The spatial scale of homogenisation and differentiation in Chihuahuan Desert fish assemblages

Abstract

We examined riverine desert fish assemblages in the Chihuahuan Desert, U.S.A. at multiple spatial scales of similarity to assess long‐term changes to assemblage distinctiveness, identify individual species responsible for changes, and determine the importance of geographic context and species resolution in interpreting patterns of change. We used a well‐documented historical data set on fish distribution and abundance, and recent collections of fishes that provided a paired analytical design across 36 localities spanning nearly 3 decades. Patterns of faunal homogenisation and differentiation were assessed at basin‐wide, sub‐basin and river‐reach scales with species occurrence and relative abundance data. Individual species responses were examined to identify the drivers of assemblage change across time. Patterns of assemblage similarity varied across spatial scales and produced seemingly incongruous trends across time. Assemblage distinctiveness depended on the spatial extent of the analyses, the geographical structuring of the fish assemblages, and whether occurrence or relative abundance data were used. These dependencies led to interesting and conflicting patterns of homogenisation and differentiation. The Rio Grande sub‐basin showed strong homogenisation with convergence between upstream and downstream reaches that corresponded to declining water quality and quantity from the Rio Conchos in Mexico. In contrast, the Pecos River sub‐basin showed strong differentiation between upstream and downstream reaches that corresponded to the successful colonisation and spread of the non‐native gulf killifish (Fundulus grandis) in the upper reach, where water quantity and quality have declined. Spatial variability in fish assemblages and their degree of change from historical conditions were largely congruent with anthropogenic modifications to the flow regime and variability in the success of invasive gulf killifish in the basin. The use of species occurrence as opposed to relative abundance data, and the spatial scale of analysis are crucial choices in studies of faunal homogenisation and differentiation, and we have demonstrated how these choices lead to variable results for our study system. Our multi‐scale approach and examination of individual species responses identified the ultimate drivers of these differences and illustrated the importance of scale‐dependent effects and geographical context on patterns of assemblage distinctiveness, especially with regard to species invasion, species loss and relative abundance shifts.

Increased landscape disturbance and streamflow variability threaten fish biodiversity in the Red River catchment, USA

“…Miyazono et al (2015) found a reduction in reach‐scale native fish relative abundance and species diversity in the Rio Grande River subbasin following a decrease in flow magnitude and increase in salinity. Similarly, Taylor et al (2019) reported changes in the fish assemblage in reaches of the Pecos River subbasin and homogenization in the Rio Grande River subbasin following reduction in flow magnitude and water quality. The homogenization of fish assemblages due to landscape development has been documented globally in both freshwater (Scott & Helfman, 2001; Villéger et al, 2014) and marine systems (Magurran et al, 2015; Richardson et al, 2018).…”

Scale dependency in fish beta diversity–hydrology linkages in lowland rivers

“…The limited evidence we have suggests that beta diversity responses to hydrology are variable across different aspects of dissimilarity (e.g. Dai et al, 2020;Taylor et al, 2019). By considering how beta diversity responses to hydrology vary across spatial extents, among…”

Temporal dynamics of freshwater fish assemblages, their background and methods of quantifications—A synthesis

“…However, based on the example of two neighbouring stream fish assemblages, Ross et al (1985) demonstrated that differences in similarity values can occur even at the same spatial and time scales. Consequently, the choice of the spatial scale in analyses could influence the conclusion about the temporal dynamics of fish assemblages (Taylor et al, 2019). We thus argue that considering spatial components and disentangling the role of the regionally and locally acting factors are essential for a better understanding of the drivers of assemblage dynamics.…”