Bird species co-occurrence patterns in an Alpine environment supports the stress gradient hypothesis

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.