Distribution Patterns of Melampus Bidentatus and Their Implications on the Effects of Climate Change on Salt Marsh Animal Populations.

Abstract

Climate change and its associated effects such as sea-level rise and increased temperature are critical concerns for salt marsh ecosystems. We have just begun to understand the effects of climate change on salt marsh plant communities, and while work has been done to determine the effects of these changes on different bird species, none has focused on salt marsh invertebrates. The coffee-bean snail, Melampus bidentatus, is a numerically dominant high-marsh invertebrate with a wide geographic distribution. Given that it is a pulmonate ('air-breathing') gastropod and susceptible to desiccation effects, it may be a useful animal model for understanding potential effects of sea-level rise, which will increase inundation and temperature of animal communities. One prediction of sea-level rise is a shift in marsh vegetation patterns, which could create a change in Melampus distribution. We first sampled eight marshes to determine density and size-class patterns of Melampus across three habitats: creek-side Spartina alterniflora, Sparina patens, and short-form Spartina alterniflora. We found that significantly larger Melampus were found in the short-form Spartina alterniflora while smaller Melampus were found in the Spartina patens, suggesting that Spartina patens acts as a nursery habitat for this species. Melampus were not found in the creek-side Spartina alterniflora. We then conducted a series of experimental field and lab manipulations to assess potential mechanisms driving these patterns and to assess the potential impact of climate change on this abundant marsh gastropod. Thermal, desiccation, and predation stress were all important factors in producing the size-class distribution pattern of Melampus, preventing small Melampus from inhabiting the short-form Spartina alterniflora and all Melampus from inhabiting the creek-side Spartina alterniflora. Our results suggest snails will physiologically be able to survive increased inundation, however, will be exposed to increased predation. Melampus will be more susceptible to increased temperatures and desiccation stress through shifts in vegetation patterns. We know that with climate change, vegetation patterns may shift to increased short-form Spartina alterniflora. Therefore, this could lead to a change in the size pattern distribution of Melampus on salt marshes and possibly a decrease in population due to lower survivorship through increased thermal stress. Changes in the Melampus population could be used to determine the effects of climate change on other salt marsh invertebrate populations such as the saltmarsh amphipod, Orchestia grillus, and the salt marsh isopod, Philoscia vittata, which are also susceptible to thermal and predation stresses. A loss of these species over time could lead to a change in ecosystem function.