Synergistic and antagonistic interactions of future land use and climate change on river fish assemblages

Multiple environmental stressors affect predation pressure in a tropical freshwater system

Environmental change can alter predator-prey dynamics. However, studying predators in the context of co-occurring environmental stressors remains rare, especially under field conditions. Using in situ filming, we examined how multiple stressors, including temperature and turbidity, impact the distribution and behaviour of wild fish predators of Trinidadian guppies (Poecilia reticulata). The measured environmental variables accounted for 17.6% of variance in predator species composition. While predator species differed in their associations with environmental variables, the overall prevalence of predators was greatest in slow flowing, deeper, warmer and less turbid habitats. Moreover, these warmer and less turbid habitats were associated with earlier visits to the prey stimulus by predators, and more frequent predator visits and attacks. Our findings highlight the need to consider ecological complexity, such as co-occurring stressors, to better understand how environmental change affects predator-prey interactions.

Complex interactions of ENSO and local conditions buffer the poleward shift of migratory fish in a subtropical seascape

Ocean warming is associated with the tropicalization of fish towards higher latitudes. However, the influence of global climatic phenomena like the El Niño Southern Oscillation (ENSO) and its warm (El Niño) and cold (La Niña) phases on tropicalization has been overlooked. Understanding the combined effects of global climatic forces together with local variability on the distribution and abundance of tropical fish is essential for building more accurate predictive models of species on the move. This is particularly important in regions where ENSO-related impacts are known to be major drivers of ecosystem change, and is compounded by predictions that El Niño is becoming more frequent and intense under current ocean warming. In this study, we used long-term time series of monthly standardized sampling (August 1996 to February 2020) to investigate how ocean warming, ENSO and local environmental variability influence the abundance of an estuarine dependent tropical fish species (white mullet Mugil curema) at subtropical latitudes in the southwestern Atlantic Ocean. Our work revealed a significant increasing trend in surface water temperature in shallow waters (<1.5 m) at estuarine and marine sites. However, against our initial expectation, we did not observe an increasing trend in the abundance of this tropical mullet species. Generalized Additive Models revealed complex, non-linear relationships between species abundance and environmental factors operating at large (ENSO's warm and cold phases), regional (freshwater discharge in the coastal lagoon's drainage basin) and local (temperature and salinity) scales across the estuarine marine gradient. These results demonstrate that fish responses to global climate change can be complex and multifaceted. More specifically, our findings suggested that the interaction among global and local driving forces dampen the expected effect of tropicalization for this mullet species in a subtropical seascape.

Agricultural conservation may not help Midwestern US freshwater biodiversity in a changing climate

Global climate change and agricultural disturbance often drive freshwater biodiversity changes at the regional level, particularly in the Midwestern US. Agricultural conservation practices have been implemented to reduce sediment and nutrient loading (e.g., crop rotation, cover crops, reduced tillage, and modified fertilizer application) for long-term economic sustainability and environmental resilience. However, the effectiveness of these efforts on freshwater biodiversity is not conclusive. In this study, we used the Kaskaskia River Watershed, Illinois as an example to evaluate how agricultural conservation practices affects both taxonomic and functional diversity under climate changes. The measures of trait-based functional diversity provide mechanistic explanations of biological changes. In specific, we model and predict 1) species richness (SR), 2) functional dispersion (FDis), and 3) functional evenness (FEve). FDis and FEve were based on ecology (life history, habitat preference, and trophic level) and physiology (thermal preference, swimming preference, etc.). The best random-forest regression models showed that flow, temperature, nitrate, and the watershed area were among the top predictors of the three biodiversity measures. We then used the models to predict the changes of SR and FDis under RCP8.5 climate change scenarios. SR and FDis were predicted to decrease in most sites, up to 20 % and 4 % by 2099, respectively. When agricultural conservation practices were considered together with climate changes, the decreasing trends of SR and FDis remained, suggesting climate change outweighed potential agriculture conservation efforts. Thus, climate-change effects on temperature and flow regimes need to be incorporated into the design of agricultural practices for freshwater biodiversity conservation.

Changes in the Suitable Habitats of Three Endemic Fishes to Climate Change in Tibet

As one of the most sensitive regions to global climate change, Tibet is subject to remarkable changes in biota over the past decades, including endemic fish species. However, no study has attempted to predict the changes in the distribution of Tibetan fishes, leaving a great blank for aquatic conservation in Tibet. Based on the Maximum Entropy model (MaxEnt), this study predicted the changes in the suitable habitats of three endemic fish species, including two species mainly inhabiting the rivers (Glyptosternon maculatum, Oxygymnocypris stewartii) and one species mainly inhabiting lakes (Gymnocypris selincuoensis) in Tibet under two representative concentration pathways (RCP2.6 and RCP8.5) under two future scenarios (2050 and 2090), and explored the impact of the barrier effects of hydropower projects on the suitable habitats of fish. The results showed that under the four scenarios, the net change in the suitable habitats of the G. maculatum was negative (-2.0--18.8%), while the suitable habitats of the O. stewartii and G. selincuoensis would be expanded, with the net change of 60.0-238.3% and 46.4-56.9%, respectively. Under different scenarios, the suitable habitats of the three species had a tendency to migrate to a higher elevation, and the largest expansion in the range of migration was projected to occur under the 2090-RCP8.5 scenario. In addition, due to the impact of the hydropower projects, the ability of G. maculatum to obtain new suitable habitats from climate change would be reduced by 2.0-8.1%, which was less than the loss induced by climate change (5.5-25.1%), while the suitable habitats of O. stewartii would be reduced by 3.0-9.7%, which was more than the impact of climate change (about 1%). The results of this study have guiding significance for the conservation and management of fish resources diversity in the Yarlung Tsangpo River basin and Siling Co basin of Tibet, and also provide a reference for the coordination and scientific planning of hydropower projects in Tibet.

Warming temperatures drive at least half of the magnitude of long-term trait changes in European birds

Climate change is impacting wild populations, but its relative importance compared to other causes of change is still unclear. Many studies assume that changes in traits primarily reflect effects of climate change, but this assumption is rarely tested. We show that in European birds global warming was likely the single most important contributor to temporal trends in laying date, body condition, and offspring number. However, nontemperature factors were also important and acted in the same direction, implying that attributing temporal trends solely to rising temperatures overestimates the impact of climate warming. Differences among species in the amount of trait change were predominantly determined by these nontemperature effects, suggesting that species differences are not due to variation in sensitivity to temperature.

Many wild populations are experiencing temporal changes in life-history and other phenotypic traits, and these changes are frequently assumed to be driven by climate change rather than nonclimatic drivers. However, this assumption relies on three conditions: that local climate is changing, traits are sensitive to climate variability, and other drivers are not also changing over time. Although many studies acknowledge one or more of these conditions, all three are rarely checked simultaneously. Consequently, the relative contribution of climate change to trait change, and the variation in this contribution across traits and species, remain unclear. We used long-term datasets on 60 bird species in Europe to test the three conditions in laying date, offspring number, and body condition and used a method that quantifies the contribution of warming temperatures to changes in traits relative to other effects. Across species, approximately half of the magnitude of changes in traits could be attributed to rising mean temperature, suggesting that increasing temperatures are likely the single most important contributor to temporal trends and emphasizes the impact that global warming is having on natural populations. There were also substantial nontemperature-related temporal trends (presumably due to other changes such as urbanization), which generally caused trait change in the same direction as warming. Attributing temporal trends solely to warming thus overestimates the impact of warming. Furthermore, contributions from nontemperature drivers explained most of the interspecific variation in trait changes, raising concerns about comparative studies that attribute differences in temporal trends to species differences in climate-change sensitivity.

Projecting the effects of agricultural conservation practices on stream fish communities in a changing climate

How anticipated climate change might affect long-term outcomes of present-day agricultural conservation practices remains a key uncertainty that could benefit water quality and biodiversity conservation planning. To explore this issue, we forecasted how the stream fish communities in the Western Lake Erie Basin (WLEB) would respond to increasing amounts of agricultural conservation practice (ACP) implementation under two IPCC future greenhouse gas emission scenarios (RCP4.5: moderate reductions; RCP8.5: business-as-usual conditions) during 2020-2065. We used output from 19 General Circulation Models to drive linked agricultural land use (APEX), watershed hydrology (SWAT), and stream fish distribution (boosted regression tree) models, subsequently analyzing how projected changes in habitat would influence fish community composition and functional trait diversity. Our models predicted both positive and negative effects of climate change and ACP implementation on WLEB stream fishes. For most species, climate and ACPs influenced species in the same direction, with climate effects outweighing those of ACP implementation. Functional trait analysis helped clarify the varied responses among species, indicating that more extreme climate change would reduce available habitat for large-bodied, cool-water species with equilibrium life-histories, many of which also are of importance to recreational fishing (e.g., northern pike, smallmouth bass). By contrast, available habitat for warm-water, benthic species with more periodic or opportunistic life-histories (e.g., northern hogsucker, greater redhorse, greenside darter) was predicted to increase. Further, ACP implementation was projected to hasten these shifts, suggesting that efforts to improve water quality could come with costs to other ecosystem services (e.g., recreational fishing opportunities). Collectively, our findings demonstrate the need to consider biological outcomes when developing strategies to mitigate water quality impairment and highlight the value of physical-biological modeling approaches to agricultural and biological conservation planning in a changing climate.

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.

Threshold responses of riverine fish communities to land use conversion across regions of the world

The growing human enterprise has sparked greater interest in identifying ecological thresholds in land use conversion beyond which populations or communities demonstrate abrupt nonlinear or substantive change in species composition. Such knowledge remains fundamental to understanding ecosystem resilience to environmental degradation and informing land use planning into the future. Confronting this challenge has been largely limited to inferring thresholds in univariate metrics of species richness and indices of biotic integrity and has largely ignored how land use legacies of the past may shape community responses of today. By leveraging data for 13,069 riverine sites from temperate, subtropical, and boreal climate zones on four continents, we characterize patterns of community change along diverse gradients of urbanization and agricultural land use, and identity threshold values beyond which significant alterations in species composition exists. Our results demonstrate the apparent universality by which freshwater fish communities are sensitive to even low levels of watershed urbanization (range of threshold values: 1%-12%), but consistently higher (and more variable) levels of agricultural development (2%-37%). We demonstrated that fish community compositional thresholds occurred, in general, at lower levels of watershed urbanization and agriculture when compared to threshold responses in species richness. This supports the notion that aggregated taxon-specific responses may better reflect the complexity of assemblage responses to land use development. We further revealed that the ghost of land use past plays an important role in moderating how current-day fish communities respond to land use intensification. Subbasins of the United States experiencing greater rates of past land use change demonstrated higher current-day thresholds. Threshold responses of community composition, such as those identified in our study, illustrate the need for globally coordinated efforts to prioritize country-specific management and policy initiatives that ensure that freshwater fish diversity is not inevitably lost in the future.

Landscape context and nutrients modify the effects of coastal urbanisation

Estuaries are focal points for coastal cities worldwide, their habitats frequently transformed into engineered shorelines abutting waters with elevated nutrients in an urbanised landscape. Here we test for relationships between shoreline armouring and nutrients on the diversity and trophic composition of fish assemblages across 22 estuaries in eastern Australia. Urbanisation was associated with fish diversity and abundance, but there were differences in the effects of shoreline armouring and nutrient level on the trophic composition of fish assemblages. Fish diversity and the abundance of most trophic groups, particularly omnivores, zoobenthivores and detritivores, was greatest in highly urban estuaries. We show that estuarine fish assemblages are associated with urbanisation in more nuanced ways than simple habitat transformation would suggest, but this depends on the broader environmental context. Our findings have wider implications for estuarine conservation and restoration, emphasizing that ecological benefits of habitat measures may depend on both landscape attributes and water quality in urban settings.

Disentangling the potential effects of land-use and climate change on stream conditions

Land-use and climate change are significantly affecting stream ecosystems, yet understanding of their long-term impacts is hindered by the few studies that have simultaneously investigated their interaction and high variability among future projections. We modeled possible effects of a suite of 2030, 2060, and 2090 land-use and climate scenarios on the condition of 70,772 small streams in the Chesapeake Bay watershed, United States. The Chesapeake Basin-wide Index of Biotic Integrity, a benthic macroinvertebrate multimetric index, was used to represent stream condition. Land-use scenarios included four Special Report on Emissions Scenarios (A1B, A2, B1, and B2) representing a range of potential landscape futures. Future climate scenarios included quartiles of future climate changes from downscaled Coupled Model Intercomparison Project - Phase 5 (CMIP5) and a watershed-wide uniform scenario (Lynch2016). We employed random forests analysis to model individual and combined effects of land-use and climate change on stream conditions. Individual scenarios suggest that by 2090, watershed-wide conditions may exhibit anywhere from large degradations (e.g., scenarios A1B, A2, and the CMIP5 25th percentile) to small degradations (e.g., scenarios B1, B2, and Lynch2016). Combined land-use and climate change scenarios highlighted their interaction and predicted, by 2090, watershed-wide degradation in 16.2% (A2 CMIP5 25th percentile) to 1.0% (B2 Lynch2016) of stream kilometers. A goal for the Chesapeake Bay watershed is to restore 10% of stream kilometers over a 2008 baseline; our results suggest meeting and sustaining this goal until 2090 may require improvement in 11.0%-26.2% of stream kilometers, dependent on land-use and climate scenario. These results highlight inherent variability among scenarios and the resultant uncertainty of predicted conditions, which reinforces the need to incorporate multiple scenarios of both land-use (e.g., development, agriculture, etc.) and climate change in future studies to encapsulate the range of potential future conditions.

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.

From topography to hydrology-The modifiable area unit problem impacts freshwater species distribution models

Species distribution models (SDMs) are statistical tools to identify potentially suitable habitats for species. For SDMs in river ecosystems, species occurrences and predictor data are often aggregated across subcatchments that serve as modeling units. The level of aggregation (i.e., model resolution) influences the statistical relationships between species occurrences and environmental predictors-a phenomenon known as the modifiable area unit problem (MAUP), making model outputs directly contingent on the model resolution. Here, we test how model performance, predictor importance, and the spatial congruence of species predictions depend on the model resolution (i.e., average subcatchment size) of SDMs. We modeled the potential habitat suitability of 50 native fish species in the upper Danube catchment at 10 different model resolutions. Model resolutions were derived using a 90-m digital-elevation model by using the GRASS-GIS module r.watershed. Here, we decreased the average subcatchment size gradually from 632 to 2 km2. We then ran ensemble SDMs based on five algorithms using topographical, climatic, hydrological, and land-use predictors for each species and resolution. Model evaluation scores were consistently high, as sensitivity and True Skill Statistic values ranged from 86.1-93.2 and 0.61-0.73, respectively. The most contributing predictor changed from topography at coarse, to hydrology at fine resolutions. Climate predictors played an intermediate role for all resolutions, while land use was of little importance. Regarding the predicted habitat suitability, we identified a spatial filtering from coarse to intermediate resolutions. The predicted habitat suitability within a coarse resolution was not ported to all smaller, nested subcatchments, but only to a fraction that held the suitable environmental conditions. Across finer resolutions, the mapped predictions were spatially congruent without such filter effect. We show that freshwater SDM predictions can have consistently high evaluation scores while mapped predictions differ significantly and are highly contingent on the underlying subcatchment size. We encourage building freshwater SDMs across multiple catchment sizes, to assess model variability and uncertainties in model outcomes emerging from the MAUP.

The interactive effects of climate change and land use on boreal stream fish communities

Ongoing and projected climate change is likely to greatly alter co-occurring stressor mechanisms, yet these potential interactions remain poorly understood in natural freshwater systems worldwide. As the global biodiversity crisis deepens, successful conservation efforts will hinge on developing mechanistic multiple stressor frameworks that have been ground-truthed in natural systems containing complex species dynamics and ecological processes. Our study examined the combined and interacting effects of potential climate and land use stressors on boreal stream fishes using data from over 300 catchments across a broad 250,000 km² region. To characterize boreal fish community health, we examined four indicators including species richness, total catch per unit effort, the proportion of lithophilic spawners (fish sensitive to sedimentation), and the assemblage tolerance index which provides a measurement of the overall community tolerance to disturbance. Land use stressors included total anthropogenic land use area and linear disturbance at multiple watershed scales as well as two site-specific habitat degradation indicators (dissolved oxygen and the proportion of fine substrate). Overall community richness and productivity were not negatively related to land use changes indicating potential compensatory dynamics (e.g. where intolerant species are replaced with more tolerant species as habitat quality degrades). In contrast, we observed declines for sensitive species, including highly valued salmonids, that varied depending on interactions between local climate, land use, and stream type. Sensitive species declines were concentrated in regions experiencing increased land use and warming, whereas increases were observed in cooler regions consistent with a subsidy-stress response. In addition, lithophilic spawners declined in watersheds experiencing warmer and wetter conditions owing to potential indirect effects on spawning habitat quality. Results from our study provide novel insight into complex climate and land use interactions occurring across a broad, real-world landscape, and highlight the potential for amplified species declines under future warming and land use scenarios.

Integrating dynamic environmental predictors and species occurrences: Toward true dynamic species distribution models

While biological distributions are not static and change/evolve through space and time, nonstationarity of climatic and land-use conditions is frequently neglected in species distribution models. Even recent techniques accounting for spatiotemporal variation of species occurrence basically consider the environmental predictors as static; specifically, in most studies using species distribution models, predictor values are averaged over a 50- or 30-year time period. This could lead to a strong bias due to monthly/annual variation between the climatic conditions in which species' locations were recorded and those used to develop species distribution models or even a complete mismatch if locations have been recorded more recently. Moreover, the impact of land-use change has only recently begun to be fully explored in species distribution models, but again without considering year-specific values. Excluding dynamic climate and land-use predictors could provide misleading estimation of species distribution. In recent years, however, open-access spatially explicit databases that provide high-resolution monthly and annual variation in climate (for the period 1901-2016) and land-use (for the period 1992-2015) conditions at a global scale have become available. Combining species locations collected in a given month of a given year with the relative climatic and land-use predictors derived from these datasets would thus lead to the development of true dynamic species distribution models (D-SDMs), improving predictive accuracy and avoiding mismatch between species locations and predictor variables. Thus, we strongly encourage modelers to develop D-SDMs using month- and year-specific climatic data as well as year-specific land-use data that match the period in which species data were collected.

The Landscape Ecology of Rivers: from Patch-Based to Spatial Network Analyses

Purpose of Review

We synthesize recent methodological and conceptual advances in the field of riverscape ecology, emphasizing areas of synergy with current research in landscape ecology.

Recent Findings

Recent advances in riverscape ecology highlight the need for spatially explicit examinations of how network structure influences ecological pattern and process, instead of the simple linear (upstream-downstream) view. Developments in GIS, remote sensing, and computer technologies already offer powerful tools for the application of patch- and gradient-based models for characterizing abiotic and biotic heterogeneity across a range of spatial and temporal scales. Along with graph-based analyses and spatial statistical stream network models (i.e., geostatistical modelling), these approaches offer improved capabilities for quantifying spatial and temporal heterogeneity and connectivity relationships, thereby allowing for rigorous and high-resolution analyses of pattern, process, and scale relationships.

Summary

Spatially explicit network approaches are able to quantify and predict biogeochemical, hydromorphological, and ecological patterns and processes more precisely than models based on longitudinal or lateral riverine gradients alone. Currently, local habitat characteristics appear to be more important than spatial effects in determining population and community dynamics, but this conclusion may change with direct quantification of the movement of materials, energy, and organisms along channels and across ecosystem boundaries—a key to improving riverscape ecology. Coupling spatially explicit riverscape models with optimization approaches will improve land protection and water management efforts, and help to resolve the land sharing vs. land sparing debate.

Fish assemblages under climate change in Lithuanian rivers

Alterations of abiotic factors (e.g., river water temperature and discharge) will definitely affect the fundamental processes of aquatic ecosystems. The purpose of this study was to examine the impact of climate change on the structure of fish assemblages in fast-flowing rivers belonging to the catchment of the major Eastern European river, the Nemunas. Five catchments of semi-natural rivers were selected for the study. Projections of abiotic factors were developed for the near (2016-2035) and far future (2081-2100) periods, according to four RCP scenarios and three climate models using the HBV hydrological modelling tool. Fish metric projections were developed based on a multiple regression using spatial data. No significant changes in projections of abiotic and biotic variables are generally expected in the near future. In the far future period, the abiotic factors are projected to change significantly, i.e., river water temperature is going to increase by 4.0-5.1 °C, and river discharge is projected to decrease by 16.7-40.6%, according to RCP8.5. By the end of century, the relative abundance of stenothermal fish is projected to decline from 24 to 51% in the reference period to 0-20% under RCP8.5. Eurythermal fish should benefit from climate change, and their abundance is likely to increase from 16 to 38% in the reference period to 38-65% under RCP8.5. Future alterations of river water temperature will have significantly more influence on the abundance of the analysed fish assemblages than river discharge.

Susceptibility of European freshwater fish to climate change: Species profiling based on life-history and environmental characteristics

Climate change is expected to strongly affect freshwater fish communities. Combined with other anthropogenic drivers, the impacts may alter species spatio-temporal distributions and contribute to population declines and local extinctions. To provide timely management and conservation of fishes, it is relevant to identify species that will be most impacted by climate change and those that will be resilient. Species traits are considered a promising source of information on characteristics that influence resilience to various environmental conditions and impacts. To this end, we collated life-history traits and climatic niches of 443 European freshwater fish species and compared those identified as susceptible to climate change to those that are considered to be resilient. Significant differences were observed between the two groups in their distribution, life history, and climatic niche, with climate-change-susceptible species being distributed within the Mediterranean region, and being characterized by greater threat levels, lesser commercial relevance, lower vulnerability to fishing, smaller body and range size, and warmer thermal envelopes. Based on our results, we establish a list of species of highest priority for further research and monitoring regarding climate-change susceptibility within Europe. The presented approach represents a promising tool to efficiently assess large groups of species regarding their susceptibility to climate change and other threats, and to identify research and management priorities.

An ecologically constrained procedure for sensitivity analysis of Artificial Neural Networks and other empirical models

Sensitivity analysis applied to Artificial Neural Networks (ANNs) as well as to other types of empirical ecological models allows assessing the importance of environmental predictive variables in affecting species distribution or other target variables. However, approaches that only consider values of the environmental variables that are likely to be observed in real-world conditions, given the underlying ecological relationships with other variables, have not yet been proposed. Here, a constrained sensitivity analysis procedure is presented, which evaluates the importance of the environmental variables considering only their plausible changes, thereby exploring only ecological meaningful scenarios. To demonstrate the procedure, we applied it to an ANN model predicting fish species richness, as identifying relationships between environmental variables and fish species occurrence in river ecosystems is a recurring topic in freshwater ecology. Results showed that several environmental variables played a less relevant role in driving the model output when that sensitivity analysis allowed them to vary only within an ecologically meaningful range of values, i.e. avoiding values that the model would never handle in its practical applications. By comparing percent changes in MSE between constrained and unconstrained sensitivity analysis, the relative importance of environmental variables was found to be different, with habitat descriptors and urbanization factors that played a more relevant role according to the constrained procedure. The ecologically constrained procedure can be applied to any sensitivity analysis method for ANNs, but obviously it can also be applied to other types of empirical ecological models.

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.

An empirical test of the relative and combined effects of land-cover and climate change on local colonization and extinction

Land-cover and climate change are two main drivers of changes in species ranges. Yet, the majority of studies investigating the impacts of global change on biodiversity focus on one global change driver and usually use simulations to project biodiversity responses to future conditions. We conduct an empirical test of the relative and combined effects of land-cover and climate change on species occurrence changes. Specifically, we examine whether observed local colonization and extinctions of North American birds between 1981-1985 and 2001-2005 are correlated with land-cover and climate change and whether bird life history and ecological traits explain interspecific variation in observed occurrence changes. We fit logistic regression models to test the impact of physical land-cover change, changes in net primary productivity, winter precipitation, mean summer temperature, and mean winter temperature on the probability of Ontario breeding bird local colonization and extinction. Models with climate change, land-cover change, and the combination of these two drivers were the top ranked models of local colonization for 30%, 27%, and 29% of species, respectively. Conversely, models with climate change, land-cover change, and the combination of these two drivers were the top ranked models of local extinction for 61%, 7%, and 9% of species, respectively. The quantitative impacts of land-cover and climate change variables also vary among bird species. We then fit linear regression models to test whether the variation in regional colonization and extinction rate could be explained by mean body mass, migratory strategy, and habitat preference of birds. Overall, species traits were weakly correlated with heterogeneity in species occurrence changes. We provide empirical evidence showing that land-cover change, climate change, and the combination of multiple global change drivers can differentially explain observed species local colonization and extinction.

Disentangling multiple pressures on fish assemblages in large rivers

European large rivers are exposed to multiple human pressures and maintained as waterways for inland navigation. However, little is known on the dominance and interactions of multiple pressures in large rivers and in particular inland navigation has been ignored in multi-pressure analyzes so far. We determined the response of ten fish population metrics (FPM, related to densities of diagnostic guilds and biodiversity) to 11 prevailing pressures including navigation intensity at 76 sites in eight European large rivers. Thereby, we aimed to derive indicative FPM for the most influential pressures that can serve for fish-based assessments. Pressures' influences, impacts and interactions were determined for each FPM using bootstrapped regression tree models. Increased flow velocity, navigation intensity and the loss of floodplains had the highest influences on guild densities and biodiversity. Interactions between navigation intensity and loss of floodplains and between navigation intensity and increased flow velocity were most frequent, each affecting 80% of the FPM. Further, increased sedimentation, channelization, organic siltation, the presence of artificial embankments and the presence of barriers had strong influences on at least one FPM. Thereby, each FPM was influenced by up to five pressures. However, some diagnostic FPM could be derived: Species richness, Shannon and Simpson Indices, the Fish Region Index and lithophilic and psammophilic guilds specifically indicate rhithralisation of the potamal region of large rivers. Lithophilic, phytophilic and psammophilic guilds indicate disturbance of shoreline habitats through both (i) wave action induced by passing vessels and (ii) hydromorphological degradation of the river channel that comes along with inland navigation. In European large rivers, inland navigation constitutes a highly influential pressure that adds on top of the prevailing hydromorphological degradation. Therefore, river management has to consider river hydromorphology and inland navigation to efficiently rehabilitate the potamal region of large rives.

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

Barriers represent one of the largest anthropogenic impacts on the ecological status of rivers, and they also potentially restrict fishes' ability to respond to future environmental changes. Thus, river management aims to restore the longitudinal connectivity of rivers to allow continuous migration and movement of water, sediments and biota. However, it is often unclear whether the targeted barriers are also those most relevant for fish species, particularly to track future habitat shifts caused by environmental change. In this study, we applied species distribution models and the GIS-based fish dispersal model FIDIMO to evaluate the impacts of barriers (e.g. weirs and dams) on the dispersal of 17 native fish species in the European River Elbe with a particular focus on climate- and land use-induced habitat shifts. Specifically, we compared three scenarios of longitudinal connectivity: (i) current longitudinal connectivity, (ii) connectivity improvements as planned by river managers for 2021 and (iii) a reference with full longitudinal connectivity. The models indicated that barriers restricted the movement of two modeled fish species on average, thus impeding fishes' abilities to track future habitat shifts. Moreover, the number of species affected by barriers increased downstream. For the River Elbe, our results suggest that river management has most likely identified the most relevant barriers in respect to the modeled species and future environmental change. We emphasize that river management and barrier prioritization must thoroughly consider species-specific movement and dispersal abilities, as well as the specific spatial arrangement of barriers in the river system in relation to the spatial distribution of species' populations and suitable habitats.

The Potential Impacts of Climate Change on Biodiversity in Flowing Freshwater Systems

Ongoing increases in air temperature and changing precipitation patterns are altering water temperatures and flow regimes in lotic freshwater systems, and these changes are expected to continue in the coming century. Freshwater taxa are responding to these changes at all levels of biological organization. The generation of appropriate hydrologic and water temperature projections is critical to accurately predict the impacts of climate change on freshwater systems in the coming decade. The goal of this review is to provide an overview of how changes in climate affect hydrologic processes and how climate-induced changes in freshwater habitat can impact the life histories and traits of individuals, and the distributions of freshwater populations and biodiversity. Projections of biological responses during the coming century will depend on accurately representing the spatially varying sensitivity of physical systems to changes in climate, as well as acknowledging the spatially varying sensitivity of freshwater taxa to changes in environmental conditions.

The future distribution of river fish: The complex interplay of climate and land use changes, species dispersal and movement barriers

The future distribution of river fishes will be jointly affected by climate and land use changes forcing species to move in space. However, little is known whether fish species will be able to keep pace with predicted climate and land use-driven habitat shifts, in particular in fragmented river networks. In this study, we coupled species distribution models (stepwise boosted regression trees) of 17 fish species with species-specific models of their dispersal (fish dispersal model FIDIMO) in the European River Elbe catchment. We quantified (i) the extent and direction (up- vs. downstream) of predicted habitat shifts under coupled "moderate" and "severe" climate and land use change scenarios for 2050, and (ii) the dispersal abilities of fishes to track predicted habitat shifts while explicitly considering movement barriers (e.g., weirs, dams). Our results revealed median net losses of suitable habitats of 24 and 94 river kilometers per species for the moderate and severe future scenarios, respectively. Predicted habitat gains and losses and the direction of habitat shifts were highly variable among species. Habitat gains were negatively related to fish body size, i.e., suitable habitats were projected to expand for smaller-bodied fishes and to contract for larger-bodied fishes. Moreover, habitats of lowland fish species were predicted to shift downstream, whereas those of headwater species showed upstream shifts. The dispersal model indicated that suitable habitats are likely to shift faster than species might disperse. In particular, smaller-bodied fish (<200 mm) seem most vulnerable and least able to track future environmental change as their habitat shifted most and they are typically weaker dispersers. Furthermore, fishes and particularly larger-bodied species might substantially be restricted by movement barriers to respond to predicted climate and land use changes, while smaller-bodied species are rather restricted by their specific dispersal ability.

Exposure to a heat wave under food limitation makes an agricultural insecticide lethal: a mechanistic laboratory experiment

Extreme temperatures and exposure to agricultural pesticides are becoming more frequent and intense under global change. Their combination may be especially problematic when animals suffer food limitation. We exposed Coenagrion puella damselfly larvae to a simulated heat wave combined with food limitation and subsequently to a widespread agricultural pesticide (chlorpyrifos) in an indoor laboratory experiment designed to obtain mechanistic insights in the direct effects of these stressors in isolation and when combined. The heat wave reduced immune function (activity of phenoloxidase, PO) and metabolic rate (activity of the electron transport system, ETS). Starvation had both immediate and delayed negative sublethal effects on growth rate and physiology (reductions in Hsp70 levels, total fat content, and activity levels of PO and ETS). Exposure to chlorpyrifos negatively affected all response variables. While the immediate effects of the heat wave were subtle, our results indicate the importance of delayed effects in shaping the total fitness impact of a heat wave when followed by pesticide exposure. Firstly, the combination of delayed negative effects of the heat wave and starvation, and the immediate negative effect of chlorpyrifos considerably (71%) reduced larval growth rate. Secondly and more strikingly, chlorpyrifos only caused considerable (ca. 48%) mortality in larvae that were previously exposed to the combination of the heat wave and starvation. This strong delayed synergism for mortality could be explained by the cumulative metabolic depression caused by each of these stressors. Further studies with increased realism are needed to evaluate the consequences of the here-identified delayed synergisms at the level of populations and communities. This is especially important as this synergism provides a novel explanation for the poorly understood potential of heat waves and of sublethal pesticide concentrations to cause mass mortality.