DOI: 10.5061/dryad.np5hqbzth
title: Bird species co-occurrence patterns in an Alpine environment supports the stress gradient hypothesis

description: 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.
description: 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.