Plant–plant spatial association networks in gypsophilous communities: the influence of aridity and grazing and the role of gypsophytes in its structure
In stressful environments many plant species are only able to survive if they benefit from the facilitative effect of "nurse" species. Typically, these nurses are species adapted to the stressful environmental conditions that favor the formation of vegetation patches, where other, less-adapted species can be established. However, ecological interactions can be influenced by abiotic and biotic factors. In this study we quantified the effect of grazing and aridity on the patch structure of gypsophilous plant communities and the role that gypsophytes, species adapted to gypsum soils, play in structuring these communities. Specifically, we created signed networks (networks with positive and negative links) at grazed and ungrazed sites in two areas in the middle Ebro Valley, Spain, that differed in aridity. We built networks connecting plant species with positive and negative links derived from the spatial associations between species. Then, we divided networks in partitions which represented the different vegetation patches present in the community. We found that vegetation patches were more specific (same species always were associated in the same patch type) in high aridity and grazed sites, where environmental conditions were the most stressful and many species persisted by associating with nurse species. Gypsophytes were more important aggregating species than nongypsophytes in grazed high aridity sites. Independently of study sites, gypsophyte shrubs acted as nurses, but small gypsophytes segregated from other species and formed monospecific patches. In conclusion, grazing and aridity influenced the patch structure of gypsophilous plant communities. Gypsophytes played an important role structuring the patch community, but this importance depended on environmental conditions and the identity of gypsophyte.