Improved river continuity facilitates fishes' abilities to track future environmental changes

Neglecting biodiversity baselines in longitudinal river connectivity restoration impacts priority setting

River habitats are fragmented by barriers which impede the movement and dispersal of aquatic organisms. Restoring habitat connectivity is a primary objective of nature conservation plans with multiple efforts to strategically restore connectivity at local, regional, and global scales. However, current approaches to prioritize connectivity restoration do not typically consider how barriers spatially fragment species' populations. Additionally, we lack knowledge on biodiversity baselines to predict which species would find suitable habitat after restoring connectivity. In this paper, we asked how neglecting these biodiversity baselines in river barrier removals impacts priority setting for conservation planning. We applied a novel modelling approach combining predictions of species distributions with network connectivity models to prioritize conservation actions in rivers of the Rhine-Aare system in Switzerland. Our results show that the high number and density of barriers has reduced structural and functional connectivity across representative catchments within the system. We show that fragmentation decreases habitat suitability for species and that using expected distributions as biodiversity baselines significantly affects priority settings for connectivity restorations compared to species-agnostic metrics based on river length. This indicates that priorities for barrier removals are ranked higher within the expected distributions of species to maximize functional connectivity while barriers in unsuitable regions are given lower importance scores. Our work highlights that the joint consideration of existing barriers and species past and current distributions are critical for restoration plans to ensure the recovery and persistence of riverine fish diversity.

Local environment and fragmentation by drought and damming shape different components of native and non-native fish beta diversity across pool refuges

Pool refuges are critical for maintaining stream fish diversity in increasingly intermittent streams. Yet, the patterns and drivers of beta diversity of native and non-native fish in pool refuges remain poorly known. Focusing on Mediterranean streams, we decomposed beta diversity of native and non-native fish into richness difference (RichDiff) and species replacement (Repl), and local (LCBD, LCBDRichDiff and LCBDRepl) and species (SCBD) contributions. We assessed the influence of environmental and spatial factors associated with drought and damming fragmentations on beta diversity components and LCBDs, and of local species richness and occupancy on LCBDs and SCBD, respectively. Overall, non-native species showed a more limited occupancy of pool refuges than native fish. RichDiff dominated beta diversity, though it was influenced by drought and damming fragmentations for native fish and local environment for non-native fish. Repl for native fish was slightly influenced by local environment, but for non-native fish was largely driven by drought and damming, albeit with a contribution of local environment as well. LCBD and LCBDRichDiff increased in pools in low order streams for native fish and at low elevations for non-native fish, and with high or low species richness. SCBD was higher for native species with intermediated pool occupancy, but for non-native species with low occupancy. Our results suggest that stream fragmentation may drive native species loss and non-native species replacement in pool refuges, and that environmental filtering may shape non-native species loss. Pools in lower order streams harbouring unique species-rich or species-poor assemblages should be prioritize for conservation and restoration, respectively, and pools at low elevation with unique non-native assemblages should deserve control efforts. We encourage the partitioning of beta diversity and individual analysis of native and non-native fish in intermittent streams, which may be key in stressing the importance of pool refuges in safeguarding native fish diversity.

Unifying climate change biology across realms and taxa

A major challenge in modern biology is to understand extinction risk from climate change across all realms. Recent research has revealed that physiological tolerance, behavioral thermoregulation, and small elevation shifts are dominant coping strategies on land, whereas large-scale latitudinal shifts are more important in the ocean. Freshwater taxa may face the highest global extinction risks. Nevertheless, some species in each realm face similar risks because of shared adaptive, dispersal, or physiological tolerances and abilities. Taking a cross-realm perspective offers unique research opportunities because confounding physical factors in one realm are often disaggregated in another realm. Cross-realm, across taxa, and other forms of climate change biology synthesis are needed to advance our understanding of emergent patterns of risk across all life.

Restoration recommendations for mitigating habitat fragmentation of a river corridor

Flow discharge and anthropogenic activities influence the composition and configuration of habitat patches in river ecosystems. Understanding the response of habitat landscapes and the corresponding fish habitat quality is crucial for river management. We investigated the reaction of fish habitat suitability and variant flow discharge performance in examining aquatic habitat patch fragmentation. The hydraulic simulation and fish habitat calculation were used to determine the flow characteristics, habitat conditions, and river landscapes. FRAGSTATS was applied to explore the composition and configuration of habitat patches. Cluster analysis and logistic regression were employed to compute the spatiotemporal variabilities of riverscape indices and establish the relationship between riverscape attributes and fish habitat quality. The results indicate that the changes in specific habitat features are associated with the riverscape indices of total edge (TE), mean nearest-neighbor distance (MNN), interspersion and juxtaposition index (IJI), mean patch size (MPS), and area-weighted mean patch fractal dimension (AWMPFD). The flow discharge is the key to determining habitat fragmentation in rivers, with natural barriers occurring at low flow. In contrast, weirs are anthropogenic obstacles that have significant adverse effects on the downstream corridor. A priority restoration activity to conserve river habitat is to create refuge pools during dry seasons by modifying channel morphology. The positive correlation between habitat suitability and MPS and the negative relationship between habitat suitability and AWMPFD highlight the patch size and shape complexity that are critical indices for pool creation. The prediction of the landscape attributes of the outcomes under different scenarios could support the decision-making in river management. The innovative integrated method presented in this study provides a solid foundation and supports the implementation of nature-based solutions for sustainable river management.

Improving river hydromorphological assessment through better integration of riparian vegetation: Scientific evidence and guidelines

River hydromorphology has long been subjected to huge anthropogenic pressures with severe negative impacts on related ecosystems' functioning and water quality. Therefore, improving river hydromorphological conditions represents a priority task in sustainable river management and requires proper assessment tools. It is well known that riparian vegetation plays a crucial role in sustaining river hydromorphological conditions. However, it has been nearly neglected in most hydromorphological assessment protocols, including the European Water Framework Directive (WFD). This paper reviews and synthesizes the relevance of riparian vegetation for river hydromorphology, focusing on its contribution to streamflow and sediment regime conditions. We also examine how riparian vegetation is considered in the WFD and how it is included in national hydromorphological protocols currently in use. Our findings point to a temporal mismatch between the date when the WFD came into force and the emergence of scientific and technologic advances in riparian vegetation dynamism and bio-geomorphic modeling. To overcome this misalignment, we present promising approaches for the characterization and assessment of riparian vegetation, which include the identification of vegetation units and indicators at multiple scales to support management and restoration measures. We discuss the complexity of riparian vegetation assessment, particularly with respect to the establishment of river-type-based reference conditions and the monitoring and management targets, and propose some attributes that can serve as novel indicators of the naturalness vs. artificiality of riparian vegetation. We argue that the hydromorphological context of the WFD should be revisited and offer guidance to integrate riparian vegetation in river hydromorphological monitoring and assessment.

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

Understanding how global change and connectivity will jointly modify the distribution of riverine species is crucial for conservation biology and environmental management. However, little is known about the interaction between climate change and fragmentation and how movement barriers might impede native species from adjusting their distributions versus limit the further spread of alien species. In this study, we modelled the current and future distributions of 11 native and five alien fishes in the large and heavily fragmented Ebro River, located within the Mediterranean region, which has many freshwater endemics severely threatened by global change. We considered 10 climate change models and five modelling algorithms and assessed the effects of connectivity on the accessibility of future suitable habitats. Thereby, we identify most conflict-prone river reaches, that is, where barriers pose a particular trade-off between isolating and negatively impacting native species versus potentially reducing the risk of alien species spread. Our results projected upstream habitat shifts for the vast majority of the species. Climate change affected species differently, with alien species generally showing larger habitat gains compared to natives. Most pronounced distributional changes (i.e. losses of native species and gains of alien species) and compositional turnover might be expected in the lower and mid reaches of large tributaries of the Ebro River. The role of anthropogenic barriers in this context is often ambiguous but rather unfavourable, as they not only restrict native fishes but also alter stream habitats and flow conditions. However, with our spatial modelling framework, we could identify specific river reaches where the connectivity trade-off in the context of climate change is particularly relevant. Overall, our findings emphasize the importance of the complex effects that climate change, riverine connectivity and alien species are expected to impose on river communities and the urgent need to adapt management strategies accordingly.

The combined effects of climate change and river fragmentation on the distribution of Andean Amazon fishes

Upstream range shifts of freshwater fishes have been documented in recent years due to ongoing climate change. River fragmentation by dams, presenting physical barriers, can limit the climatically induced spatial redistribution of fishes. Andean freshwater ecosystems in the Neotropical region are expected to be highly affected by these future disturbances. However, proper evaluations are still missing. Combining species distribution models and functional traits of Andean Amazon fishes, coupled with dam locations and climatic projections (2070s), we (a) evaluated the potential impacts of future climate on species ranges, (b) investigated the combined impact of river fragmentation and climate change and (c) tested the relationships between these impacts and species functional traits. Results show that climate change will induce range contraction for most of the Andean Amazon fish species, particularly those inhabiting highlands. Dams are not predicted to greatly limit future range shifts for most species (i.e., the Barrier effect). However, some of these barriers should prevent upstream shifts for a considerable number of species, reducing future potential diversity in some basins. River fragmentation is predicted to act jointly with climate change in promoting a considerable decrease in the probability of species to persist in the long-term because of splitting species ranges in smaller fragments (i.e., the Isolation effect). Benthic and fast-flowing water adapted species with hydrodynamic bodies are significantly associated with severe range contractions from climate change.

Landscape resistance mediates native fish species distribution shifts and vulnerability to climate change in riverscapes

A broader understanding of how landscape resistance influences climate change vulnerability for many species is needed, as is an understanding of how barriers to dispersal may impact vulnerability. Freshwater biodiversity is at particular risk, but previous studies have focused on popular cold-water fishes (e.g., salmon, trout, and char) with relatively large body sizes and mobility. Those fishes may be able to track habitat change more adeptly than less mobile species. Smaller, less mobile fishes are rarely represented in studies demonstrating effects of climate change, but depending on their thermal tolerance, they may be particularly vulnerable to environmental change. By revisiting 280 sites over a 20 year interval throughout a warming riverscape, we described changes in occupancy (i.e., site extirpation and colonization probabilities) and assessed the environmental conditions associated with those changes for four fishes spanning a range of body sizes, thermal and habitat preferences. Two larger-bodied trout species exhibited small changes in site occupancy, with bull trout experiencing a 9.2% (95% CI = 8.3%-10.1%) reduction, mostly in warmer stream reaches, and westslope cutthroat trout experiencing a nonsignificant 1% increase. The small-bodied cool water slimy sculpin was originally distributed broadly throughout the network and experienced a 48.0% (95% CI = 42.0%-54.0%) reduction in site occupancy with declines common in warmer stream reaches and areas subject to wildfire disturbances. The small-bodied comparatively warmer water longnose dace primarily occupied larger streams and increased its occurrence in the lower portions of connected tributaries during the study period. Distribution shifts for sculpin and dace were significantly constrained by barriers, which included anthropogenic water diversions, natural step-pools and cascades in steeper upstream reaches. Our results suggest that aquatic communities exhibit a range of responses to climate change, and that improving passage and fluvial connectivity will be important climate adaptation tactics for conserving aquatic biodiversity.

Predicting impacts of future climate change and hydropower development towards habitats of native and non-native fishes

Climate change and hydropower development are two primary stressors affecting riverine ecosystems and both stressors facilitate invasions by non-native species. However, little study has focused on how habitats of native and non-native fishes may be affected by independent or combined impacts of such stressors. Here we used the Jinsha River as an example to predict habitat change and distributional shift of native and non-native fishes with species distribution models. The Jinsha River Basin has nearly 40 cascade dams constructed or planned and located in the Tibetan Plateau, which is sensitive to future climate change. Two climate change scenarios and future hydropower development were combined to produce five scenarios of future changes. Under the impacts of independent extreme climate change or hydropower development, non-native fishes showed greater habitat gain in total, while native fishes shifted their distribution into tributaries and higher elevations, and impacts were stronger in combined scenarios. Habitat overlap between the two groups also increased in future scenarios. Certain fish traits correlated with stressors in habitat change prediction. River basins with hydropower development were shown to face higher risk of non-native fishes invasion under future climate change. As the most biodiverse river basins globally are threatened by hydropower development, our results emphasize the importance of regulating non-native fish introduction in reservoirs. Our approaches are also applicable to other systems globally to better understand how hydropower development and climate change may increase invasion risk, and therefore help conserve native species effectively.

Hydro-morphological parameters generate lifespan maps for stream restoration management

Anthropogenic, eco-morphological degradation of lotic waters necessitates laws, directives, and voluntary actions involving stream restoration and habitat enhancement. Research and engineering efforts are establishing a vast number of stream restoration planning approaches, design testing frameworks, construction techniques, and performance evaluation methods. As the practice of restoration scales up from an individual action at a single site to sequences of actions at many sites in a long river segment, a primary question arises as to the lifespan of such a sequence. This study develops a new framework to identify relevant parameters, design criteria and survival thresholds for ten multidisciplinary restoration techniques, adequate for site-scale to segment-scale application, in a comprehensive review: (1) bar and floodplain grading; (2) berm setback; (3) vegetation plantings; (4) riprap placement; (5) sediment replenishment; (6) side cavities; (7) side channel and anabranches; (8) streambed reshaping; (9) structure removal; and (10) placement of wood in the shape of engineered logjams and rootstocks. Survival thresholds are applied to a sequence of proposed habitat enhancement features for the lower Yuba River in California, USA. Spatially explicit hydraulic and sediment data, together with numerical model predictions of the measures, were vetted against the survival thresholds to produce discharge-dependent lifespan maps. Discharges related to specific flood-return periods enabled probabilistic estimates of the longevity of particular design features. Thus, the lifespan maps indicate the temporal stability of particular stream restoration and habitat enhancement features and techniques. Areas with particularly low or high lifespans help planners optimise the design and positioning of restoration features.

Environmental and spatial correlates of hydrologic alteration in a large Mediterranean river catchment

The natural flow regime is of central importance to the ecological integrity of rivers. Many rivers are heavily regulated and their flow regime has been severely affected by weirs and dams. However, information on hydrologic alteration is often not readily available or is only available for specific locations that may not coincide spatially with biological sampling sites, which restricts the analysis of the relationship between species and their riverine environment on large spatial scales. In this study on the Ebro River catchment, we applied boosted regression tree analyses to reveal significant environmental and spatial correlates of hydrologic alteration (i.e., differences between observed altered flow and modelled natural flow). Specifically, we used 37 variables related to climate, land use, topology and dams that can be easily derived in GIS systems to assess their association with three indices of hydrologic alteration describing changes in: (i) annual discharge, (ii) summer flow, and (iii) flow seasonality at 220 sites. Our results revealed highly variable spatial patterns of flow alteration in the Mediterranean catchment, which were mainly related to climate (dryness and seasonality), land use patterns, and upstream catchment size. The distance to the next upstream dam and reservoir surface area were the most relevant dam-related predictors of the investigated indices of hydrologic alteration, with the strongest effects of the distance to the next dam being on summer flows. The study also found potential limitations of using simulated, natural flow data from hydrologic models, which might be prone to uncertainties, to assess hydrologic alterations. We therefore (i) suggest that methods need to be improved to appropriately model natural flow regimes and quantify flow alteration, especially for data-limited and ungauged water bodies; and (ii) encourage future research on how global change interacts with river regulation, jointly affecting flow alteration.