Phylogenetic relatedness, ecological strategy, and stress determine interspecific interactions within a salt marsh community

Abstract

Species interactions in nature can be positive or negative. The stress gradient hypothesis (SGH) states that the strength of positive interactions increases with increasing stress. The phylogenetic limiting similarity hypothesis (PLSH) states that competition intensity is likely to be greater between closely related species than between distantly related species. Testing the SGH, the PLSH and determining the factors that influence species interactions with changing stress levels are important for ecosystem conservation and restoration. In the following study we conducted experiments to investigate the effects of salinity stress, phylogenetic relatedness (i.e., the sum of branch lengths separating species on a phylogenetic tree), and species ecological strategy on interspecific interactions using 11 species found with in a salt marsh located in the Yellow River Delta, China. We found most of the species interactions across increasing salinity levels to be inconsistent with the SGH. The net outcomes of interspecific interactions were significantly affected by multiple factors, including salinity stress, phylogenetic distance, ecological strategy, and the interaction between salinity and phylogenetic distance. Importantly, with increasing phylogenetic distance separating a pair species, the likelihood of facilitative interactions was increased and the likelihood of competitive interactions was reduced; this relationship was especially strong at medium and high salinities and supports the PLSH.