Discussion
These data confirm the multifaceted changes in environmental conditions
along Himalayan rivers over their large altitudinal range (Manel et al.
2001), in turn accompanied by pronounced variations in the community
composition and trait character of river birds. Functional distances
between co-existing species decreased with increasing elevation after
controlling for species richness such that only a subset of traits
persisted (Fig. 2). Communities at higher altitudes shifted towards
species with smaller bodies, shorter tarsi, smaller bills and a greater
tendency to feed as insectivores in the ripariang zone or on aquatic
prey. These patterns are consistent with the hypothesis that altitudinal
trends affect these communities through environmental filtering (Dehling
et al. 2014; Vollstädt et al. 2017; Hanz et al. 2019). They also echo
similar filtering effects for example, on ground beetles along a land
disturbance gradient (Ribera et al. 2001), plants along a salinity
gradient (Pavoine et al. 2011), ants along a complexity gradient
(Weischer et al. 2012), bats across a gradient of forest fragmentation
(Farneda et al. 2015) and birds with urbanization (Evans et al. 2018).
Beyond these filtering effects, however, phylogenetic dispersion also
declined with increasing altitude, illustrating for the first time that
bird species composition along Himalayan rivers is constrained by
phylogenetic origins: passerines, and specifically muscicapids or their
near-relatives, dominated higher altitude rivers.
A range of caveats affect interpretation in studies like ours where
survey data are used to test hypotheses. Above all, the evolutionary
phenomena implied in our analyses occur over temporal and spatial scales
that preclude straightforward experimentation. Large-scale surveys of
this type provide one of the few pragmatic methods of capturing
large-scale phenomena, but need appropriate design to eliminate
potential confounds as well as data that corroborate the ecological or
evolutionary processes inferred from correlations (Manel et al. 2000).
In support of our approach, our design involved surveys that were
replicated across regions, and observations that we took as robust
representations of past evolutionary processes – such as phylogenetic
relatedness or trait expression. Nevertheless, there are well-known
challenges in understanding how trait data or phylogenies reflect
ecological processes (Cadotte et al. 2019). Furthermore, functional
approaches fail to account for within-species variations across
populations while the phylogenetic approach can inflate signals related
to certain traits (Zhao et al. 2020). At a more empirical level, some
parts of our analysis would have been improved by more detailed data.
Feeding traits, for example, were represented only crudely by
categorisations of prey use, yet river birds can make precise selection
for different prey types when foraging. This includes targeting prey of
specific size, elemental composition, accessibility and ease of handling
(Ormerod and Tyler 1991a). Potentially even more important in the
context of niche use and limiting similarity is the extent to which
riparian and river birds in the Himalayan mountains use subtly different
components of the available prey base. This is apparent among the common
insectivores whose diet appears to be partitioned along dimensions of
prey size, taxonomic composition and capture method (aerial vs
terrestrial vs aquatic) (Buckton and Ormerod 2008). However, incomplete
dietary information from several of the species in our study precluded
more detailed dietary assessment. In similar vein, measurements of the
availability or abundance of prey used by any of the species were beyond
the capabilities of this study even though prey abundance is known to
influence the density of river birds (Ormerod 1985; Ormerod and Tyler
1991b). A further limitation is that both our field surveys and data
analysis focussed on the breeding season, yet several of the species in
our study are altitudinal migrants that descend to lower elevations in
winter. As a consequence, our investigation is likely to have reflected
evolutionary effects during the breeding period when resources demands
and selection pressures are likely to be large (Verhulst and Nilsson
2008).
Notwithstanding these caveats, our study revealed clear relationships
among river character, species traits and community composition of river
birds in the Himalayan mountains aided by the increasingly used RLQ
analysis (Ribera et al. 2001). Here, over the largest altitudinal range
on Earth, species composition and trait representation changed
dramatically as several species of kingfishers, River Lapwing, Common
Sandpiper, Blue Whistling-thrush and Spotted Forktail gave way at higher
altitude to a generally smaller, insectivorous and functionally
clustered array of species such as Plumbeous Water Redstart,
White-capped Water Redstart, Brown Dipper and Little Forktail. The
latter group of passerines that breed along high elevation river reaches
come from several genera (Fig. 1(c)) and are morphologically adapted for
different foraging techniques such as fly catching, ground gleaning and
aquatic foraging in aquatic, bankside and riparian habitats. Besides
tracking the trends in habitat structure and vegetation pattern assessed
here, these community changes also reflect well known altitudinal trends
in temperature, nutrient status, oxygen concentrations, discharge
patterns and sediment regimes that have major effects on fish densities,
invertebrate abundances and other factors influencing prey availability
(Ormerod et al. 1994). The resulting heterogeneity in habitat character
and productivity in this region has given rise to the greatest diversity
of specialist river birds on Earth in which selective habitat use,
foraging methods and niche partitioning are consistent with resource
segregation (Buckton and Ormerod, 2002; 2008). These established
patterns add to the support from our data for the hypothesis that river
habitat templates have influenced trait distributions within river bird
communities through the evolutionary history of the species involved.
In addition to major altitudinal trends in community composition and
trait expression among Himalayan river birds, we found that species were
assembled non-randomly along the elevation gradient into communities
with distinct phylogenetic origins and functional character. Although
reflecting patterns among a small group of species, the strength of this
phylogenetic signal implies that historical contingency has influenced
trait-environment relationships and river bird communities in the
Himalaya, particularly at high altitude. It is particularly noteworthy
that three of the four species most abundant at high altitudes were
Muscicapidae - Plumbeous Water Redstart, White-capped Water Redstart and
Little Forktail – an Old-World family with large richness across the
Himalayan region in general (Sinha 2021). When communities of organisms
are shaped predominantly by environmental conditions, their composition
is typically aggregated by similar trait compositions in similar
habitats, irrespective of evolutionary history or phylogenetic
relatedness (Southwood 1977, Poff 1997). This contrasts with our case
where a strong phylogenetic signal in composition and functional traits
reflected circumstances where related species co-occurred because of
shared environmental requirements, similar general morphology and
behavioural character (Webb et al. 2002). Assuming that trait values
(body mass, bill length and tarsus length) reflected niche occupancy,
the strong phylogenetic signals in our data suggest that functional
traits and niche occupancy were constrained by phylogeny, at least at
higher elevation. Interestingly, however, some aspects of trait
expression departed from the expectations of phylogenetic effects more
than others: phylogeny was reflected in body mass, bill size and tarsus
length more than in body size, tail length and breeding traits (Table
1). We suggest that community assembly in high altitude river birds must
therefore reflect a blend of phylogenetic constraint and habitat
filtering coupled with some proximate niche-based selection of trait
character for specialization (Reif et al. 2015; Morelli et al.2019). This effect is particularly well illustrated in the forktails
(Enicurus spp.), in which tail length in the Little Forktail is
substantially reduced in comparison to its congeners and potentially
linked to its highly specialised foraging niche around the splash zone
of large boulders in highly turbulent flows (Buckton and Ormerod 2008).
Similarly, White-capped Water Redstart and Plumbeous Redstart contrast
in body size, with smaller size in the latter potentially facilitating
energy efficiency in its extensive use of aerial foraging. Further
detailed assessments of trait expression and function among Himalayan
river birds would prove interesting.