Non-native fishes homogenize native fish communities and reduce ecosystem multifunctionality in tropical lakes over 16 years

Human land use and non-native fish species erode ecosystem services by changing community size structure

Organism body size influences ecosystem services, and human pressures alter the size structure of ecological communities. However, our understanding of how different human-induced pressures (such as land use and biotic invasion) interact to drive community size structure and ecosystem services remains limited. Combining 21 years of fish size spectrum data and fishery potential (fishery monetary value in the Upper Paraná River Floodplain, Brazil), we demonstrate that the size spectrum exponent of native species has become more negative over time, indicating a relative decrease in the biomass of large versus small individuals. Conversely, the size spectrum exponent of non-native species has become less negative over time owing to the increased abundance of large species. Overall, fishery potential declined by more than 50% over time. Human land use replaced the coverage of natural environments, indirectly reducing native richness. This scenario decreased the exponent of the native size spectrum, indirectly reducing fishery potential. Our study illustrates how intensification of human land use alters the size structure of communities, favouring non-native individuals and suppressing ecosystem services.

Typology of the ecological impacts of biological invasions

Biological invasions alter ecosystems by disrupting ecological processes that can degrade biodiversity, harm human health, and cause massive economic burdens. Existing frameworks to classify the ecological impacts either miss many types of impact or conflate mechanisms (causes) with the impacts themselves (consequences). We propose a comprehensive typology of 19 types of ecological impact across six levels of ecological organisation. This allows more accurate diagnosis of the cause of impact and can help triage management options to tackle each impact-mechanism combination. We integrated the typology with broad ecological concepts such as energy, mass, and information flow and storage. By highlighting cascading effects across multiple levels, this typology provides a clearer framework for documenting, and communicating invasion impacts, thereby improving management and research.

Anthropogenic influences reshape lentic fish diversity: Patterns of homogenization and differentiation across a Mediterranean biodiversity hotspot

Freshwater ecosystems are increasingly impacted by human activities, resulting in species invasions and extinctions, disrupting biodiversity and ecosystem functions. This study investigates the patterns of taxonomic and functional homogenization and differentiation in fish assemblages within 103 lentic ecosystems across a Mediterranean biodiversity hotspot (Greece). We focus on how non-native species have altered the taxonomic and functional diversity at both national and ecoregional levels while exploring the drivers and pathways behind these changes. Our analysis combined historical and current species data, evaluating the effects of species introductions and local extinctions on biodiversity of distinct freshwater ecoregions. We calculated taxonomic and functional similarities using Jaccard and Gower dissimilarity indices and employed generalized linear models (GLMs) to assess the significance of changes over time. Results reveal significant taxonomic homogenization across lentic ecosystems, primarily driven by the introduction of widespread alien and translocated fish species. In contrast, functional homogenization was less pronounced, with some regions exhibiting differentiation, including increases in species richness due to the introduction of species with distinct ecological traits. Non-native species introduced through angling, ornamental trade, and unintentional pathways were the primary contributors to homogenization. The findings highlight prominent regional differences and vulnerabilities: mainland ecoregions experienced stronger homogenization, while insular ecoregions have experienced taxonomic differentiation. Additionally, the study reveals a decoupling of taxonomic and functional changes, emphasizing the need to consider both in biodiversity assessments and conservation management. This research contributes to the broader understanding of how species invasions reshape biodiversity patterns and ecosystem functions in freshwater systems. Our approach provides a useful framework for assessing biotic homogenization and differentiation, with implications for conservation and management strategies worldwide.

Long-term contamination by non-native fish assemblages in a Neotropical floodplain

Biological invasions are a major threat to biodiversity in species-rich regions. Therefore, it is important to understand mechanisms behind the long-term establishment of non-native fish species in aquatic environments in the Neotropical region. Here, we associated fish biomass, species richness, and the proportion of non-native species (contamination and Kempton's indices) to quantify the non-native pressure over fish biodiversity in lakes and rivers of the Parana River floodplain, seasonally, from 2000 to 2017. We divided species into native and non-native assemblages sampled in spatio-temporal gradients. Temporal trends were examined using linear regressions and generalised additive models. Fish biomass in gillnets increased for both native and non-native fish species, but their Kempton indices were inversely correlated. Extinction of native species occurred locally with biotic differentiation of non-native species in lakes, rivers, and ecosystem contamination. A constant increase in fish biomass resulted in overwhelming biodiversity of non-natives at the end of the time series evaluated. Native biotic resistance to introductions was not detected in deterministic trends. The observed patterns were consistent with previous studies showing native biotic homogenisation and extinction of species in response to biological invasions, landscape fragmentation, and riverine impoundments. Increases in abundance and species richness of non-native fish were the biodiversity drivers that resulted in non-native species outweighing native species in the Parana floodplain.

Universal beta-diversity-functioning relationships are neither observed nor expected

Widespread evidence shows that local species richness (α-diversity) loss hampers the biomass production and stability of ecosystems. β-Diversity, namely the variation of species compositions among different ecological communities, represents another important biodiversity component, but studies on how it drives ecosystem functioning show mixed results. We argue that to better understand the importance of β-diversity we need to consider it across contexts. We focus on three scenarios that cause gradients in β-diversity: changes in (i) abiotic heterogeneity, (ii) habitat isolation, and (iii) species pool richness. We show that across these scenarios we should not expect universally positive relationships between β-diversity, production, and ecosystem stability. Nevertheless, predictable relationships between β-diversity and ecosystem functioning do exist in specific contexts, and can reconcile seemingly contrasting empirical relationships.

How does invasion degree shape alpha and beta diversity of freshwater fish at a regional scale?

Freshwater ecosystems appear more vulnerable to biodiversity loss due to several anthropogenic disturbances and freshwater fish are particularly vulnerable to these impacts. We aimed to (1) identify the contribution of land use, spatial variables, and invasion degree in determining freshwater fish alpha (i.e., species richness) and beta (i.e., local contributions to beta diversity, LCBD) diversity, evaluating also the relationship between invasion degree and nestedness (β nes) and turnover (β sim) components of beta diversity. (2) Investigate the relationship between alpha diversity and LCBD, under the hypothesis that alpha diversity and LCBD correlate negatively and (3) investigate the relationship between species contributions to beta diversity (SCBD) and species occurrence, hypothesizing that non-native species show a lower contribution to beta diversity. The linear mixed models and the partition of R 2 retained the invasion degree as the most important variables explaining alpha and beta diversity, having a positive relationship with both diversity components. Furthermore, land use related to human impacts had a positive influence on alpha diversity, whereas it showed a negative effect on LCBD. Regression model further showed that invasion degree related positively withβ sim, but negatively withβ nes, suggesting that non-native species were involved in the replacement of native species in the fish community. Alpha diversity and LCBD showed a weak positive correlation, meaning that sites with low species richness have higher LCBD. SCBD scaled positively with species occurrence highlighting that rarer species contribute less to SCBD. Finally, native and exotic species contributed similarly to beta diversity. These results suggest that invasion degree plays a central role in shaping alpha and beta diversity in stream fish, more than land use features reflecting habitat alteration or other geospatial variables. Furthermore, it is important to evaluate separately the native and the non-native components of biotic communities to identify linkages between invasion dynamics and biodiversity loss.

Pervasive decline of subtropical aquatic insects over 20 years driven by water transparency, non-native fish and stoichiometric imbalance

Insect abundance and diversity are declining worldwide. Although recent research found freshwater insect populations to be increasing in some regions, there is a critical lack of data from tropical and subtropical regions. Here, we examine a 20-year monitoring dataset of freshwater insects from a subtropical floodplain comprising a diverse suite of rivers, shallow lakes, channels and backwaters. We found a pervasive decline in abundance of all major insect orders (Odonata, Ephemeroptera, Trichoptera, Megaloptera, Coleoptera, Hemiptera and Diptera) and families, regardless of their functional role or body size. Similarly, Chironomidae species richness decreased over the same time period. The main drivers of this pervasive insect decline were increased concurrent invasions of non-native insectivorous fish, water transparency and changes to water stoichiometry (i.e. N : P ratios) over time. All these drivers represent human impacts caused by reservoir construction. This work sheds light on the importance of long-term studies for a deeper understanding of human-induced impacts on aquatic insects. We highlight that extended anthropogenic impact monitoring and mitigation actions are pivotal in maintaining freshwater ecosystem integrity.