Generalist plants are more competitive and more functionally similar to each other than specialist plants: insights from network analyses

Aim: Ecological specialization is a property of species associated with the variety of contexts they occupy. Identifying the mechanisms influencing specialization is critical to understand species coexistence and biodiversity patterns. However, the functional attributes leading to specialization are still unknown. Similarly, there is contrasting evidence between the level of specialization and local abundance of species. We ask whether plant specialist and generalist species (i) are associated with distinct functional profiles, using core plant functional traits and strategies, (ii) show comparable functional variation, and how (iii) they perform at local scale.

Location: France, Countrywide scale.

Taxon: Herbaceous plants.

Results: We identified five major modules in the bipartite network, related to different environmental conditions and composed of species displaying different functional attributes. Species that were more specialist were less competitive, had smaller stature, higher stress-tolerance and stronger resource conservation, while generalist species were taller. Generalists were also more similar among themselves than specialists. In addition, specialist species had higher local abundances and occurred in communities with plants of similar height.

Main conclusions: We found distinctive functional signatures of specialist and generalist species in grassland communities across diverse environments at regional and community scales. Network metrics can benefit community ecology to test classical macro-ecological hypotheses by identifying distinct ecological unit at large scale and quantifying the links developed by species.

Methods
Our approach is based on the structure of a bipartite network integrating the occurrences of ~2.900 plant species in ~90.000 sites. We identified ecologically coherent sets of species and sites, called modules. To define a metrics of specialization, we quantified the occurrences of species in sites belonging to one or several modules. We used functional traits related to resource acquisition, competition for light, dispersal abilities and indices of competitive, stress-tolerance and ruderal strategies.