The majestic Eastern Himalayas, one of the world's biodiversity hotspots, are also among the most vulnerable landscapes on Earth. The mountains are facing threats from climate change and intense human activities, like damming and river channel modification, which threaten the extinction of many species. Among the organisms facing pressures of a changing ecosystem are the region's unique freshwater fish communities, which have evolved over millennia through periods of isolation and reconnection. To protect these vulnerable communities, researchers must first understand what controls the distribution of these fish: is it the physical environment, or their ability to move between streams?
A new study focusing on the Teesta and Jaldhaka river basins, two adjacent systems in the Eastern Himalayas, has tackled this question. Researchers from the Indian Institute of Science Education and Research (IISER) Kolkata, the University of Calcutta, Kolkata, and the Zoological Survey of India (ZSI), Port Blair, spent five years studying the factors that affect the health of local freshwater fish communities. Their study reveals that while habitat is the primary driver, the specific rules of assembly are surprisingly inconsistent between the two neighbouring rivers.
Did You Know? The Eastern Himalayan rivers, including the Teesta and Jaldhaka, originate in snow-clad mountains and glacial lakes before flowing south to meet the Brahmaputra and Ganges Rivers. The formation of these river basins is linked to the dramatic Miocene uplifting of Eastern Tibet, caused by the collision of the Indian plate with the Burma and Chinese plates. |
The researchers collected data between 2015 and 2020 from 40 accessible sampling sites across the two basins. At each site, they recorded the presence or absence of fish species and measured 11 environmental variables, including elevation, stream width, water temperature, dissolved oxygen, and total dissolved solutes. They also compiled a list of nine eco-morphological traits for each species, such as maximum body length, body shape, and oral gape position, which are hypothesised to relate to a fish's habitat and feeding ecology.
To understand the complex web of ecological interactions occurring in the basins, the researchers employed a statistical tool called Joint Species Distribution Models (JSDMs), explicitly using the Hierarchical Multi-Species Model (HMSC) framework. The HMSC framework enables researchers to model the entire fish community simultaneously, testing the relative importance of multiple factors: environmental variables (the "filters"), spatial distance (dispersal), species traits, and phylogenetic relationships (evolutionary history). By fitting the JSDM to their comprehensive dataset, they could calculate how much of the fish community structure was explained by each factor.
The researchers found that the distribution of fish communities in both the Teesta and Jaldhaka basins is primarily shaped by environmental filtering, meaning the local habitat conditions are the strongest factor determining which species can survive in a given location. The effects of dispersal, or the distance between sites, were found to be weaker than the environmental factors. This finding emphasises that for these lotic (flowing water) fish communities along subtropical elevational gradients, the physical and chemical characteristics of the stream are the primary gatekeepers.
However, the specific environmental factors that matter most differ significantly between the two basins. In the Teesta Basin, the most important variables were total dissolved solids (TDS), stream width, and water velocity. In contrast, the fish communities in the Jaldhaka Basin were most strongly influenced by elevation. This variability makes it challenging to create broad, one-size-fits-all conservation plans across the entire region.
When examining the role of dispersal, the study found only weak evidence for spatial autocorrelation, or the similarity of assemblages over distance, in the Teesta Basin. This is a key finding, as the Teesta is known to be heavily impacted by human activities like damming and hydropower generation, which can obstruct river flow and degrade water quality. The Jaldhaka Basin, which is less disturbed, showed a slightly stronger signal of community similarity over distance (up to about 10 km), suggesting that human impacts in the Teesta may be limiting the fish's ability to move and disperse.
The researchers also investigated the role of eco-morphological traits, physical characteristics like body length, eye position, and fin shape, in predicting where species occur. Traits explained a relatively small amount of the variation in species niches (8% in Teesta, 14% in Jaldhaka). The only consistent trait-niche relationship found across both basins was that fish with a greater maximum body length (MBL) showed a stronger positive response to higher water velocity.
Interestingly, the study found a moderate to strong residual phylogenetic signal in both basins. This means that while the measured traits didn't explain everything, evolutionary history still plays a role, suggesting that other, unmeasured traits (like life history or physiology) that are correlated with a species' lineage are likely important in determining where a fish can live. Finally, the study found that differences in traits between species pairs generally did not predict whether those species would co-occur (be found together).
By employing a trait-based approach and a multi-species model, the new research offers a deeper understanding of the actual mechanisms underlying community assembly. While previous work had identified elevation and hydrology as key modulators, this study affirms those findings while adding the crucial detail of basin-specific variation.
However, the inconsistency in results between the Teesta and Jaldhaka basins makes broad generalisations across freshwater systems difficult. The moderate to strong residual phylogenetic signal suggests that the nine measured traits are not the whole story, and that other, unmeasured traits are likely playing a stronger role. The authors suggest that future analyses incorporate more life history traits and abundance data to improve the model's performance.
Nonetheless, this research provides critical, actionable insights for conservation in the Eastern Himalayas. The finding that environmental filtering is the dominant force means that conservation strategies must prioritise protecting and restoring local habitat quality, such as maintaining healthy water chemistry (TDS, DO) and natural stream morphology (width, velocity). Furthermore, the clear differences between the Teesta and Jaldhaka basins demonstrate that a singular, unifying conservation strategy is unsuitable. Instead, management plans must be tailored to the specific ecological drivers of each river basin.
This article was written with the help of generative AI and edited by an editor at Research Matters.