Functional Structure Of Fish Assemblages Research Articles

Metropolitan pressures: Significant biodiversity declines and strong filtering of functional traits in fish assemblages

Accelerating global urbanization is leading to drastic losses and restructuring of biodiversity. Although it is crucial to understand urban impacts on biodiversity to develop mitigation strategies, there is a dearth of knowledge on the functional structure of fish assemblages spanning the entire city-scale spectrum of urbanization intensity. Here, using environmental DNA sampled from 109 water sites in Beijing, we investigated the taxonomic and functional diversity patterns of fish assemblages across the city and uncovered community-, trait-, and species-level responses to various environmental stressors. By ranking sampling sites into three disturbance levels according to water physiochemical and landcover conditions, we found that both native and non-native fish taxonomic and functional α-diversity decreased significantly with elevating disturbance, as strong disturbance led to the disappearance of many species. However, the quantitative taxonomic and functional β-diversity components of native and non-native fish showed distinct patterns; assemblage turnover dominated native fish β-diversity and decreased with increasing disturbance, whereas species/trait richness differences dominated non-native fish β-diversity and increased with disturbance intensity particularly in lotic waters. RLQ and fourth-corner analyses revealed that fish size, fecundity, diet, and reproductive behaviors were significantly correlated with water quality, with pollution-tolerant, larger-sized native and omnivorous non-native fishes being urban winners, which indicates strong trait-dependent environmental filtering. Potential ecological indicator species were identified based on the sensitivity of fish responses to pollution loads; these were mostly small native species, and many have bivalve-dependent reproduction. Our results demonstrate that, along with native fish assemblage simplification and homogenization, urban stressors exert profound impacts on community trait composition, highlighting the need to consider both biodiversity loss and functional reorganization in combating disturbance of aquatic ecosystems under global urbanization. Furthermore, correlations between cropland cover and water nutrient level suggested that the management of agricultural runoff might be critically important for safeguarding urban water quality.

Hydro-morphological features and functional structure of fish assemblages mediate species isotopic niches in estuaries

Despite the popularity of stable isotope analysis (Carbon and Nitrogen), the drivers of species isotopic niches and their consequences on food web functioning remained poorly described, especially in estuaries. Here, we hypothesised that species niche characteristics are influenced by ecosystem hydro-morphological features and the functional structure of fish assemblages. The trophic niches of four fish species (Dicentrarchus labrax, Pomatoschistus minutus, Sprattus and Platichthys flesus) and overlap between them were compared during two consecutive autumns in eight estuarine ecosystems of the northeast Atlantic Ocean. Our findings demonstrated a relative steadiness of trophic positions of fish in estuaries, despite a high level of interspecific variability. The seabass generally occupied the higher trophic position, followed by the sand goby, while the flounder and sprat were at the lowest position in the food web. Species isotopic niches and overlap were mediated by some estuarine features, including estuary size and intertidal extent, but also by biotic interactions at the intra- and inter-specific levels, as reflected by the influence of fish abundance and functional diversity metrics. Our results support the statement that the intertidal area is a pivotal factor in regulating trophic interactions, by promoting niche partitioning and diversification of trophic resources between species. Moreover, niche size and dietary divergence of species decreased with increasing regularity of trophic strategies within assemblages, suggesting that species occupied more restricted trophic niches when they were less subjected to competition for the same food resources. These findings emphasize the importance of local conditions and the functional composition of assemblages in modulating the trophic resources consumed by fish in estuarine environments.

Prolonged drought influences the taxonomic and functional structure of fish assemblages in estuaries along the Brazilian semiarid coast

Prolonged drought influences the taxonomic and functional structure of fish assemblages in estuaries along the Brazilian semiarid coast

Fish assemblage shifts in an Ozark river over 80 years amidst a mosaic of forest regeneration and persistent pasture

AbstractFish assemblages are useful in evaluating the health of aquatic ecosystems. Over time, fish assemblages will respond to anthropogenic disturbances but may also simultaneously respond to conservation efforts in a watershed, shifting the taxonomic and functional structure of the assemblage. We quantified temporal changes in taxonomic and functional structure of fish assemblages in the Current River, Missouri, over an 80‐year time span and related these changes to forest regeneration and persistent pasture land use within the watershed. We found shifts in taxonomic and functional assemblage structure in two time‐steps from the 1940s to 2020 related to land use within the watershed. Reforested reaches exhibited increases in benthic species, rheophilic species and invertivorous feeding guilds. Higher pasture reaches exhibited decreases in benthic species and native centrarchid species while generalist centrarchid species increased. Ultimately, fish assemblage structure in the Current River appears to be shifting to an alternative state (i.e. increased presence/abundance of benthic species) in response to forest regeneration and persistent pasture land use.

Contemporary environment and historical legacy explain functional diversity of freshwater fishes in the world rivers

AbstractAimRegional taxonomic diversity (species richness) is strongly influenced by a joint effect of the current processes (habitat and energy availability) and historical legacies (past climate and geography), but it is still unclear how those historical and current environmental drivers have shaped the functional diversity of species assemblages.Major taxa studiedFreshwater fish.LocationGlobal.Time period1960s–2000s.MethodsWe combined the spatial occurrences over 2,400 river basins world‐wide and the functional traits measured on 10,682 freshwater fish species to quantify the relative role of the habitat, climate and historical processes on the current global fish functional diversity. To avoid any correlation between taxonomic diversity and functional diversity, we controlled for differences in the number of species (species richness) between rivers. Functional diversity was considered through three complementary facets: functional richness, functional dispersion and functional identity.ResultsThe habitat‐related variables explained most of the gradient in functional richness, verifying the habitat size–diversity hypothesis. In contrast, the historical climate–geography legacies markedly imprinted the functional dispersion and functional identity patterns, leading to a balanced influence of the current and historical processes. Indeed, the distribution of morphological traits related to fish dispersal was explained largely by the glaciation events during the Quaternary, leading to strong latitudinal gradients.Main conclusionsThis study provides new insights into the role of historical and current environmental determinants on the functional structure of fish assemblages and strengthens the proposal that the independence of facets of functional diversity from the species richness makes them essential biodiversity variables to understand the structure of communities and their responses to global changes.

Environmental filtering and low trait redundancy characterise fish assemblages in Lake Mamo, Orinoco River floodplain, Venezuela

The study of assembly rules of fish communities in Neotropical floodplain lakes represents a major interest in community ecology due to their high species diversity and environmental variation, both spatial and temporal. In this study, assembly rules of freshwater fishes in Lake Mamo were explored. The main goals of this research were to analyse the seasonal variations in taxonomic and functional diversities of fishes in sandbanks and patches of aquatic macrophytes, in order to identify whether the fish assemblages are organised according to environmental filtering or limiting similarity, and to explore the relationships between taxonomic and functional diversity to determine if fish assemblages are either functionally redundant or complementary. In both mesohabitats, fishes were collected using seine nets during four hydrological seasons corresponding to low waters, rising waters, high waters and falling waters. Taxonomic and functional structure of fish assemblages in each habitat and across hydrological seasons was examined through multidimensional scaling (MDS), PERMANOVAs and principal co-ordinate analyses. Seasonal variations in taxonomic diversity were determined through Fligner tests of Shannon diversity indices, and for functional diversity, of functional richness (FRich), functional dispersion (FDisp), functional evenness (FEve) and functional divergence (FDiv). Null model analyses were used to determine if the functional indices diverged from randomness across hydrological seasons. Finally, relationships between functional indices and Shannon diversity were explored using linear, power, asymptotic and logistic models. There were significant differences in taxonomic and functional composition between mesohabitats and more markedly across hydrological seasons. All fish assemblages were significantly underdispersed at all instances, while functional evenness was higher than expected in both mesohabitats during low waters, signaling both a strong environmental filtering and niche packing. There were significant linear associations between the Shannon diversity and functional dispersion and between the Shannon diversity and functional evenness, suggesting that functional redundancy in these assemblages is low. In Lake Mamo, fish assemblages are filtered first by seasonality and then by habitat type. As more species are added to the assemblages, niches are more packed in the functional space, but the number of traits increases at a constant rate. This means that if a lake becomes impaired as a consequence of potential oil spills, the loss of fish species could represent also the loss of traits and functions in the ecosystem.

How does conversion from forest to pasture affect the taxonomic and functional structure of the fish assemblages in Amazonian streams?

AbstractClearing land for the establishment of pasture is the principal cause of deforestation in the Amazon basin. We investigated how stream habitats and the taxonomic and functional structure of the fish fauna respond to the effects of the conversion of forest to pasture in the eastern Brazilian Amazon. We sampled 13 streams located in areas of forest and 13 streams within a pasture matrix. The structure of the stream habitats, and the taxonomic and functional structure of the fish assemblages were compared between forest and pasture, using univariate and multivariate analyses. Species richness did not vary between environments, although abundance was greater in forest streams. Functional divergence was lower in the pasture streams, indicating a reduced degree of niche differentiation. The pasture streams presented a higher percentage of artificial shelters, such as pipes and plastic containers, and riffles, while the forest streams had greater canopy cover formed by large trees, and a denser substrate formed by leaf litter. Environmental variables influenced the distribution of species based on their functional traits. The greater canopy cover in forest streams benefitted benthic species. The ongoing increase in the area of pasture in the Amazon region is causing changes in stream habitats, and in the taxonomic and functional structure of the local fish assemblages. The impacted streams did not favour the occurrence of species with parental care or piscivorous, invertivorous, and benthic habits. Overall, protecting forests and the stream margins will promote adequate conditions to maintain the equilibrium of Amazonian stream ecosystems.

When roads cross streams: fish assemblage responses to fluvial fragmentation in lowland Amazonian streams

ABSTRACT Roads affect biodiversity by increasing mortality rates, habitat loss, and natural landscape fragmentation. Poorly installed culverts can impound streams, changing the environmental conditions, and affecting aquatic communities. We evaluated the effects of road crossings on the taxonomic composition and functional structure of fish assemblages in lowland eastern Amazonian streams, Brazil. We tested the hypothesis that the presence of road-derived impoundments affects assemblage taxonomic and functional composition and structure. Two predictions were addressed: (1) Species and functional group composition will differ in impounded reaches in relation to lotic reaches; (2) Assemblages in downstream lotic reaches will be richer in species and functional groups in relation to upstream lotic and impoundment reaches. We sampled five streams crossed by roads presenting impoundment formation, conducted by visual census (day, dusk, and night) in 200 m line transects. Assemblage composition from lentic reaches was different from lotic reaches, and Shannon diversity from downstream reaches was different from upstream and impoundment, however, beta diversity partitioning between lotic reaches showed higher nestedness contribution, reinforcing the role of impoundment in limiting fish dispersal between lotic reaches. These results suggest that impoundments impose environmental and dispersal constraints to fish, affecting their longitudinal distribution in streams fragmented by roads.

Simulation of over‐exploitation of ornamental fish and its consequences for the functional structure of assemblages of Amazonian streams

AbstractThe consequences of over‐exploitation may seriously impair the integrity and functioning of ecosystems. When loss of species is accompanied by the loss of ecological processes, the effects are no longer just taxonomic and may affect the stability of the environments. Ornamental fishing is one of the main economic activities of the middle Negro River. Such activity is directed to a small number of species and has the Cardinal Tetra (Paracheirodon axelrodi) as the main target species. Given the potential effects this pressure and the lack of information on its consequences, the present study analysed, through simulations, the possible effects of depletion of ornamental fish populations on the functional structure of fish assemblages, represented by functional diversity (FD) and functional redundancy (FR) indexes. We sampled 13 streams exploited by the ornamental fishery, where we collected 4,286 specimens of 110 species, of which 22 were targeted as ornamental fishing. We found that the ornamental species corresponded, on average, to 27.8% of the species and 33.3% of the FD of the assemblages. In a scenario of complete exclusion of ornamental species, local communities would lose 24.6% of the FD and reduce up to 12% of the FR. With these results, it is possible to infer that the local extinction of exploited species would cause negative impacts on the multifunctionality and the resilience of the streams ecosystems. In this way, we emphasise the need for proper management of the ornamental fisheries aiming to the sustainability of the activity and to the conservation of ecosystem functionality.

Species richness and functional structure of fish assemblages in three freshwater habitats: effects of environmental factors and management.

In this study, the inverted trophic hypothesis was tested in the freshwater fish communities of a reservoir. The distribution of fish species in three freshwater habitats in the Jurumirim Reservoir, Brazil, was examined using both species richness and the relative proportions of different trophic groups. These groups were used as a proxy for functional structure in an attempt to test the ability of these measures to assess fish diversity. Assemblage structures were first described using non-metric multidimensional scaling (NMDS). The influence of environmental conditions for multiple fish assemblage response variables (richness, total abundance and abundance per trophic group) was tested using generalised linear mixed models (GLMM). The metric typically employed to describe diversity; that is, species richness, was not related to environmental conditions. However, absolute species abundance was relatively well explained with up to 54% of the variation in the observed data accounted for. Differences in the dominance of trophic groups were most apparent in response to the presence of introduced fish species: the iliophagous and piscivorous trophic groups were positively associated, while detritivores and herbivores were negatively associated, with the alien species. This suggests that monitoring functional diversity might be more valuable than species diversity for assessing effects of disturbances and managements policies on the fish community.

How deforestation drives stream habitat changes and the functional structure of fish assemblages in different tropical regions

Abstract Deforestation can modify stream habitat and the functional structure of fish assemblages. The aims of this study were (a) to identify whether deforestation has a similar effect on local habitats from two biogeographically distinct river basins in a tropical region; (b) to identify how fish trait–habitat relationships were influenced by deforestation; and (c) to compare functional redundancy patterns in these basins. Environmental and biological variables were obtained for 160 stream reaches, 85 located in the Alto Paraná River basin and 75 in the Machado River basin. Traits were associated with body size, habitat preference, food items and foraging period. Linear models were constructed to verify the environmental convergence of habitats across streams from the Alto Paraná and the Machado basins. An RLQ and a fourth‐corner analysis were conducted to investigate how deforestation affected habitat variables and trait–habitat relationships. The nearest relative index (NRI) was used as a functional redundancy measure. Deforestation led to a similar habitat gradient from streams with a higher proportion of coarse roots in more forested catchments to streams with streamside grasses, bare soil and unconsolidated substrate in more deforested catchments; however, a general pattern of fish trait–habitat relationships was not identified across basins. Functional redundancy was associated with a long history of habitat loss in the Alto Paraná streams, whereas functional complementarity was related to more recent and less intense habitat loss in Machado streams. We believe that differences in regional species pools and historical processes between the basins influenced the fish functional responses to deforestation. Nevertheless, the results highlighted the importance of stream habitat heterogeneity and the presence of preserved forest fragments in a region to prevent the loss of unique traits. Decision‐makers should therefore maintain large forest fragments and restore riparian forests to preserve stream habitat and the functional structure of fish assemblages.

Threshold Responses in the Taxonomic and Functional Structure of Fish Assemblages to Land Use and Water Quality: A Case Study from the Taizi River

Biological functional traits help to understand specific stressors that are ignored in taxonomic data analysis. A combination of biological functional traits and taxonomic data is helpful in determining specific stressors which are of significance for fish conservation and river basin management. In the current study, the Taizi River was used as a case study to understand the relationships between the taxonomic and functional structure of fish and land use and water quality, in addition to determining the thresholds of these stressors. The results showed that taxonomic structure was significantly affected by the proportion of urban land and specific conductivity levels, while functional metrics were influenced by the proportions of farmland and forest. Threshold indicator taxa analysis found that Phoxinus lagowskii, Barbatula barbatula nuda, Odontobutis obscura, and Cobitis granoei had negative threshold responses along the gradients of urban developments and specific conductivity. There was a significant change in fish taxonomic composition when the proportion of urban land exceeded a threshold of 2.6–3.1%, or specific conductivity exceeded a threshold of 369.5–484.5 μS/cm. Three functional features—habitat preference, tolerance to disturbances, and spawning traits—showed threshold responses to the proportion of farmland and forest. The abundance of sensitive species should be monitored as part of watershed management, as sensitive species exhibit an earlier and stronger response to stressors than other functional metrics. Sensitive species had a positive threshold response to the proportion of forest at 80.1%. These species exhibited a negative threshold response to the proportion of farmland at 13.3%. The results of the current study suggest that the taxonomic and functional structure of fish assemblages are affected by land use and water quality. These parameters should be integrated into routine monitoring for fish conservation and river basin management in the Taizi River. In addition, corresponding measures for improving river habitat and water quality should be implemented according to the thresholds of these parameters.

Trait–environment relationships in Amazon stream fish assemblages

AbstractStudies of trait–environment relationships provide important tools for the prediction of the response of biological communities to environmental alterations. The Amazon basin presents enormous potential for the development of research on this type of relationship, given the diversity of both its fish fauna and the aquatic ecosystems this fauna inhabits. The present study investigated the association between local environmental variables and the functional traits of fish in 54 streams of six major Amazonian basins. We identified the relationship between the characteristics of the streams (channel morphology, channel habitat units, riparian vegetation cover, large woody fragments and instream cover for aquatic organisms) and fish traits related to locomotion, habitat use and feeding behaviour. The fish fauna of the broader, deeper and more slowly flowing streams was dominated by nektobenthic species that exploit autochthonous resources such as fish and invertebrates. In narrow, fast‐flowing streams, by contrast, there was a predominance of benthic fishes with varying feeding habits, including periphytivorous and invertivorous species. Narrow, shady streams were inhabited by nektonic species adapted for the exploitation of resources from the marginal vegetation. The results of this study contribute to the understanding of the association between the functional structure of fish assemblages and local environmental variables in Amazonian streams. We hope that these findings will stimulate further research into the natural variation in stream fish assemblages that will ensure the development of more effective management strategies that better protect these important aquatic ecosystems.

Influence of intertidal seascape on the functional structure of fish assemblages: Implications for habitat conservation in estuarine ecosystems

Abstract Habitat loss and fragmentation remain major threats in estuaries, particularly with respect to the erosion of intertidal areas that provide shelter and food resources for numerous fish species. Improving our understanding of the fauna–intertidal seascape relationship is thus of critical importance. Here, the seascapes of 24 tidal estuaries were compared at multiple spatial scales (from estuary to local) to determine how the spatial structure of intertidal patches affects the abundance of several fish functional groups. For the majority of fish groups, our results support the statement that a large intertidal area can support high fish abundance at both of the studied scales; however, the spatial distribution of patches within estuaries, and especially the presence of large intertidal patches close to the estuary mouth, appeared to be an important feature for both marine and estuarine‐resident fish. It was also shown that habitat fragmentation at the estuary scale is associated with a decline in the abundance of several fish groups, probably as a result of reduced intertidal suitability and accessibility. At the local scale, the area of intertidal–subtidal interface was positively related to fish densities, emphasizing the importance of fish having permanently flooded refuges close to feeding grounds. This study provides essential insights to guide conservation and restoration actions for intertidal areas at both estuary and local scales (10–100 m2), by stressing differences in species sensitivity to seascape modification.

Functional redundancy and sensitivity of fish assemblages in European rivers, lakes and estuarine ecosystems

The impact of species loss on ecosystems functioning depends on the amount of trait similarity between species, i.e. functional redundancy, but it is also influenced by the order in which species are lost. Here we investigated redundancy and sensitivity patterns across fish assemblages in lakes, rivers and estuaries. Several scenarios of species extinction were simulated to determine whether the loss of vulnerable species (with high propensity of extinction when facing threats) causes a greater functional alteration than random extinction. Our results indicate that the functional redundancy tended to increase with species richness in lakes and rivers, but not in estuaries. We demonstrated that i) in the three systems, some combinations of functional traits are supported by non-redundant species, ii) rare species in rivers and estuaries support singular functions not shared by dominant species, iii) the loss of vulnerable species can induce greater functional alteration in rivers than in lakes and estuaries. Overall, the functional structure of fish assemblages in rivers is weakly buffered against species extinction because vulnerable species support singular functions. More specifically, a hotspot of functional sensitivity was highlighted in the Iberian Peninsula, which emphasizes the usefulness of quantitative criteria to determine conservation priorities.

Effects of pasture conversion to sugarcane for biofuel production on stream fish assemblages in tropical agroecosystems

Abstract One of the main causes of biodiversity loss in aquatic ecosystems is instream habitat change associated with land‐use change in the drainage basin. Biofuel crop production is an increasingly important driver of land‐use change in existing agricultural drainage basins. Conceptually, biofuels such as sugarcane (Saccharum spp.: Poaceea) can provide a sustainable alternative to fossil fuels, but little is known about the consequences of biofuel crop production for biodiversity. Thus, our aim was to investigate if and how the degree of further environmental changes at regional and local scales can influence taxonomic and functional structure of fish assemblages in agroecosystem streams. We quantified drainage basin land use, instream and riparian habitat, and fish assemblages in 38 agroecosystem streams in Brazil with drainage basins experiencing rapid expansion of sugarcane production, and calculated degree of environmental change and corresponding response in taxonomic and functional structure of stream fish assemblages. Procrustes and redundancy analyses tested for correlation between land use and instream habitat and environmental factors correlated with fish assemblage structure, respectively. Previous studies demonstrated relationships between land‐use change, instream habitat change and taxonomic and functional structure of stream fish assemblages. Therefore, we used regression to test the hypothesis that degree of environmental change is positively correlated with magnitude of change in taxonomic and functional structure of fish assemblages. Primary gradients of environmental change included conversion of pasture to sugarcane and reduction of stream width, depth and volume. Although drainage basin and instream attributes were not correlated within a year, we detected a legacy effect of land use on instream habitat. The degree of environmental change was not significantly correlated with taxonomic or functional changes in stream fish assemblages in most cases. However, abundances of resistant taxa increased, whereas nektonic functional groups decreased, and the biological response also presented evidence of a legacy effect. Our findings demonstrate that instream habitat and aquatic biodiversity in streams with agricultural drainage basins undergoing rapid conversion to biofuel production likely will not experience the full magnitude of responses in the short term. Therefore, repeated sampling of sufficient duration to account for legacy effects or time lags is needed to test for effects of biofuel expansion on biodiversity, and similarly to assess whether management practices such as riparian zone preservation can benefit biodiversity in agroecosystems.

What governs the functional diversity patterns of fishes in the headwater streams of the humid forest enclaves: environmental conditions, taxonomic diversity or biotic interactions?

The relationship between functional and taxonomic diversity is a major issue in ecology. Biodiversity in aquatic environments is strongly influenced by environmental gradients that act as dispersion and niche barriers. Environmental conditions act as filters to select functional traits, but biotic interactions also play a role in assemblage structure. In headwater streams, the relationship between functional and taxonomic diversity remains unclear. In this study we investigated how environmental conditions, taxonomic diversity and biotic interactions influence the spatial distribution of traits and functional diversity in stream fish species. Standardized sampling of fish species was carried out in 50 m sections of 16 streams located in rainforest enclaves in a semiarid region of Brazil (Caatinga biome). The functional diversity indices displayed different responses to the predictor variables used. Functional richness was mainly influenced by environmental conditions, while functional evenness was mostly determined by taxonomic diversity. On the other hand, functional dispersion was explained by a combination of environmental conditions and taxonomic diversity. The spatial distribution of fish species with the same functional traits was random, indicating that biotic interactions are not a strong predictor in these ecosystems. Channel width, pH and substrate were the most important variables in the spatial distribution of the functional traits of the fish species. Our results suggest that the functional structure of fish assemblages in headwater streams depends mainly on environmental conditions and taxonomic diversity.

Taxonomic and functional distinctness of the fish assemblages in three coastal environments (bays, coastal lagoons and oceanic beaches) in Southeastern Brazil

Several species of marine fish use different coastal systems especially during their early development. However, these habitats are jeopardized by anthropogenic influences threatening the success of fish populations, and urgent measures are needed to priorize areas to protect their sustainability. We applied taxonomic (Δ+) and functional (X+) distinctiveness indices that represent taxonomic composition and functional roles to assess biodiversity of three different costal systems: bays, coastal lagoons and oceanic beaches. We hypothesized that difference in habitat characteristics, especially in the more dynamism and habitat homogeneity of oceanic beaches compared with more habitat diversity and sheltered conditions of bays and coastal lagoons results in differences in fish richness and taxonomic and functional diversity. The main premise is that communities phylogenetically and functionally more distinct have more interest in conservation policies. Significant differences (P < 0.004) were found in the species richness, Δ+ and X+ among the three systems according to PERMANOVA. Fish richness was higher in bays compared with the coastal lagoons and oceanic beaches. Higher Δ+ was found for the coastal lagoons compared with the bays and oceanic beaches, with the bays having some values below the confidence limit. Similar patterns were found for X+, although all values were within the confidence limits for the bays, suggesting that the absence of some taxa does not interfere in functional diversity. The hypothesis that taxonomic and functional structure of fish assemblages differ among the three systems was accepted and we suggest that coastal lagoons should be priorized in conservation programs because they support more taxonomic and functional distinctiveness.

Disentangling the pathways of land use impacts on the functional structure of fish assemblages in Amazon streams.

Agricultural land use is a primary driver of environmental impacts on streams. However, the causal processes that shape these impacts operate through multiple pathways and at several spatial scales. This complexity undermines the development of more effective management approaches, and illustrates the need for more in-depth studies to assess the mechanisms that determine changes in stream biodiversity. Here we present results of the most comprehensive multi-scale assessment of the biological condition of streams in the Amazon to date, examining functional responses of fish assemblages to land use. We sampled fish assemblages from two large human-modified regions, and characterized stream conditions by physical habitat attributes and key landscape-change variables, including density of road crossings (i.e. riverscape fragmentation), deforestation, and agricultural intensification. Fish species were functionally characterized using ecomorphological traits describing feeding, locomotion, and habitat preferences, and these traits were used to derive indices that quantitatively describe the functional structure of the assemblages. Using structural equation modeling, we disentangled multiple drivers operating at different spatial scales, identifying causal pathways that significantly affect stream condition and the structure of the fish assemblages. Deforestation at catchment and riparian network scales altered the channel morphology and the stream bottom structure, changing the functional identity of assemblages. Local deforestation reduced the functional evenness of assemblages (i.e. increased dominance of specific trait combinations) mediated by expansion of aquatic vegetation cover. Riverscape fragmentation reduced functional richness, evenness and divergence, suggesting a trend toward functional homogenization and a reduced range of ecological niches within assemblages following the loss of regional connectivity. These results underscore the often-unrecognized importance of different land use changes, each of which can have marked effects on stream biodiversity. We draw on the relationships observed herein to suggest priorities for the improved management of stream systems in the multiple-use landscapes that predominate in human-modified tropical forests.

Fish complementarity is associated to forests in Amazonian streams

The functional structure of communities is commonly measured by the variability in functional traits, which may demonstrate complementarity or redundancy patterns. In this study, we tested the influence of environmental variables on the functional structure of fish assemblages in Amazonian streams within a deforestation gradient. We calculated six ecomorphological traits related to habitat use from each fish species, and used them to calculate the net relatedness index (NRI) and the nearest taxon index (NTI). The set of species that used the habitat differently (complementary or overdispersed assemblages) occurred in sites with a greater proportion of forests. The set of species that used the habitat in a similar way (redundant or clustered assemblages) occurred in sites with a greater proportion of grasses in the stream banks. Therefore, the deforestation of entire watersheds, which has occurred in many Amazonian regions, may be a central factor for the functional homogenization of fish fauna.