The Response of Two Salt Marsh Molluscs, Littorina irrorata and Geukensia demissa, to Field Manipulations of Density and Spartina Litter

Abstract

There is little research on processes limiting macroconsumer species in salt marshes. We investigated the significance of competition for space and food resources in three North Carolina salt marshes by altering densities of the molluscs Littorina irrorata and Geukensia demissa, and by manipulating Spartina litter. Morality and weight gain were the two principal responses measured. Enclosures (7.5 m2) were used to contain Littorina and marsh grass. Molluscan densities and Spartina litter levels were one—half, control, and double control levels. The field experiments ran from February 1975 through March 1977. The overall mortality and growth patterns of both species were similar within each marsh, but differed among marshes. The molluscs exhibited high mortality and slow growth in Tar Landing marsh, high mortality and rapid growth in Causeway marsh, and low mortality and slow growth in Calico marsh (enriched by sewage effluent). In Tar Landing survival and weight gain in both species were reduced by crowding. Increasing Spartina litter produced significant weight gain of medium—sized Littorina and small Geukensia. In Causeway marsh crowding decreased Littorina and Geukensia growth rates, but increased mortality only in the mussel population. Litter manipulations primarily affected weight gain in Littorina and mortality in Geukensia. In the enriched Calico marsh crowding depressed Geukensia survivorship and Littorina weight gain. Only in the mussel population was weight gain enhanced (1st yr only) by the addition of Spartina litter. We conclude that competition for food is an important limiting process, particularly for larger Littorina in Tar Landing marsh. In Causeway marsh competition for space appears more significant. In Calico marsh space limitation in Littorina populations and competition for food resources in Geukensia appear to be the principal processes. Variation in these competition—limiting mechanisms between two distant marsh consumers, a grazer and a filter feeder, may represent different procedures for dealing with the variable resource levels existing among salt marshes.