Biotic interactions are the dominant drivers of phylogenetic and functional structure in bird communities along a tropical elevational gradient

Abstract

Understanding how biotic and abiotic interactions influence community assembly and composition is a fundamental goal in community ecology. Addressing this issue is particularly tractable along elevational gradients in tropical mountains that feature substantial abiotic gradients and rates of species turnover. We examined elevational patterns of avian community structure on 2 mountains in Malaysian Borneo to assess changes in the relative strength of biotic interactions and abiotic constraints. In particular, we used metrics based on (1) phylogenetic relatedness and (2) functional traits associated with both resource acquisition and tolerance of abiotic challenges to identify patterns and causes of elevational differences in community structure. High elevation communities were composed of more phylogenetically and functionally similar species than would be expected by chance. Resource acquisition traits, in particular, were clustered at high elevations, suggesting low resource and habitat diversity were important drivers of those communities. Traits typically associated with tolerance of cold temperatures and low atmospheric pressure showed no elevational patterns. All traits were neutral or overdispersed at low elevations suggesting an absence of strong abiotic filters or an increased influence of interspecific competition. However, relative bill size, which is important for thermoregulation, was larger in low elevation communities, suggesting abiotic factors were also influential there. Regardless of metric, clustered and neutral communities were more frequent than overdispersed communities overall, implying that interspecific competition among close relatives may not be a pervasive driver of elevational distribution and community structure of tropical birds. Overall, our analyses reveal that a diverse set of predominantly biotic factors underlie elevational variation in community structure on tropical mountains.

Biotic interactions help explain variation in elevational range limits of birds among Bornean mountains

“…Interestingly, though, our results mesh well with inferences from studies of phylogenetic community structure. These studies of the degree to which species in communities are phylogenetically related indicate that competition is more important at low elevations, whereas physiological constraints, either direct or indirect, are more important at high elevations (Boyce et al, ; Graham, Parra, Rahbek, & McGuire, ; Machac, Janda, Dunn, & Sanders, ). Our results provide a complementary test of these results.…”

“…To test for possible phylogenetic signal in the direction and magnitude of species range shift values, we used a maximum likelihood tree of Bornean birds based on a concatenated dataset of two mitochondrial and two nuclear loci from Boyce, Shakya, Sheldon, Moyle, and Martin () and package ‘phylosignal’ (Keck, Rimet, Bouchez, & Franc, ) to calculate Moran's I for each pair of mountains. Moran's I is a measure of autocorrelation of traits along the tree (Gittleman & Kot, ).…”

Evidence for intraspecific phenotypic variation in songbirds along elevation gradients in central Europe

“…Along an elevation gradient, temperature (the variable hypothesized to explain Bergmann’s Rule) might not be the only determining factor shaping bird morphology. Indeed, in tropical mountain systems, it has been shown to be of lower importance for trait variations along elevation gradients (Freeman 2017, Boyce et al 2019); instead, flying in thin air (lower air pressure at high elevations) and the necessity to increase flight performance (due to stronger winds) might select for longer wings (Altshuler & Dudley 2006, Bears et al 2008). The seasonality of resource availability in higher elevation habitats, e.g.…”

“…For example, Freeman (2017) found little evidence for body size clines within tropical bird species, but where there were differences, they were inconsistent with Bergmann’s Rule. Similarly, Boyce et al (2019) found that body mass of Bornean mountain birds decreased with elevation, whereas relative tarsus length increased. The authors argued that phenotypic adaptation is more likely to be driven by the foraging mode of high‐elevation species, rather than temperature (Boyce et al 2019).…”

“…Similarly, Boyce et al (2019) found that body mass of Bornean mountain birds decreased with elevation, whereas relative tarsus length increased. The authors argued that phenotypic adaptation is more likely to be driven by the foraging mode of high‐elevation species, rather than temperature (Boyce et al 2019). In both of these studies, wing length was not related to elevation, even though other studies have argued that longer wings reduce the cost of flying at lower air pressure (Scott 2011) and may increase aerodynamic performance (Altshuler & Dudley 2006).…”