A new study on the endemic freshwater fish of India's Western Ghats Escarpment (WGE) reveals that the distribution and ecological roles of these species differ significantly between west-flowing and east-flowing river basins, influencing community structure [1]. Researchers Rohitashva Shukla, Ada Fontrodona-Eslava, Vidyadhar Atkore, Anuradha Bhat, Neelesh Dahanukar, Jeyaraj Antony Johnson, Rajeev Raghavan, Maria Dornelas, Anne E. Magurran, and Kartik Shanker reported that geographically widespread, trait-distinct endemic fish are disproportionately present in the west-flowing basins, where they contribute to overall taxonomic and functional convergence [1]. Conversely, in the east-flowing basins, a lower-than-expected presence of these same species supports taxonomic divergence and functional convergence [1]. This ecological heterogeneity is attributed to factors like higher ecosystem productivity and temporary lateral connectivity in the west-flowing basins, which facilitate fish dispersal and support trait distinctiveness [1]. The findings, published in 2026 in the journal Proc. R. Soc. B, highlight how different dimensions of biodiversity interact to form ecological structure, which is crucial for ecosystem resilience [1].
The Western Ghats Escarpment, recognized as one of the world's 'hottest hotspots' for endemism, is a continental drainage divide in Peninsular India known for its high levels of biodiversity, including freshwater fishes [1]. Endemic species are unique to these hotspots and are vital for ecosystem services and local livelihoods, yet freshwater species globally are experiencing rapid population declines [1]. Despite the availability of more data, understanding the ecological processes by which endemic taxa shape macroecological patterns remains a challenge, hindering effective conservation policy development [1].
To investigate these patterns, the research team employed a bidimensional framework that classifies species based on their geographical extent and the distinctiveness of their traits relative to co-occurring taxa [1]. The study area encompassed 91 river basins within the WGE, covering 240,056 km2, with particular attention paid to the contrasting geomorphology and climatic conditions of the east- and west-flowing rivers [1]. West-flowing basins typically exhibit a high flow gradient and greater annual rainfall, while east-flowing basins have a low flow gradient and less precipitation [1].
The researchers compiled an extensive dataset of freshwater fish occurrence records from 59 river basins (35 west-flowing and 24 east-flowing) covering 160,991 km2, which is 67% of the total delineated area [1]. They assigned endemic or non-endemic status to species and gathered morphological trait data, including eleven body measurements from fish photographs, which were then converted into nine unitless ratios describing species morphology [1]. An additional trait, maximum body length, was obtained from FishBase [1]. These 10 variables are commonly used in fish functional trait analyses because they characterize ecological niche dimensions such as food acquisition and locomotion [1].
Species were categorized into four ecological roles by quantifying their geographical restrictedness (Ri) and global functional distinctiveness (Di) [1]. Geographical restrictedness was measured as one minus the proportion of basins a species occupied, with higher Ri values indicating restriction to fewer basins [1]. Functional distinctiveness was calculated as the average distance between a species and all other species in the WGE, scaled between 0 and 1, where higher values signified more infrequent trait combinations [1]. Using the median values of Ri (0.9) and Di (0.2) as cut-offs, species were classified into four groups: geographically widespread and trait-distinct (GW-TD), geographically restricted and trait-distinct (GR-TD or rare), geographically widespread and trait-redundant (GW-TR or common), and geographically restricted and trait-redundant (GR-TR) [1].
A null model analysis was conducted to assess endemic species richness across the WGE's basins, controlling for total species richness [1]. This involved 999 iterations of an independent swap algorithm to randomize the basin-species community data matrix, from which standardized effect sizes (SES) were calculated for nine measures of endemic species richness [1]. The study found a strong correlation between endemic species richness and total species richness, with a Pearson’s r of 0.9 (d.f. = 57, p < 0.01) [1].
The analysis revealed that the distribution of these ecological roles differed significantly between the two sides of the escarpment [1]. The higher occurrence of geographically widespread, trait-distinct endemic species in west-flowing basins leads to greater similarity in both species composition and functional traits among those communities, a phenomenon known as overall convergence [1]. In contrast, the lower prevalence of these species in east-flowing basins results in taxonomically dissimilar communities but with similar functional traits, indicating taxonomic divergence and functional convergence [1].
The study's framework provides a valuable tool for understanding the ecological processes that shape biodiversity patterns at macroecological scales [1]. This approach has direct applications in conservation and policy, offering guidance for global initiatives aimed at protecting endemic biodiversity in vulnerable hotspots [1].
