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.