Positive interactions of the smooth cordgrass Spartina alterniflora on the mud snail Heleobia australis, in South Western Atlantic salt marshes

The role of positive interactions is often crucial in communities with intense abiotic stress such as intertidal environments. Grasses acting as ecosystem engineers, for example, may ameliorate intertidal harsh physical conditions and modify the community structure. The mud snails Heleobia australis d'Orbigny frequently inhabit the SW Atlantic marshes, mainly associated to intertidal marsh plants (mainly the smooth cordgrass Spartina alterniflora Loisel) probably due to the plant indirect effects. The purpose of this work was to investigate the magnitude of these association and the processes that generate the pattern. Samples of the snail abundance in six SW Atlantic coastal marshes show that H. australis is associated to coastal areas of low energy and low or none freshwater input. This result is important because this species is being used as bioindicator of coastal estuarine systems during the Holocene. Thus the paleontological interpretation based on this species should be revised. Within the studied areas, snails are associated to intertidal marsh plants. However, stable isotope analysis shows that neither plant nor their epiphytes are their main food sources. Field experiments show that snails actively select areas with plants, although tethering experiments show that plants do not provide shelter from predators. However, plants do buffer physical stress factors such as temperature, which generate important mortality outside plants covered areas. These positive interactions have large effects on H. australis distributions in marsh communities; increasing the habitats available for colonization and affecting their local distribution. (c) 2007 Elsevier B.V. All rights reserved.