Interspecific facilitation influences tidal marsh structure and function by ameliorating stress, thus filling an important role in estuarine ecosystems. Research in salt marshes dominated by the grass Spartina alterniflora indicates that plant characteristics affect fiddler crab burrowing and in turn, crab activity can enhance primary productivity by increasing soil oxygen and nutrient cycling. Crab-plant interactions have not been well studied in microtidal Gulf Coast marshes where S. alterniflora is restricted to a narrow band along the low intertidal zone, the rush Juncus roemerianus forms the most extensive vegetation zone, which grades upslope into a narrow fresh marsh, and hypersaline salt pannes are patchily distributed. It is unknown how structure of these dominant vegetation zones affects density of crab burrows and how burrows may influence primary productivity. We hypothesized that fiddler crabs would be most abundant in marsh zones with intermediate substrate hardness and vegetation density (Goldilocks Hypothesis). To determine fiddler crab usage of these zones, we conducted a seasonal habitat preference study in tidal marshes at Grand Bay National Estuarine Research Reserve in coastal Mississippi using burrow density as a proxy for crab abundance. We also sampled plant above- and below-ground biomass, burrow proximity to vegetation and soil hardness as potential drivers of fiddler crab populations.
Results/Conclusions
Our results indicated that fiddler crabs burrow in all four zones, but to varying degrees and that burrow density was highest during our autumn survey. The fresh marsh had the highest average density of burrows as well as vegetation and soil parameters most representative of intermediate habitat, thereby supporting our hypothesis. The brackish marsh also proved to be important fiddler crab habitat. Given potential facilitative effects of fiddler crab bioturbation on the growth of tidal marsh plants, it is critical to better understand their association before it becomes potentially altered by shifting habitat. Preferential fiddler crab usage of upslope habitat like fresh and brackish marsh in our Gulf Coast sites suggests that transitions to higher elevations with sea level rise may be relatively smooth.