Relationships betweenSpartina alterniflora andLittoraria irrorata in a South Carolina salt marsh
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Bottom-up factors such as nutrient availability have long been thought to be the primary regulators of plant growth in salt marshes. However, that paradigm has been challenged by investigations showing that grazing by the periwinkle snailLittoraria irrorata regulatesSpartina alterniflora growth through top-down forces. Our investigation was conducted between November 2003 and March 2007 within the North Inlet Estuary (NIE) salt marsh to examine relationships betweenS. alterniflora andL. irrorata. The primary goal was to determine if observed in situ densities ofL. irrorata significantly decreasedS. alterniflora productivity at the study sites. The results indicate thatS. alterniflora productivity positively correlated withL. irrorata snail density during winter. However, no correlation was observed during summer. Hence, there was no inverse relationship between snail density and primary productivity and no support for the hypothesis of ‘top-down’ control of marsh plant production. A significant relationship betweenS. alterniflora stem density andL. irrorata density was observed during summer and winter, suggesting that stem density may play a key role in determining the distribution of periwinkle snails in NIE. These results challenge the applicability of theL. irrorata-S. alterniflora top-down control model as a generalized phenomenon in southeastern salt marshes.
Key Wordsbottom-up detritivore herbivory top-down
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