Impacts of small-scale engineering projects on Neotropical freshwater fishes

Franco Teixeira de Mello¹ , Carla Simone Pavanelli², José Luis Olivan Birindelli³, Ana Cristina Petry⁴, Andréa Bialetzki², Caroline C. Arantes⁵, Fernando Rogério Carvalho⁶, Paulo Santos Pompeu⁷ and Fernando Mayer Pelicice⁸

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Associate Editor: Lilian Casatti

Section Editor: Bruno Melo

Editor-in-chief: Luiz Roberto Malabarba

Abstract​

Introduction​

The inland aquatic ecosystems of the Neotropics harbor some of the richest fish diversity in the planet (Albert et al., 2020), playing a fundamental role in ecosystem functioning and providing essential Nature’s Contributions to People ecosystem services to human societies (Díaz et al., 2018; Pelicice et al., 2023). Unfortunately, this exceptional biodiversity is increasingly under threat as the region’s river systems have been degraded at an accelerating rate. Small rivers and streams, which represent the bulk of the hydrological network, have been especially affected by intensive transformation due to a wide variety of engineering works and small-scale infrastructure projects that often proceed without sufficient environmental oversight from public policies and agencies. Due to urbanization, infrastructure development, and farming, these systems have been altered, channelized, deepened, and dammed, in both their headwaters and midsections, resulting in major impacts on biodiversity and ecosystem functioning (Freitas et al., 2022; Tesitore, Teixeira de Mello, 2022). Biological lineages that have evolved for millions of years in lotic or semi-lotic environments have encountered their habitats completely transformed within only a few months or years.

Large-scale interventions such as dams and reservoirs have significant, well-documented environmental impacts on Neotropics’ environments and biodiversity (Agostinho et al., 2016), including a previous Special Issue of Neotropical Ichthyology devoted to this topic (Pelicice et al., 2021 and articles therein). Yet, smaller engineering projects, including aqueducts, canals, road crossings, culverts, small reservoirs, or cutwaters, also affect biodiversity in diverse ways, depending on how they are implemented and managed. Modifications resulting from human infrastructure may harm natural fish communities by fragmenting, transforming original habitats and creating new conditions that benefit opportunistic or invasive species. These small-scale interventions sometimes filter regional diversity by limiting native species spreading (Mariano et al., 2012) or, conversely, favoring the colonization and expansion of exotic species (Santos et al., 2021). They also constitute physical barriers, impeding fish movement, shifting metacommunity dynamics, changing thermal conditions, flow regime, and longitudinal and lateral connectivity in rivers (Sousa et al., 2018; Borthagaray et al., 2025). Yet, in certain contexts, small-scale interventions may generate a variety of aquatic habitats in highly modified rural or urban landscapes, boosting local diversity and facilitating species dispersal at the landscape level (Siqueira et al., 2021). Similarly, channelizing waterways for water transport or agricultural irrigation can, in some cases assist fish in moving between environments or host particular communities that play relevant ecological or functional roles (Camara et al., 2018; Guimarães et al., 2022).

In this context, understanding the ecological impacts of small-scale engineering projects on fish diversity within Neotropical systems becomes a priority. Despite its relevance to watershed planning and management, this topic has so far received little scientific attention. Therefore, the Special Issue of Neotropical Ichthyology entitled “Impacts of Small-Scale Engineering Projects on Neotropical Freshwater Fish Diversity” was developed to advance knowledge in this area. This collection brings together research findings from different Neotropical basins, examining how engineering interventions influence the taxonomic, functional, and trophic diversity of fish communities. Together, these studies provide an empirical synthesis vital to guide conservation and management strategies in landscapes modified by human activities, highlighting a range of often-overlooked effects, with relevant consequences for the ecological integrity of aquatic ecosystems in the region.

The Special Issue

This special issue comprises eleven original articles focused on the impacts of small-scale engineering projects on Neotropical freshwater fish diversity. In total, 65 authors affiliated with 21 institutions, mainly universities from Brazil, Uruguay, the UK, and the USA, contributed to this issue, with a predominance of Brazilian institutions. The Special Issue was coordinated by nine editors of Neotropical Ichthyology.

The included studies addressed different types of anthropogenic interventions on lotic and lentic environments, from urban channelization to dams, roads, water extraction, artificial drainage, and mining, covering a wide representation of aquatic ecoregions in Brazil. Together, they provide a complementary view of the effects of fragmentation, hydrological alteration, and landscape transformation on Neotropical ichthyofauna, with approaches ranging from local scales (sites or micro-watersheds) to assessment of effects at regional scales. Although each study focuses on a specific type of impact, common patterns of ecological response emerged, including biotic homogenization, loss of sensitive species, dominance of tolerant or generalist taxa, and alteration of reproductive and trophic processes. Some studies also reported positive effects of human interventions on fish diversity, indicating that biotic responses are complex and context-dependent.

The integrated analysis of keywords demonstrates significant thematic alignment, articulated around three principal conceptual axes: physical and hydrological impacts on river system structure and connectivity, such as dams, channelization, roads, deviation of the original riverbed and water abstraction; biological and community-level responses of ichthyofauna, including loss of species richness, trophic changes, homogenization, and reproductive alterations; and implications for management and conservation, including ecological monitoring, mitigation strategies, and recognition of the ecological potential of artificial habitats.

Types of interventions and impacts analyzed

The impacts addressed in the eleven studies are grouped into five main categories: (a) Hydraulic transformations and urban channelization, where Begnini et al. (2025) analyzed the immediate effects of urban stream channelization in the Iguaçu and Piquiri watersheds, evidencing reductions in richness and biomass; (b) Fluvial fragmentation and hydrological alterations by dams and hydropower, examined in three studies: Ribas et al. (2025) compared the impacts of small and large dams; Ticiani et al. (2025) assessed ichthyoplankton in a run-of-river cascade dam system; and Salvador et al. (2025) analyzed fragmentation by small dams in the upper Paraná River under the River Continuum Concept and the Serial Discontinuity Concept; c) Landscape modifications and terrestrial infrastructure, with Drager et al. (2025) and Mascarenhas et al. (2025) assessing the effects of roads on the body condition, feeding and reproduction of Amazonian fish; Costa et al. (2025) analyzing the ecological value of drainage ditches in the Atlantic Forest, and Soinski et al. (2025) evaluating disturbance of habitat integrity due to road construction in a Atlantic Forest stream; d) Resource extraction and intensive agriculture, where Reynalte-Tataje et al. (2025) examined direct fish mortality and loss of early stages associated with water extraction for irrigation; and e) Mining and land use, assessed by Penha et al. (2025), who linked mining infrastructure with changes in fish composition of Amazonian streams; and Lima et al. (2025) who compared fish communities between natural and artificial wetlands in South Brazil, highlighting the potential of the latter as alternative habitats.

Ecological effects and common patterns

Although scenarios differ, research consistently highlights recurring ecological patterns and their underlying mechanisms. The most widespread pattern is the structural and functional simplification of fish communities. Urban channelization (Begnini et al., 2025) led to immediate reductions in richness and biomass, with dominance of small, tolerant species. In parallel, fragmentation by dams (Ribas et al., 2025; Salvador et al., 2025; Ticiani et al., 2025) and flow alterations resulted in more homogeneous and less diverse communities across sites. Artificial wetlands (Lima et al., 2025) are dominated by the same small opportunistic Characiformes and Cichliformes species, which represent only a fraction of the regional diversity, reflecting a functional loss of diversity.

Effects on welfare and trophic structure were also evident. Drager et al. (2025) showed that road density reduces the condition factor of insectivorous species, possibly due to reduced food availability and changes in flow. In contrast, detritivorous and omnivorous species showed no alteration in body condition in degraded landscapes. Similarly, Mascarenhas et al. (2025) reported that road-induced impoundments in Amazonian streams negatively affect fish feeding patterns, reproductive output, and body condition, revealing additional physiological and ecological costs associated with road infrastructure. Complementarily, Soinski et al. (2025) found that road construction activities in Atlantic Forest streams altered water quality and temperature, while reducing fish species richness. Sensitive species such as Scleromystax barbatus and Rhamdia aff. quelen declined in abundance, whereas tolerant taxa like Phalloceros lucenorum and Cambeva zonata maintained high densities even after the onset of construction, highlighting contrasting responses of fish species within assemblages. In hydroelectric systems (Ribas et al., 2025; Salvador et al., 2025; Ticiani et al., 2025), flow reduction was observed to alter hydrodynamic signals necessary for spawning and restrict dispersal. In a complementary way, Reynalte-Tataje et al. (2025) evidenced direct losses of eggs, larvae, and juveniles by suction during water abstraction for irrigation, affecting population recruitment. Some studies suggest local resilience or complementary effects, especially in artificial or modified environments. Anthropogenic wetlands (Lima et al., 2025) showed comparable diversity to natural wetlands, and roadside ditches (Costa et al., 2025) hosted threatened native species alongside with exotics, indicating that these habitats can play a complementary role in conservation if they maintain connectivity and environmental quality.

Multivariate analyses conducted by Penha et al. (2025) showed that habitats’ structural heterogeneity, including factors as fine roots presence, depth variation, and dissolved oxygen levels, is key to maintaining diversity. This finding aligns with results from studies on channelization and dams (Begnini et al., 2025; Ticiani et al., 2025), where the loss of microhabitats seems to explain the convergence towards simplified communities dominated by generalist species. These trends align with other works at different scales showing the relevance of environmental heterogeneity for fish communities (Barrios, Teixeira de Mello, 2022; Moi, Teixeira de Mello, 2022; Moi et al., 2024). On the other hand, Ribas et al. (2025) highlighted how traditional BACI designs can underestimate the effects of dams, whereas the Regression Discontinuity in Time (RDiT) approach offers a more robust alternative for detecting ecological discontinuities.

Conclusions and perspectives

The studies presented in this Special Issue indicate that small-scale anthropic interventions, whether urban, agricultural, hydroelectric, or extractive, result in complex and multidimensional impacts on Neotropical freshwater ichthyofauna. Notable consequences include a decrease in richness and biomass of sensitive species, loss of habitat heterogeneity, increased prevalence of tolerant and generalist species, disruption of reproductive processes, and trophic and functional homogenization at the landscape level. However, these effects are not strictly linear or monotonic. Some artificial or secondary environments (Costa et al., 2025; Lima et al., 2025) maintain levels of diversity comparable to those found in natural environments, highlighting potential for mitigation through compensatory habitat design, if connectivity and water quality are preserved.

Collectively, the evidence reinforces that maintaining the ecological integrity of river systems requires attention to both habitat structure and the dynamics of the surrounding landscape. Local impacts such as channelization or water abstraction, and regional disturbances including fragmentation caused by dams and roads, act synergistically, diminishing fish community diversity and function. Although specific drivers may vary, their impacts generally converge to reduce system complexity and resilience. Considering these findings, effective management and conservation of Neotropical fish communities should adopt integrated management models that explicitly account for small-scale impacts, considering both their physical dimensions and the small systems they affect. Such sites can simultaneously serve as refuges for native fish or sources of invasive species, depending on their connectivity, design, and environmental context, underscoring the need to incorporate these assessments into conservation and land-use planning policies systematically.

It is evident that South American aquatic ecosystems have been exposed to a continuum of anthropogenic pressures, ranging from rapid habitat modification to disruption of essential ecological processes, but small-scale interventions remain comparatively understudied. The eleven articles included in this Special Issue represent only a fraction of the invited contributions, underscoring the limited research currently available in this area. This highlights a pressing need for further investigation, particularly given the ongoing expansion of urban centers, roads, and agricultural projects. For instance, Uruguay has more than 180,000 dams, 160,000 of which are small (< 1 hectare), generally built without prior environmental assessments, although systematic studies of these systems are only beginning to be conducted. Similarly, recent legislative changes in Brazil have reduced requirements for rigorous environmental assessments of small-scale projects (PL–2159, the so-called “PL da Devastação”). In both countries, the impacts of small-scale interventions are consistently underestimated by authorities and the broader public. The findings presented in this Special Issue aim to draw greater attention to and stimulate discussion of this important topic, promoting advances in scientific research and in the formulation of public policies aimed at the conservation of aquatic environments, especially at this time of profound global environmental change.

Acknowledgments​

We thank all the authors and reviewers who contributed to the Special Issue, and the two anonymous reviewers for their helpful comments.

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Authors

Franco Teixeira de Mello¹ , Carla Simone Pavanelli², José Luis Olivan Birindelli³, Ana Cristina Petry⁴, Andréa Bialetzki², Caroline C. Arantes⁵, Fernando Rogério Carvalho⁶, Paulo Santos Pompeu⁷ and Fernando Mayer Pelicice⁸

^[1]    Departamento de Ecología y Gestión Ambiental, Centro Universitario Regional Este, Universidad de la República, Maldonado, Uruguay. (FTM) frantei@cure.edu.uy (corresponding author).

^[2]    Universidade Estadual de Maringá, Av. Colombo, 5790, 87020-900, Maringá, PR, Brazil. (CSP) carlasp@nupelia.uem.br, (AB) bialetzki@nupelia.uem.br.

^[3]    Universidade Estadual de Londrina, Rodovia Celso Garcia Cid, km 380, Rodovia Celso Garcia Cid, km 380, 86057-970, Londrina, PR, Brazil. (JLOB) josebirindelli@uel.br.

^[4]    Universidade Federal do Rio de Janeiro, Instituto de Biodiversidade e Sustentabilidade, Av. Amaro Reinaldo dos Santos Silva, 764, 27965-045, Macaé, RJ, Brazil. (ACP) petryanacristina@gmail.com.

^[5]    West Virginia University, Morgantown, USA. (CCA) caroline.arantes@mail.wvu.edu.

^[6]    Universidade Federal de Mato Grosso do Sul, Câmpus de Três Lagoas, Avenida Ranulpho Marques Leal 3484, 79613-000, Três Lagoas, MS, Brazil. (FRC) carvalhofr@gmail.com.

^[7]    Universidade Federal de Lavras, Trevo Rotatório Professor Edmir Sá Santos, 37203-202, Lavras, MG, Brazil. (PSP) pompeu@ufla.br.

^[8]    Universidade Federal do Tocantins, 77500-000, Porto Nacional, TO, Brazil. (FMP) pelicice.editor@gmail.com.

Authors’ Contribution

Franco Teixeira de Mello: Conceptualization, Funding acquisition, Project administration, Writing-original draft, Writing-review and editing.

Carla Simone Pavanelli: Conceptualization, Funding acquisition, Project administration, Writing-original draft, Writing-review and editing.

José Luis Olivan Birindelli: Funding acquisition, Project administration, Writing-original draft, Writing-review and editing.

Ana Cristina Petry: Writing-original draft, Writing-review and editing.

Andréa Bialetzki: Writing-original draft, Writing-review and editing.

Caroline C. Arantes: Writing-original draft, Writing-review and editing.

Fernando Rogério Carvalho: Writing-original draft, Writing-review and editing.

Paulo Santos Pompeu: Writing-original draft, Writing-review and editing.

Fernando Mayer Pelicice: Conceptualization, Funding acquisition, Project administration, Supervision, Writing-original draft, Writing-review and editing.

Ethical Statement​

Not applicable.

Competing Interests

The author declares no competing interests.

Data availability statement

Not applicable. The review includes the articles published in this special volume.

AI statement

Grammarly AI was used to copyedit the English text.

Funding

FTM received support from the SNI (Agencia Nacional de Investigación e Innovación, ANII, Uruguay) and PEDECIBA. CSP received grant from CNPq (process 307124/2023–1), and financial support from Fundação Araucária (NAPI Taxonline process 164/2024). JLB received grant from CNPq (process 08846/2023–0), and financial support from Fundação Araucária (processes CP 23/2024, NAPI Taxonline 68/2024), and FINEP (process 3086–24). ACP received grant from CNPq (315924/2023–3), and financial support from Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (E–26/210.557/2024). AB received grant from Fundação Araucária (Process CP 23/2023). CCA received support from USDA National Institute of Food and Agriculture, McIntire-Stennis project (1026124). PSP received grant from CNPq (process 302328/2022–0). FMP received grant from CNPq (process 309025/2023–0)

How to cite this article

Teixeira de Mello F, Pavanelli CS, Birindelli JLO, Petry AC, Bialetzki A, Arantes CC, Carvalho FR, Pompeu PS, Pelicice FM. Impacts of small-scale engineering projects on Neotropical freshwater fishes. Neotrop Ichthyol. 2025; 23(4):e250188. https://doi.org/10.1590/1982-0224-2025-0188

Copyright​

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