Functional diversity of afrotropical fish communities across river gradients in the Republic of Congo, west central Africa

Abstract

Characterizing functional freshwater biodiversity patterns and understanding community determinants can aid in predicting changes and prioritizing specific conservation management actions. Afrotropical freshwater systems support diverse ecological communities, including many taxonomically undescribed species, yet few studies have been undertaken in this region and they remain poorly understood. Here, we use functional trait-based approaches to describe the functional diversity of fish assemblages in the Republic of Congo (west Central Africa) from the headwaters of a large basin (upper basin) to the coastal plains (lower basin). Functional traits were assessed using multivariate ordination, and functional diversity was calculated using functional trait-space occupation. We investigated if fish β diversity is nested regionally and at a basin-scale to understand if species assemblages exhibit non-random or environmental filtering patterns. Trait-environment relationships were modeled to determine the most significant drivers of trait diversity, and functional richness (FRic) was calculated for communities in both basins to identify whether Afrotropical systems conform to expected patterns of biodiversity along a fluvial gradient. Moderate to weak community nestedness and various traits associated with resistance and feeding were significantly correlated with habitat variables in the upper and lower basins respectively. Whilst environmental filtering is an important process of community assembly at basin scale, non-random processes are evident in the upper basin, whereas a high turnover was noted in the lower basin. Trophic traits in larger coastal plain habitats of the lower basin suggest higher trophic diversity is associated with higher spatial heterogeneity and nutrient concentrations. High functional turnover between headwaters and coastal plains suggests different functional strategies in fish assemblages between localities along the fluvial gradient. In contrast to expectations derived from temperate lotic systems, functional richness was comparable across both basinswhich suggests that headwater zones in the region harbor substantial functional diversity, especially when considering upstream catchment size. Functional diversity and environment patterns documented show unique patterns of functional richness that may assist in improving understanding of responses, vulnerabilities and conservation needs in Afrotropical systems.

The past and current distribution of native and non-native fish in the Kowie River catchment, Makhanda, Eastern Cape

“…This increase in species diversity is likely a result of downstream river widening and the presence of larger pools. These habitats are known to host increased species diversity, specifically larger and more predatory fishes (Bertora et al, 2021;Walsh et al, 2022). At both sites (R1 and R2), we found larger and more piscivorous fishes (Mozambique Tilapia, African Sharptooth Catfish, and Largemouth Bass) occupying larger pools.…”

Checklist of the fish fauna of the Munim River Basin, Maranhão, north-eastern Brazil

“…Median species richness in the Middle river basin section was lower than in the Upper and Lower river basin sections; however, the Middle section had both more sampling sites and much higher range of species richness. Environmental filtering across river gradients has a strong influence on species richness and diversity ( López‐Delgado et al 2019 , Walsh et al 2022 ). By grouping by section, we are missing local habitat-specific variables which are likely to be driving differences in fish assemblages across a highly heterogenous river network.…”

“…Further research is needed to understand the specific microhabitats and fish associations throughout the river basin as this is undoubtedly a driving factor of diversity. For example, river slope and flow conditions exert strong environmental filters on fish community and traits in Neotropical and Afrotropical freshwaters and dispersal between heterogenous habitats may be limited by side channels and swamp habitats ( Caetano et al 2021 , Walsh et al 2022 ). A higher concentration of specialist species is expected to be found in the Upper section as there is more competition for niches ( Sternberg and Kennard 2013 ).…”

Functional diversity does not explain the co‐occurrence of non‐native species within a flow‐modified African river system

“…Species functional traits can be used to better understand fish assemblage structure (Pease et al, 2012; Walsh et al, 2022), quantify the trait‐environment relationships (Brind'Amour et al, 2011; dos Santos et al, 2017; Santos et al, 2019) and assess the response of ecosystem to disturbances, including the introduction of non‐native species into novel environments (Angulo‐Valencia et al, 2022; Dala‐Corte et al, 2016; Lin et al, 2021; Miiller et al, 2021). Consistent with the first hypothesis that functional traits would reflect broad differences in species and their habitat associations within the Great Fish River system, the results of the present study revealed evidence of interspecific variation in functional traits, which appeared to reflect spatial differences in the distribution patterns and species‐environmental relationships.…”