Understanding the relative contribution of different biotic interactions in shaping species assemblages constitutes a major goal in community ecology and consequently, multiple methods aimed at inferring the nature of these associations have emerged during the last decade. In this framework, the stress-gradient hypothesis (SGH) predicts that prevalent biotic interactions shift from competition to facilitation as abiotic stress increases (and productivity decreases). This hypothesis originally raised by plant ecologists has been barely applied to faunal communities. Here, we take advantage of 20 years of abundance data to investigate pairwise patterns in species co-occurrence in Alpine bird communities inhabiting two contrasting habitat types; forests (high-productivity) and mountain grasslands (low-productivity). We also integrate functional data with presence-absence and quantitative matrices in order to detect the signature of processes driving community assembly and test for limiting similarity. We employed a Bayesian approach, probabilistic pairwise association tests and joint Species Distribution Models; all methods revealed a higher frequency of positive interactions in mountain grasslands in agreement with what the SGH predicts. Both the frequency of positive and negative interactions remained moderately stable over the study period in both habitat types. There was no significant relationship between the degree of co-occurrence of species pairs and their functional distance in either habitat. However, when we only considered those combinations of species whose co-occurrence pattern deviated from that expected at random, we found that co-existing species are functionally more similar than those pairs that show segregated patterns in the forest assemblages. Such a relationship may arise via selective social information use and other processes including microhabitat preferences. Overall, our findings suggest that interspecific competition does not seem to be a major force driving the structure of bird assemblages in this mountain region.

This dataset comes from the "Monitoring Häufige Brutvögel" MHB program coordinated by the Swiss Ornithological Institute.

The MHB sample consists of 267 1-km squares that are laid out as a grid across Switzerland. Most of the MHB sample squares form a subset of the larger sample (ca. 500 squares) surveyed as part of the federal programme Biodiversity Monitoring Switzerland. Fieldwork is conducted by about 200 skilled birdwatchers, most of them volunteers, whose collaboration is invaluable for this project. Avian populations are monitored using a simplified territory mapping protocol, where each square is surveyed three times and above the timberline twice, respectively, during the breeding season. Surveys are conducted along a square specific transect route that does not change over the years. For each detected species, only records that meet certain conditions (e.g. in terms of survey date) are retained.

This dataset consists of bird abundance data (plot x species format) for a 20 yr period (one file per year) for each of the habitats included in this study: FOREST and GRASSLAND habitat.