Introduction
Understanding how species assemble into communities is one of the most
fundamental themes in ecology (Weiher et al. 2011; Götzenberger, et al.
2012; Gerhold et al. 2015). Key postulates are that evolutionary forces,
environmental conditions and inter-species interactions combine to
structure local communities (Cavender-Bares et al. 2009; Webb et al.
2002), with prominent theories proposing either neutral or deterministic
processes (Swenson and Enquist 2009; Kembel 2010; Chase and Myers 2011).
Neutral theory holds that species initially have quasi- identical
requirements, and communities become structured by some dynamic balance
between species loss through extinction, immigration and speciation
through genetic drift (Kimura 1991). Conversely, niche-based concepts
emphasize how environmental factors determine assembly through filtering
mechanisms that limit the occurrence of species with similar traits
(Kraft et al. 2015). While niche overlap – also known as limiting
similarity – is expected to exclude similar species from co-existing
(Macarthur and Levin 1975), environmental filtering and niche shifts act
to moderate the extent to which similar species co-occur in similar
habitat conditions (Weiher et al. 2011; Gerhold et al. 2015; Ulrich et
al. 2018).
An important proviso in studying species and trait assembly in
communities is that species relatedness should be controlled or
represented in order to eliminate phylogeny as a potential confound
(Mayfield and Levine, 2010; Kraft et al., 2015; Cadotte and Tucker,
2017). Phylogenetic analyses can account for trait expression at the
species level thus enabling insights into the evolution of habitat
preferences, species function and distribution patterns (Webb et al.,
2002; McGill et al., 2006). Typical analyses attempt to understand
whether ecologically relevant traits are conserved or modified along any
given phylogeny thereby providing evidence about the roles environmental
filtering and competitive segregation in assembly processes
(Cavender-Bares et al. 2009; Pavoine et al. 2008; He et al. 2018;
reviewed in Cadotte et al., 2017). Ideally, investigations aimed at
understanding communities should blend field observations with some
assessment of the functional and phylogenetic identities of the
component species (McGill et al. 2006; Winemiller et al. 2015; Xu et al.
2017).
Given that ecosystem character changes in time and space, conditions
under which species assemble and co-exist must also vary (McGill et al.
2006). Such environmental gradients offer a means to test competing
assembly theories, for example by revealing relationships between
environmental conditions and the morphological, physiological or
behavioural traits of the species involved (Cavender-Bares et al. 2004;
Dehling et al. 2014). Traits also reflect species’ roles or functions
within communities and can reveal mechanisms that affect distributional
patterns along habitat gradients (Kraft et al. 2007). In terrestrial
ecosystems, for example, competition, trait expression and environmental
filtering along elevational gradients can have marked effects on bird
communities (McCain 2009; Machac et al. 2011; Dehling et al. 2014; He et
al. 2018; Ulrich et al. 2018; Ding et al. 2019; Chiu et al. 2020).
Among all ecosystems, rivers have received considerable emphasis in
community ecology (Ward et al. 1998; Robinson et al. 2002; Altermatt et
al. 2020), including seminal assessments of assembly rules,
environmental filtering and trait-based studies (Poff 1997; Heino et al.
2015). In part, this interest reflects the pronounced environmental
gradients represented by rivers both longitudinally and among
contrasting river basins that together have created a diverse habitat
template into which species have proliferated (Townsend and Hildrew
1994; Terui et al. 2021). Growing concern about the global status of
freshwater ecosystems is also prompting interest in interactions between
natural biodiversity patterns in rivers and the effects of environmental
change (see Dudgeon et al. 2006).
So far, little of the research effort into community assembly has
focussed on high-energy river systems in mountain landscapes, where
large altitudinal ranges, complex topography and geomorphological
dynamism give rise to pronounced ecological gradients with large species
turnover (Ormerod et al. 1994; Jacobsen et al. 1997). Moreover, despite
being conspicuous components of the global riverine fauna, river birds
have been neglected in fundamental studies of mechanisms structuring
communities, especially in mountainous areas (Manel et al. 2000; Sinha
et al. 2019). One such region, the Himalayan Mountains, has the most
diverse communities of specialist river birds on Earth (Buckton and
Ormerod 2002) – thus prompting questions about evolutionary mechanisms
that have allowed their coexistence. Marked diversity and distinctness
in habitat use has led to some speculation about the roles of
environmental filtering and niche partitioning, but there has been no
formal analysis using current methods, and no attempts to assess
phylogenetic effects in community assembly (Buckton and Ormerod 2008).
In this paper, we use specialist river birds to examine Southwood’s
original premise (1977, 1988), restated for rivers by Townsend &
Hildrew (1994), that habitats provide the templet through which
evolutionary forces act with phylogeny to determine species’
life-history. In turn, the resulting contrasts in species’ traits act to
determine how communities can contain multiple species while also
influencing how communities change along environmental gradients.
Specifically, we investigated river birds along multiple headstreams in
the north-west Himalayan mountains of India,
hypothesizing that river bird
communities reflect detectable trait–environment relationships arising
from environmental filtering. We asked (1) are there non-random patterns
in species distribution and species’ traits that reflect
trait-environment relationships? and (2) are local species pools a
result of common phylogenetic ancestry or convergence in response to
environmental or biotic filters acting on regional communities?
Addressing the first of these questions allowed us to quantify community
change largely in relation to elevation while the second helped to
identify how trait expression along this elevation gradient reflected
filtering beyond the constraints of phylogeny.