Aim: Mountains are biodiversity hotspots and are among the most sensitive eco- systems to ongoing global change being thus of conservation concern. Under this scenario, assessing how biological communities vary over time along elevational gra- dients and the relative effects of niche-based deterministic processes and stochastic events in structuring assemblages is essential. Here, we examined how the temporal trends of bird communities vary with elevation over a 20 year-period (1999–2018). We also tested for differences in temporal dynamics among habitat types (among-community variability) and functional groups (within-community variability).

Taxon: 97 species of common breeding birds.

Location: Swiss Alps.

Methods: We used abundance data from the Swiss breeding bird survey to compute different temporal dynamic metrics (temporal turnover, synchrony, rate of community change and community-level of covariance among species). We also examined the relative contribution of deterministic and stochastic processes in community assembly using the Raup-Crick method and the normalized stochasticity ratio. Results: We found that, with greater elevation, temporal species turnover increased while the rate of overall community change over successive years decreased, suggesting that high-elevation communities display more erratic dynamics with no clear trend. Despite this, we found a more deterministic assembly of alpine communities in comparison to those located at lower elevations. Deterministic processes had greater influence than stochastic processes on community assembly along the entire elevational gradient (80% of communities). Forest communities exhibited higher synchrony in comparison to the remaining habitats likely because they consisted of species with greater functional redundancy, whereas alpine communities were the least stable as a result of their low taxonomic richness (‘portfolio’ effect).

Main conclusions: Community-level synchrony was overall positive supporting the idea that compensatory mechanisms are rare in natural biological communities. Our results suggest that rather than competition, the existence of differences in the eco-logical strategies of species may have a stabilizing effect on bird communities by weakening the concordance of species responses to fluctuations in environmental conditions (i.e. enhanced interspecific temporal asynchrony). This study provides evidence that, although species turnover in metacommunities is frequent, a high tem- poral turnover does not necessarily imply the overriding importance of stochastic processes.

Relative abundance data (number of territories per site) collected during two decades (period 1999-2018) in 131 plots along a ~2,300 m elevational gradient (range: 380–2,710 m) in the Swiss Alps. A total of 97 common species (one columen per species) were considered in this study. The first two columns indicate plot ID and study year, respectively. 

Data comes from the ‘Monitoring HäufigeBrutvögel’ (MHB) scheme, a volunteer-based programme aimed at annual monitoring of population dynamics of common breeding species in Switzerland. This program is coordinated by the Swiss Ornithological Institute (www.vogelwarte.ch).