Broader implications: conservation and environmental change
As well as their relevance to evolutionary influences on river birds over the large altitudinal range of the Himalayan Mountains, our findings have broader implications for biodiversity conservation. Human impacts on rivers tend to simplify structural complexity, reduce connectivity and impair water quality, and across the world these processes are contributing to the decline or elimination of specialist organisms and population reductions that are among the fastest of any global ecosystem (Evans et al. 2018; Bower and Winemiller 2019; Tickner et al. 2020). These effects arise because river catchment ecosystems are both hotpots for biological diversity and hotspots for resource exploitation (Strayer and Dudgeon 2010). Both these factors have parallels with our data. First, at a global level, the association between overall bird richness and habitat heterogeneity is a well-known phenomenon, especially for species that are specialized for particular habitat types – in this case high-energy rivers (Robinson et al. 2002; Larsen et al. 2010). Specialist river birds have developed unparalleled richness and niche specificity in the Himalaya reflecting both the complex relief and productivity in this region so that major habitat impairment could have effects of global significance (Buckton and Ormerod 2002). Secondly, these same river environments face multiple pressures, for example, from climate change, catchment conversion to agriculture, pollution, hydropower and water-resource exploitation (Manel et al. 2000; Sinha et al. 2019). Some species in our study were associated with the least modified river reaches where bank vegetation, geomorphological structure and flow patterns were unimpaired and expected to support abundant prey (Ormerod and Tyler 1987,1991b; Sinha et al. 2019). Possible effects of habitat modification were also apparent in the different river basins surveyed, for example where river reaches in the Bhagirathi basin were modified for hydropower development (Fig.1 (a)). If our interpretation is correct – that riparian and riverine habitat features act as environmental filters that structure river bird assem­blages locally – it is likely that anthropogenic effects on rivers will modify these filtering processes and alter community composition unless checked by conservation action. Particular phylogenetic groups of species are at risk.