Functional diversity and redundancy of freshwater fish communities across biogeographic and environmental gradients

Influence of habitat structures on fish abundances and diversity: comparing mainstream and tributary communities in the urban uMsunduze Catchment, KwaZulu-Natal, South Africa

Various factors drive the decline of freshwater vertebrate biodiversity. These include changing landscape and urbanisation, introduced invasive species, altered habitat, water quality deterioration, instream barriers, and climate change. In the present study, we evaluated the impact of different habitat features on the fish assemblages in an urban river using catch per unit effort (CPUE) as a proxy for fish assemblage per site and season. We sampled 17 main sites and 21 ad-hoc sites in the uMsunduze Catchment in Pietermaritzburg, uMgungundlovu District, KwaZulu-Natal, South Africa, during 2022-2023. We collected data using an electro-shocker, fyke nets, and gill nets, and we also recorded and calculated habitat features such as substrate types, hydraulic biotopes, in-situ water quality, ecohydraulics, average depth, and velocity. We used Generalised Linear Models to determine the habitat features driving fish communities. We calculated the Shannon-Weiner and Pielou diversity indices to compare between rivers. We used the Fish Response Assessment Index (FRAI) tool to understand each site's ecological integrity per season. Our results indicated that various features, including substrate (mud, sand, gravel), fast intermediate and fast deep ecohydraulics, electrical conductivity, habitat (glide, pool), and average velocity significantly impacted the CPUE of fish. There was no variation in diversity indices between tributaries, but there was a significant difference in fish diversity between the uMsunduze mainstream and its tributaries. The FRAI scores showed great deterioration in the system's ecological health, and most sites, especially the mainstream sites, were critically or extremely modified. We suggest that the relevant authorities take action to mitigate the pressures compromising the uMsunduze Catchment's ecological integrity. There is an urgent need for conservation measures for the two "near threatened" species, Enteromius gurneyi and Amphilius natalensis, the former now extirpated as per our study.

Taxonomic and functional diversities reveal different fish assemblage dynamics of stow net fishery in Haizhou Bay

Biodiversity is the cornerstone of marine fisheries. To ensure the prosperity of stow net fishery in Haizhou Bay, regular investigations of fishery resources are essential. However, most studies have primarily focused on taxonomic diversity while overlooking functional diversity. In this study, we examined both the taxonomic and functional diversity of fish assemblages based on abundance and functional traits from 2013 to 2018. Significant differences in taxonomic diversity were observed only between two seasons, whereas functional diversity showed significant differences across years, seasons and groups. Diversity indices exhibited negative linear relationships with catch per unit effort, except for functional evenness (FEve) and functional divergence. Twenty-seven out of 30 correlations between the two types of diversity indices were predominantly positive and synclastic, whether linear or nonlinear. Functional dispersion showed the most positive relationships with taxonomic diversities, while FEve exhibited gentle slopes. The functional redundancy curves indicated that the ecological stability and resilience of fish assemblages were vulnerable. The non-target fish group demonstrated a higher overlap in functions compared to the target fish group and the total fish group. In summary, the taxonomic and functional diversities revealed inconsistent statuses and trends of fish assemblages, with an evident decreasing trend in the non-target fish group requiring special attention. This study highlights that both taxonomic and functional diversity should be considered simultaneously in fish biodiversity investigations, which is crucial for establishing effective fish conservation systems.

Exploring the potential effects of forest urbanization on the interplay between small mammal communities and their gut microbiota

Urbanization significantly impacts wild populations, favoring urban dweller species over those that are unable to adapt to rapid changes. These differential adaptative abilities could be mediated by the microbiome, which may modulate the host phenotype rapidly through a high degree of flexibility. Conversely, under anthropic perturbations, the microbiota of some species could be disrupted, resulting in dysbiosis and negative impacts on host fitness. The links between the impact of urbanization on host communities and their gut microbiota (GM) have only been scarcely explored. In this study, we tested the hypothesis that the bacterial composition of the GM could play a role in host adaptation to urban environments. We described the GM of several species of small terrestrial mammals sampled in forested areas along a gradient of urbanization, using a 16S metabarcoding approach. We tested whether urbanization led to changes in small mammal communities and in their GM, considering the presence and abundance of bacterial taxa and their putative functions. This enabled to decipher the processes underlying these changes. We found potential impacts of urbanization on small mammal communities and their GM. The urban dweller species had a lower bacterial taxonomic diversity but a higher functional diversity and a different composition compared to urban adapter species. Their GM assembly was mostly governed by stochastic effects, potentially indicating dysbiosis. Selection processes and an overabundance of functions were detected that could be associated with adaptation to urban environments despite dysbiosis. In urban adapter species, the GM functional diversity and composition remained relatively stable along the urbanization gradient. This observation can be explained by functional redundancy, where certain taxa express the same function. This could favor the adaptation of urban adapter species in various environments, including urban settings. We can therefore assume that there are feedbacks between the gut microbiota and host species within communities, enabling rapid adaptation.

Can fish species co-occurrence patterns be predicted by their trait dissimilarities?

Trait-based analyses have been successful in determining and predicting species association outcomes in diverse communities. Most studies have limited the scope of this approach to the biotic responses of a small number of species or geographical regions. We focused on determining whether three biologically relevant traits (body size, temperature preference and trophic level) influence the patterns of co-occurrence between multiple species. We used fish species presence/absence from 9204 lakes in Ontario, Canada, to obtain effect sizes of 2001 species-pair co-occurrence values, using a null model approach. Euclidean distances between each species-pair were calculated for each of the three traits selected. Multiple regression models and randomization tests were used to determine the direction and significance of the relationship of each trait with the observed co-occurrence values. The results show that species temperature preference was significantly related to co-occurrence patterns, indicating the effect of environmental filtering. Trophic level was significantly related to co-occurrence values for both linear and quadratic terms, suggesting that segregation between species is driven by large differences in this trait (predation effects). Unexpectedly, body size was not significantly related to the observed co-occurrence patterns. We provide a new approach to test relationships between species assemblages and trait conditions.

Understanding the relative roles of local environmental, geo-climatic and spatial factors for taxonomic, functional and phylogenetic β-diversity of stream fishes in a large basin, Northeast China

The primary objective of this study was to determine the relative roles of local environmental (Local), geo-climatic (Geo), and spatial (Spatial) factors to taxonomic, functional, and phylogenetic β-diversity of stream fish in a large basin in Northeast China. We quantified the current biodiversity patterns of fish communities in the Hun-Tai River using β-diversity. We assessed (i) corresponding contributions of turnover and nestedness within the taxonomic, functional, and phylogenetic β-diversity of fishes; (ii) correlations among β-diversity facets (i.e., taxonomic, functional, and phylogenetic facets); (iii) relative contributions of Local, Geo, and Spatial factors to β-diversity. We collected fish communities from 171 sampling sites. Mantel tests were used to examine the correlation of three facets of β-diversity and their components (i.e., total, nestedness, and turnover). Distance-based redundancy analysis and variation partitioning assess the relative contributions of Local, Geo, and Spatial factors to β-diversity. We found that turnover is the main driving mechanism for β-diversity in fish. Among the facets of β-diversity, taxonomic and phylogenetic facets have strong ecological information association. Spatial factors have a general contribution to various facets of β-diversity and its components. From aspects of fish β-diversity conservation, connectivity and habitat heterogeneity need to be maintained in the entire aquatic environment. In addition, protecting taxonomic β-diversity is helpful for maintaining phylogenetic β-diversity.

Abiotic factors influence species co-occurrence patterns of lake fishes

Abiotic factors are recognized for their strong influence on community structure. Habitat diversity is related to resource availability that influences species richness and abundance. In lakes, surface area and depth have been used as measures of the size and diversity of habitat, and have strong effects on the structure of entire communities. We tested whether abiotic variables, related to habitat size, influence co-occurrence patterns of species pairs of fishes by analysing groups of lakes within a specific area and depth categories in two regions in Ontario, Canada. We used null models to obtain co-occurrence patterns and standard effect sizes for each species pair within each area and depth category. We estimated standard effect sizes relative to lake area or depth and determined whether species co-occurrence patterns change systematically as these measures of habitat increase. We evaluated groups of species where factors such as predation and habitat filtering have been shown to structure those assemblages, and we tested whether area and depth alter the species associations and our interpretation of these relationships. We found significant differences between the observed and expected distributions of regression slopes relating co-occurrences to area and depth in both regions across all species, which indicated the strong influence of both variables on the overall co-occurrence patterns. We observed a significant negative trend of the co-occurrence patterns across lake area categories for the predator-prey species, indicating that the effect of predation was stronger in smaller lakes, but it was reduced in larger lakes, possibly due to increased habitat and resource availability. We show that pooling results as done in standard community null models can lead to Type II errors due to the 'cancellation' of opposing ecological signals. Our results demonstrate the effect of environmental variables on species co-occurrence patterns, but the divergent results obtained between geographical regions suggest that such patterns are context-dependent. This study emphasizes the importance of considering abiotic factors in null models of species co-occurrence to obtain reliable and detailed information about the association patterns between species.

Effects of different types of land-use on taxonomic and functional diversity of benthic macroinvertebrates in a subtropical river network

Expansion of agricultural and urban areas and intensification of catchment land-use increasingly affect different facets of biodiversity in aquatic communities. However, understanding the responses of taxonomic and functional diversity to specific conversion from natural forest to agriculture and urban land-use remains limited, especially in subtropical streams where biomonitoring programs and using functional traits are still under development. Here, we conducted research in a subtropical stream network to examine the responses of macroinvertebrate taxonomic and functional diversity to different types of land-use in central China. Our results showed that medium body size, univoltine, gill respiration, and slow seasonal development were much higher in natural forest sites, while certain traits related to strong resilience and resistance (e.g., small body size, fast seasonal development, bi-or multivoltine, abundant occurrence in drift, sprawler) dominated in high-intensity agriculture and urbanization sites. We further found that land-use compromised water quality (e.g., increases in total phosphate, conductivity and water temperature) and habitat conditions (e.g., high proportion of sand and silt, gravel, and channel width) accounted for the changes in trait composition based on a combination of RLQ and fourth-corner analysis. Moreover, natural forest sites presented relatively high values of functional richness than other land-use, demonstrating the importance of natural forest maintenance to promote high levels of functional diversity. However, taxonomic diversity indexes showed higher sensitivity to distinguish different types of land-use compared to functional diversity measures. Even so, given that certain trait categories showed significant relationships with specific local environmental stressors, trait-based approaches can provide reliable evidence to diagnose the cause of impairment and complement the results of the taxonomic-based approaches. Our findings support the idea that taxonomic and functional approaches should be integrated in river restoration and land-use management.

Evolutionary history drives aspects of stoichiometric niche variation and functional effects within a guild

Functional traits are characteristics of an organism that represents how it interacts with its environment and can influence the structure and function of ecosystems. Ecological stoichiometry provides a framework to understand ecosystem structure and function by modeling the coupled flow of elements (e.g. carbon [C], nitrogen [N], phosphorus [P]) between consumers and their environment. Animals tend to be homeostatic in their nutrient requirements and preferentially sequester the element in shortest supply relative to demand, and release relatively more of the element in excess. Tissue stoichiometry is an important functional trait that allows for predictions among the elemental composition of animals, their diet, and their waste products, with important effects on the cycling and availability of nutrients in ecosystems. Here, we examined the tissue stoichiometric niches (C:N:P) and nutrient recycling stoichiometries (N:P) of several filter-feeding freshwater mussels in the subfamily Ambleminae. Despite occupying the same functional-feeding group and being restricted to a single subfamily-level radiation, we found that species occupied distinct stoichiometric niches and that these niches varied, in part, as a function of their evolutionary history. The relationship between phylogenetic divergence and functional divergence suggests that evolutionary processes may be shaping niche complementarity and resource partitioning. Tissue and excretion stoichiometry were negatively correlated as predicted by stoichiometric theory. When scaled to the community, higher species richness and phylogenetic diversity resulted in greater functional evenness and reduced functional dispersion. Filter-feeding bivalves are an ecologically important guild in freshwater ecosystems globally, and our study provides a more nuanced view of the stoichiometric niches and ecological functions performed by this phylogenetically and ecologically diverse assemblage.