The role of the physical structure of Spartina densiflora Brong. in structuring macroinvertebrate assemblages

In Patagonian rocky salt marshes, the presence of the austral cordgrass Spartina densiflora provide habitat for diverse faunal assemblages. Two different mechanisms may influence the distribution and abundance patterns of these associated organisms: those generated by the biological properties of cordgrasses and those caused or mediated by the physical structure supplied by the plants. The aims of this study were: (1) to determine the effect of cordgrasses on macroinvertebrate assemblages dominating a rocky marsh and (2) to determine how much of this effect is caused by the physical structure supply by the plants. In order to achieve these objectives, we conducted two field manipulative experiments. In the first one, we manipulated the presence of defaunated transplants of cordgrass, and in the second one, we did the same with plastic cordgrass mimics simulating the architecture of Spartina densiflora. In both experiments, sessile and mobile assemblages were separately examined to evaluate whether they respond in the same way or not. Abundance and richness of mobile and sessile organisms were higher in transplant and mimic cordgrass plots compared to the controls, indicating that the presence of cordgrasses, either transplanted or artificial, has a positive effect on sessile and mobile assemblages. Furthermore, we found that the composition of mobile and sessile macroinvertebrates assemblages did not differ between transplants and mimics. Therefore, the physical structure of cordgrass was found to be the major factor influencing macroinvertebrates assemblages in the studied Patagonia rocky salt marshes. Within the period of 1 year, mobile and sessile fauna colonized several times more abundantly transplant and artificial cordgrass than the control unvegetated plots, highlighting the key role of cordgrasses in optimizing the colonization rate of macroinvertebrate communities in this rocky bottom environment. Since Spartina densiflora is invading different regions worldwide, our study may help to predict its potential effect on the invaded communities.