Altitudinal limits of Eastern Himalayan birds are created by competition past and present

Abstract

The degree to which interspecific competition structures diverse communities is an oft-debated topic. An approach to answering this question is to examine spatial patterns of coexistence among putatively competing species. The degree to which interspecies competition predominates in a community can have important effects on our ability predict the response of that community to perturbations, most notably climate change, when shifting species’ ranges may result in novel species assemblages. We present a study on the avifauna of the Eastern Himalayas. We hypothesize that in a community where competitive interactions predominate, there will be a relationship between pairwise altitudinal overlaps and morphological differences between species. Moreover, we hypothesize that both morphological traits and altitudinal traits depart from a Brownian motion evolution model, resulting in species trait covariances having a phylogenetic component. We find a significant relationship between morphological dissimilarity and altitudinal overlaps of species pairs. We also find that closely related species are significantly more altitudinally stratified than a null model would predict. However, as more distantly related species pairs are included in the analysis, this pattern disappears, indicating that competitive interactions predominate only in closely related species. This is further suggested by the fact that altitudinal ranges themselves are phylogenetically overdispersed at the genus level, as are morphological traits. This effect disappears when the entire phylogeny is examined, with morphology and altitude being phylogenetically underdispersed. Model fitting suggests that individual clades have evolved towards local clade-specific fitness peaks, while within-clade results show evidence of niche partitioning. We interpret these results as a tension between competition on shorter time scales and selection on longer time scales, where competition forces closely-related species away from fitness peaks in order to allow for niche separation and hence coexistence, suggesting that this effect is partially responsible for the recent diversification of Eastern Himalayan avifauna.

Bird species co‐occurrence patterns in an alpine environment supports the stress‐gradient hypothesis

“…Here, we examined patterns of species co‐occurrence over a 20‐year period in bird communities of the Swiss Alps applying a pluralistic analytical approach. Our results do not evince a prominent role of competition as structuring agent in the studied assemblages, which is in accordance with that reported in other mountain systems (Bornean mountains: Boyce et al 2019, Burner et al 2020, Cantabrian mountains: Bastianelli et al 2017; eastern Himalayas: Elsen et al 2017, but see Surya and Keitt 2019). However, we must stress that competitive interactions are difficult to infer using observational data.…”

An environmental habitat gradient and within-habitat segregation enable co-existence of ecologically similar bird species

“…Elevation niche partitioning was identified as a potential coexistence mechanism in over half of the species pairs in the community. Together with the estimated reduction in residual pairwise correlations in the extended warbler model (figure 4), these results provide strong evidence of niche partitioning along this Grinellian mechanism for ecologically similar bird species in the Virunga volcanoes, consistent with previous studies in the Cameroonian mountains [61], Himalayan highlands [26], tropical Andes [62,63] and in New Guinea highlands [64,65]. A recent survey of birds covering a third of the study area (Parc National des Volcans in Rwanda) identified 57% of species to be associated with narrow elevation bands (less than 300 m; [66]).…”

“…While our analyses highlight that coexistence of ecologically similar species is facilitated by variation in vertical and horizontal space use, and to a lesser extent variation in diet preferences ( table 1 ), we were unable to quantify how coexistence mechanisms vary across space [ 26 ] and time [ 69 ]. For example, consider two species ( Cinnyris regius and Cinnyris stuhlmanni ) with partial elevation overlap ( figure 3 b ): vertical forest strata niche partitioning is likely to be more important for individuals of both species within the overlapping environmental niche space compared to individuals at species optima or edges of the gradient.…”

“…Alternatively, Eltonian variables are affected by the presence of the species, operate at fine scales, and are density dependent. Ecologically similar bird species could partition their niches through attributes that characterize the Eltonian niche such as vertical stratification of feeding zones [ 21 ], variation in activity pattern [ 22 ], differences in diet and foraging behaviour [ 23 ], interspecific territoriality [ 19 , 24 ], body size variation [ 25 ] and within-habitat segregation [ 26 ], and/or through factors that characterize the Grinellian niche such as climate and habitat variables [ 27 ].…”

Intraspecific trait variability and community assembly in hawkmoths (Lepidoptera: Sphingidae) across an elevational gradient in the eastern Himalayas, India

“…New species, of even distinctive vertebrate taxa, continue to be described from the region (e.g., Athreya, 2006a; Captain et al., 2019; Mirza et al., 2020; Sinha et al., 2005; Sondhi & Ohler, 2011). The entire list of research publications on diversity patterns in the region is a short one: elevational gradient of bird diversity (Acharya et al., 2011; Price et al., 2014; Schumm et al., 2020; Surya & Keitt, 2019), tree diversity patterns and population structure (Bhuyan et al., 2003; Rana et al., 2019), and distribution and abundance of arthropods (Ghosh‐Harihar, 2013; Marathe et al., 2020; Supriya et al., 2020).…”