Positive plant-animal interactions in the high marsh of an Argentinean coastal lagoon

Although plant-plant facilitations have been shown to be important structuring forces in salt marshes, less attention has been given to the potential role played by plant-animal facilitations in structuring these communities. We used a combination of sampling and field experiments to evaluate the effect of microenvironmental changes produced by plant cover on the distribution of the burrowing crab Chasmagnathus granulata, a dominant macroinvertebrate of high marshes of the southwestern Atlantic coast. Four questions were explored. Is there a relationship between the spatial distribution of C. granulate and the spatial distribution of rooted macrophytes or distance from the marsh edge? How important is plant cover for the establishment and survival of crabs in the high marsh'? Does plant cover affect critical physical variables for crab establishment? How important are environmental conditions for the survival of crabs in the high marsh? Surveys of the marsh surface showed that: (1) there was a strong relationship between the presence of plant cover and the spatial distribution of Chasmagnathus granulata in the high marsh, and (2) both mean crab size and burrow density decreased from the marsh edge to high-marsh levels. By shading the substrate, live plants and experimental plant mimics were found to be equally efficient at buffering high temperature, dehydration, and soil hardness in the high marsh. Experimental amelioration of these harsh physical conditions led to higher crab densities. Crab burrows also buffered harsh environmental conditions, diminishing surface air temperature from similar to46degreesC to 23degreesC. Finally, tethering experiments showed that stressful heating in the high marsh is lethal for crabs, and that plant cover is crucial for the establishment and long-term success of crabs in the high marsh. No predation was observed in tethering experiments. Our results suggest that plant cover is largely responsible for determining the spatial distribution of this dominant crab in the high marsh through facilitation. Thus, our work shows that plant-animal facilitations as well as plant-plant facilitations are important structuring forces in salt marsh communities.