integrating robust scientific modelling with stakeholder values to define clear, adaptive management objectives. Finally, we call for innovative monitoring and predictive decision-making tools to navigate the uncertainties inherent in a changing climate, with the goal of ensuring the resilience and sustainability of aquatic ecosystems.

Evaluating effects of climate change on the spatial distribution of an atypical cavefish Onychostoma macrolepis

Comprehending endangered species' spatial distribution in response to global climate change (GCC) is of great importance for formulating adaptive management, conservation, and restoration plans. However, it is regrettable that previous studies mainly focused on geoclimatic species, while neglected climate-sensitive subterranean taxa to a large extent, which clearly hampered the discovery of universal principles. In view of this, taking the endemic troglophile riverine fish Onychostoma macrolepis (Bleeker, 1871) as an example, we constructed a MaxEnt (maximum-entropy) model to predict how the spatial distribution of this endangered fish would respond to future climate changes (three Global Climate Models × two Shared Socio-economic Pathways × three future time nodes) based on painstakingly collected species occurrence data and a set of bioclimatic variables, including WorldClim and ENVIREM. Model results showed that variables related to temperature rather than precipitation were more important in determining the geographic distribution of this rare and endemic fish. In addition, the suitable areas and their distribution centroids of O. macrolepis would shrink (average: 20,901.75 km2) and move toward the northeast or northwest within the study area (i.e. China). Linking our results with this species' limited dispersion potential and unique habitat requirements (i.e. karst landform is essential), we thus recommended in situ conservation to protect this relict.

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.

Operational methods for prioritizing the removal of river barriers: Synthesis and guidance

Barrier removal can be an efficient method to restore river continuity but resources available for defragmenting rivers are limited and a prioritization strategy is needed. We review methods for prioritizing barriers for removal and report on a survey asking practitioners which barrier prioritization methods they use. Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates where barriers are normally located, their size, age, condition, and likely impacts. Crucially, river fragmentation depends chiefly on the number and location of barriers, not on barrier size, while the costs of barrier removal typically increase with barrier height. Acting on many small barriers will often be more cost-efficient than acting on fewer larger structures. Barriers are not randomly distributed and a small proportion of barriers have a disproportionately high impact on fragmentation, therefore targeting these 'fragmentizers' can result in substantial gains in connectivity. Barrier prioritization methods can be grouped into six main types depending on whether they are reactive or proactive, whether they are applied at local or larger spatial scales, and whether they employ an informal or a formal approach. While mathematical optimization sets the gold standard for barrier prioritization, a hybrid approach that explicitly considers uncertainties and opportunities is likely to be the most effective. The effectiveness of barrier removal can be compromised by inaccurate stream networks, erroneous barrier coordinates, and underestimation of barrier numbers. Such uncertainties can be overcome by ground truthing via river walkovers and predictive modelling, but the cost of collecting additional information must be weighed against the cost of inaction. To increase the success of barrier removal projects, we recommend that barriers considered for removal fulfill four conditions: (1) their removal will bring about a meaningful gain in connectivity; (2) they are cost-effective to remove; (3) they will not cause significant or lasting environmental damage, and (4) they are obsolete structures. Mapping barrier removal projects according to the three axes of opportunities, costs, and gains can help locate any 'low hanging fruit.'

Mountain frog species losing out to climate change around the Sichuan Basin

Amphibians are particularly vulnerable to climate changes that are expected to cause habitat fragmentation and loss and, ultimately, local extirpations. However, little is known about how the interaction between climate change and fragmentation may impede the ability of amphibians to adapt to climate change. Here, we used the iconic mountain frog Quasipaa boulengeri as an indicator species to extrapolate climate-driven shifts in its habitat availability and connectivity in central and southern China according to the minimum and maximum representative concentration pathways. The models projected an average habitat loss of 36%-71% and the in situ and ex situ climate-change refugia to be 29%-64% and 5%-18% of the present-day suitable habitats, respectively. An increase in habitat fragmentation was reflected in a 51% decrease in core patch size, a 9% increase in the mean least-cost path (LCP) length, and a 19% increase in the cost-weighted distance. These climate-driven shifts varied spatially around the Sichuan Basin, with those in the southeast of the Basin being the most pronounced habitat and connectivity losses and those along the Basin being relatively optimistic. The effectiveness of refugia may only be maintained through a narrow passageway along the southern Sichuan Basin because of the presence of LCPs over time. Our results emphasize the need to understand how climate change and connectivity will jointly affect the distribution of mountain amphibians and to accordingly adopt conservation strategies. Further, our findings highlight the importance of identifying and preserving climate-change refugia and habitat connectivity for species persistence and conservation planning.

The Role of Climate Changes in the Spread of Freshwater Fishes: Implications for Alien Cool and Warm-Water Species in a Mediterranean Basin

In running waters, under climate change conditions, the combined effect of water warming and decreasing flow rates may encourage colonisation by invasive cool and warm-water fish species. The aim of the study was to analyze the potential climate change effects on the spread of four invasive alien fishes in the Tiber River basin, taking into account the effects of river fragmentation. Fish and environmental data collected in 91 sites over the years 1998–2018, were used to analyze temporal changes in their habitat requirements. A multivariate analysis was conducted, and the hypothesis of a range expansion towards the upstream reaches has been tested. For Barbus barbus, Gobio gobio, Padogobius bonelli and Pseudorasbora parva population abundances and body condition were analyzed. Detectability, occupancy, local extinction and colonization probabilities were estimated. We showed that B. barbus and P. bonelli have significantly extended their range toward upstream. P. parva did not move toward higher altitudes significantly, suggesting that, at this stage, the species has probably reached an equilibrium. River fragmentation, elevation, water temperature and average current speed seem to be major determinants in colonization processes, affecting the dispersal ability of the species. Not surprisingly for species introduced in relatively recent times, the colonization probabilities were much higher than extinction probabilities. Our results provided evidence for some synergistic effects between climate changes and alien fish species invasions, in terms of species range shifts mediated by rising water temperatures, although they should be interpreted cautiously, taking into account that these species most likely were not yet stabilized.

Increasing climate-driven taxonomic homogenization but functional differentiation among river macroinvertebrate assemblages

Global change is increasing biotic homogenization globally, which modifies the functioning of ecosystems. While tendencies towards taxonomic homogenization in biological communities have been extensively studied, functional homogenization remains an understudied facet of biodiversity. Here, we tested four hypotheses related to long-term changes (1991-2016) in the taxonomic and functional arrangement of freshwater macroinvertebrate assemblages across space and possible drivers of these changes. Using data collected annually at 64 river sites in mainland New Zealand, we related temporal changes in taxonomic and functional spatial β-diversity, and the contribution of individual sites to β-diversity, to a set of global, regional, catchment and reach-scale environmental descriptors. We observed long-term, mostly climate-induced, temporal trends towards taxonomic homogenization but functional differentiation among macroinvertebrate assemblages. These changes were mainly driven by replacements of species and functional traits among assemblages, rather than nested species loss. In addition, there was no difference between the mean rate of change in the taxonomic and functional facets of β-diversity. Climatic processes governed overall population and community changes in these freshwater ecosystems, but were amplified by multiple anthropogenic, topographic and biotic drivers of environmental change, acting widely across the landscape. The functional diversification of communities could potentially provide communities with greater stability, resistance and resilience capacity to environmental change, despite ongoing taxonomic homogenization. Therefore, our study highlights a need to further understand temporal trajectories in both taxonomic and functional components of species communities, which could enable a clearer picture of how biodiversity and ecosystems will respond to future global changes.