31 December 2005

Papers read this week

Musical accompaniment: Rachmaninoff: Piano Concerto No. 2 & Tchaikovsky, Piano Concerto No. 1, played by Felicja Blumental.

I have written about the almost-but-not-entirely-terrestrial snail Littoraria angulifera common in mangrove forests in Florida. The following 2 papers are about its close relative Littoraria irrorata (the periwinkle snail). The latter also lives at the edge of the sea and enters it only to spawn.

Brian R. Silliman, Johan van de Koppel, Mark D. Bertness, Lee E. Stanton, and Irving A. Mendelssohn. 2005. Drought, Snails, and Large-Scale Die-Off of Southern U.S. Salt Marshes. Science 310:1803-1806.
The authors did several experiments in salt marshes in Georgia and Louisiana to understand the causes of the recent large scale die-off of the dominant plant Spartina alterniflora (cordgrass). Their results put the blame on severe droughts (1999-2001) that stressed the plants and the enormous numbers of Littoraria irrorata, which at some sites reached mean densities of 834 individuals/m2. The figures from their paper reproduced below compare Spartina biomass in plots from where snails were excluded (solid bars) with those in lots where snails were present (open bars). The latter are hardly visible.

Interestingly, the snails don't actually eat Spartina alterniflora, but the fungi that grow on it. But the snails' grazing damages live plant tissues and consequently, facilitates their subsequent infection with fungi and thereby creating a circular and indirect process of destruction.

My first thought upon reading this paper was that it wasn't quite normal for so many snails to be in one spot. Clearly, something that would normally regulate snail population densities was obviously missing and that is the predatory control of snails. The predators of L. irrorata were discussed in an earlier paper by some of the same authors.

Brian Reed Silliman and Mark D. Bertness. 2002. A trophic cascade regulates salt marsh primary production. PNAS 99:10500-10505. Full text
The figure below, from the cited paper, is the proposed salt marsh food web. Blue crabs (Callinectes sapidus), mud crabs and terrapins (Malaclemys terrapin) eat the periwinkles, L. irrorata, which feed on the fungi growing on Spartina alterniflora.

The authors believe that over harvesting of snail predators, for example, blue crabs, may be leading to the development of very high population densities of snails and, indirectly, resulting in the disappearance of salt marshes.

Happy New Year everyone!

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