Community isolation drives lower fish biomass and species richness, but higher functional evenness, in a river metacommunity

Abstract

1. The flow of individuals among communities and their interactions with local environmental filters are increasingly recognised as determinants of biodiversity patterns in riverine ecosystems. Both incoming dispersers and local conditions are expected to systematically change along connectivity gradients from headwaters to downstream communities. However, the interplay between isolation-centrality gradients and environmental conditions as determinants of biodiversity structure and function has seldom been considered. 2. Here, we represented the dendritic structure of the Negro River basin riverscape (Uruguay) in a directed graph quantifying the isolation-centrality of each river section and evaluated the direct and indirect pathways by which riverscape structure and environmental local drivers determine fish community assembly. 3. Fish communities (n = 58) were sampled following a stratified sampling design that properly represents this isolation-centrality connectivity gradient through the riverscape. In each community, fish abundance, biomass, richness, and functional diversity were estimated, and the direct and indirect hypothesised connections among them were evaluated with structural equation models. 4. We showed that the range of isolation among river sections determines a 2-fold, 5-fold, and 25-fold variation in total fish richness, abundance, and biomass, respectively. Additionally, isolation-centrality was positively associated with local temperature and conductivity, while negatively related to local depth. These variables and taxonomic richness accounted for most of the variation in total fish biomass (81%) herein used as measurement of ecosystem function. Local fish abundance was negatively and positively associated with functional evenness and taxonomic richness, respectively. Furthermore, once the effect of isolation on biomass and richness was accounted for, an effect of diversity on biomass became evident.

Effect of environmental and spatial factors on small-sized fish assemblages in a tropical river

“…Although some studies have shown that within-assemblage abundance increases in downstream direction (Henriques-Silva et al 2019;Borthagaray et al 2020), we found a negative relationship between abundance and distance from headwaters. This was due to that three of the most abundant species (Serrapinnus cf.…”

“…Small fish are mostly short-distance migrants, and respond mainly to fine-scale environmental variations like water depth and habitat structure (Lamouroux et al 1999;Fernandes et al 2015). Likewise, the effect of the distance from headwaters also points to limited dispersal capacity, as predicted by the network position hypothesis (Henriques-Silva et al 2019;Borthagaray et al 2020). Environmental and spatial factors explained only 30% of the variation in our data, which indicates that important variables, such as functional connectivity, and ecological interactions, such as competition, known to be important in regulating composition and structure of local assemblages (Cadotte and Tucker 2017) were not considered.…”

Local hydrological conditions and spatial connectivity shape invertebrate communities after rewetting in temporary rivers

“…As all TRs were flooded during the same rainfall event, the identity of each TR lost relevance against the spatial isolation. The relevance of closeness has already been shown at larger landscape scales when assessing changes in species richness (Borthagaray et al, 2020). Interestingly, here this effect was detected at a smaller spatial scale, highlighting the potential of this metric for explaining diversity patterns in dendritic networks (Economo & Keitt, 2010;Carrara et al, 2012;Altermatt, 2013).…”

“…By considering the 'out-closeness' as our centrality metric we assume that a higher closeness value for a given site implies higher isolation (i.e. upstream reaches) within a network (Borthagaray et al, 2020). Closeness provides a solid proxy of dendritic network structure and can relate significantly to biodiversity in river networks at theoretical, experimental, observational and management levels (Estrada & Bodin, 2008;Economo & Keitt, 2010;Altermatt, 2013;Borthagaray et al, 2020).…”

“…upstream reaches) within a network (Borthagaray et al, 2020). Closeness provides a solid proxy of dendritic network structure and can relate significantly to biodiversity in river networks at theoretical, experimental, observational and management levels (Estrada & Bodin, 2008;Economo & Keitt, 2010;Altermatt, 2013;Borthagaray et al, 2020). We used closeness to characterise the potential for recolonisation through the river network by smallto large-scale drift.…”