Fiddler Crab Regulation of Spartina alterniflora Production on a New England Salt Marsh

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Abstract
While a great deal of research has been directed at understanding the high productivity of marsh grasses, little attention has been paid to the potential importance of marsh fauna in influencing marsh grass production. The mud fiddler crab, Uca pugnax, excavates burrows in Spartina alterniflora—dominated salt marshes along the East Coast of North America. Here, I examined experimentally the effect of fiddler crab burrowing on S. alterniflora production. At low intertidal heights, S. alterniflora production was robust and characterized by tall—form plants with roots and rhizomes that penetrate deeply into the substrate. At higher tidal heights, low productivity, short—form S. alterniflora with a dense superficial root mat prevailed. Fiddler crabs differentially affected grass production at different tidal heights. In tall—form S. alterniflora stands at intermediate tidal heights, experimental reduction of fiddler crab density for a single growing season decreased aboveground production by 47% and increased root mat density by 35%. In contrast, on the lower intertidal soft edge of the marsh and in the high intertidal, dense root mat, short—form S. alterniflora zone, experimental reduction of crab density did not have as marked an effect on cordgrass production. Crab burrows were shown to increase soil drainage, soil oxidation—reduction potential, and in situ decompostion of belowground plant debris. The relationship between fiddler crabs and S. alterniflora appears to represent a faculatative mutualism. On the sewart edge of marshes in soft sediment that would not otherwise support burrow structures, the roots, rhizomes, and debris of S. alterniflora provide structural support and consequently facilitate burrowing. At intermediate tidal heights, intensive burrowing activity increases aboveground grass production and prevents the establishment of a dense root mat, while crabs are largely precluded from burrowing in the dense root mat, short—form S. alterniflora stands at higher tidal heights. Therefore, while initially dependent on S. alternifora in soft sediments, U. pugnax burrowing activity appears to maintain a habitat suitable for continued burrowing, and as a byproduct, increases cordgrass production and maintains tall—form S. alterniflora stands.