Niche filling slows the diversification of Himalayan songbirds

Abstract

Speciation generally involves a three-step process--range expansion, range fragmentation and the development of reproductive isolation between spatially separated populations. Speciation relies on cycling through these three steps and each may limit the rate at which new species form. We estimate phylogenetic relationships among all Himalayan songbirds to ask whether the development of reproductive isolation and ecological competition, both factors that limit range expansions, set an ultimate limit on speciation. Based on a phylogeny for all 358 species distributed along the eastern elevational gradient, here we show that body size and shape differences evolved early in the radiation, with the elevational band occupied by a species evolving later. These results are consistent with competition for niche space limiting species accumulation. Even the elevation dimension seems to be approaching ecological saturation, because the closest relatives both inside the assemblage and elsewhere in the Himalayas are on average separated by more than five million years, which is longer than it generally takes for reproductive isolation to be completed; also, elevational distributions are well explained by resource availability, notably the abundance of arthropods, and not by differences in diversification rates in different elevational zones. Our results imply that speciation rate is ultimately set by niche filling (that is, ecological competition for resources), rather than by the rate of acquisition of reproductive isolation.

Functional traits reveal the expansion and packing of ecological niche space underlying an elevational diversity gradient in passerine birds

“…Whether this pattern holds across all birds is open to question, but it seems likely given that passerines make up more than half of all avian diversity. Moreover, our results are consistent with earlier reports that average trait distances between species are smaller at low elevations for frugivores along the same gradient [23], and that the morphovolume of Himalayan songbird assemblages actually declines towards the lowlands [36]. Previous clade-level analyses across environmental gradients also reveal that the local richness of passerine families is largely unrelated to the volume of trait space they occupy [37].…”

“…The use of normalized PC trait axes when quantifying assemblage morphospace assumes that each trait axis is of similar importance in charactering avian niche space [36,37]. To test this assumption, we estimated the contribution of each PC trait in predicting guilds by removing each axis as a term from the model and calculating the drop in classification accuracy.…”

Uplift-driven diversification in the Hengduan Mountains, a temperate biodiversity hotspot

“…31, 50, and 51). In the Himalayas, the lack of in situ diversification of songbirds has been attributed to the filling of niches by preadapted colonizing lineages (52).…”

The evolutionary convergence of avian lifestyles and their constrained coevolution with species' ecological niche

“…We chose the backbone tree based on Ericson et al [35] to obtain a 50% majority rule consensus tree in which branch length is represented by the proportion of nucleotide substitutions (electronic supplementary material, figure S2). Being a sympatric assemblage of species, the phylogenetic relationship among them should capture differentiation mechanisms limited by niche similarities [19]. We first quantified the influence of phylogeny on variation in each trait by means of K statistics [28], to assess whether traits conserved a phylogenetic signal and whether predictors (body mass, migration strategy and ecological parameters) were not changing too fast with respect to the rate of adaptation of life histories, a condition in which adaptive variation cannot take place [22].…”