Swimming with the giant: coexistence patterns of a new redfin minnow Pseudobarbus skeltoni from a global biodiversity hot spot

Abstract

Ecological niche theory predicts that coexistence is facilitated by resource partitioning mechanisms that are influenced by abiotic and biotic interactions. Alternative hypotheses suggest that under certain conditions, species may become phenotypically similar and functionally equivalent, which invokes the possibility of other mechanisms, such as habitat filtering processes. To test these hypotheses, we examined the coexistence of the giant redfin Pseudobarbus skeltoni, a newly described freshwater fish, together with its congener Pseudobabus burchelli and an anabantid Sandelia capensis by assessing their scenopoetic and bionomic patterns. We found high habitat and isotope niche overlaps between the two redfins, rendering niche partitioning a less plausible sole mechanism that drives their coexistence. By comparison, environment-trait relationships revealed differences in species-environment relationships, making habitat filtering and functional equivalence less likely alternatives. Based on P. skeltoni's high habitat niche overlap with other species, and its large isotope niche width, we inferred the likelihood of differential resource utilization at trophic level as an alternative mechanism that distinguished it from its congener. In comparison, its congener P. burchelli appeared to have a relatively small trophic niche, suggesting that its trophic niche was more conserved despite being the most abundant species. By contrast, S. capensis was distinguished by occupying a higher trophic position and by having a trophic niche that had a low probability of overlapping onto those of redfins. Therefore, trophic niche partitioning appeared to influence the coexistence between S. capensis and redfins. This study suggests that coexistence of these fishes appears to be promoted by their differences in niche adaptation mechanisms that are probably shaped by historic evolutionary and ecological processes.

Keywords: Freshwater fishes; habitat niche; morphological traits; niche overlap; trophic niche.

Figure 1

Map of the study area showing the sampled section of the Upper Riviersonderend River within the Hottentots Holland nature Reserve, Western Cape, South Africa.

Figure 2

Interspecific similarities in unified realized niche overlaps among Pseudobarbus skeltoni, Pseudobabus burchelli sp. “Breede,” and Sandelia capensis sp. “Riviersonderend” assessed by nonmetric multidimensional scaling (NMDS) based on habitat type, depth and substratum type. The contours represent depth gradient based on generalized additive model (GAM) fits.

Figure 3

Proportional abundances of Pseudobarbus skeltoni (A), Pseudobabus burchelli sp. “Breede” (B), and Sandelia capensis sp. “Riviersonderend” (C) within pool and riffles in relation to different substratum types.

Figure 4

Biplots of δ¹³C and δ¹⁵N isotope data indicating food web characteristics of Pseudobarbus skeltoni, Pseudobabus burchelli sp. “Breede,” and Sandelia capensis sp. “Riviersonderend.” Panel a displays standard ellipse areas (SEA_c) representing sample size‐corrected isotopic niche space (solid ellipses) and the convex hulls (dashed polygons) of the three fishes, and the mean δ¹³C or δ¹⁵N values of the potential food sources (black squares) and their associated standard deviations. The potential food sources included macroinvertebrate groups such as Plecoptera (Ple), Athericidae (Ath), Ephemeroptera (Eph), Megaloptera (Meg), Odonata (Odo), Trichoptera (Tri), Simuliidae (Sim), and Elmidae (Elm). Basal food sources included detritus (Det), macrophytes (Mac), and periphytic algae (Alg). Panel b displays ten random elliptical projections of trophic niche regions (NR) for each species defined by stable isotope values of δ¹³C and δ¹⁵N.

Figure 5

Comparison of posterior distribution of probabilistic trophic niche overlap for specified niche regions of Pseudobarbus skeltoni, Pseudobabus burchelli sp. “Breede,” and Sandelia capensis sp. “Riviersonderend.” Probabilities of niche overlap (mean and 95% credibility intervals) are specified as the overlap of species A (rows) onto the niche of species B (columns).

Figure 6

The estimated proportional source contributions of potential prey to the diets of Pseudobarbus skeltoni (A), Pseudobabus burchelli sp. “Breede” (B), and Sandelia capensis sp. “Riviersonderend” (C). Density estimates for the dietary source contributions were based on Bayesian inference, and the box plots for indicate 50, 75, and 95% credibility interval and the black dot indicates the mean.