Salt marshes in the Gulf of Mexico have been undergoing encroachment by mangrove trees for several decades. In some locations, salt marsh plants have been almost entirely displaced by mangroves, potentially changing the ways salt marsh basal consumers interact with their environment. Along the Gulf Coast of Texas, this most often occurs in the form of black mangroves, Avicennia germinans, displacing marsh cordgrass, Spartina alterniflora. We conducted a combination of field surveys and laboratory experiments with two dominant salt marsh basal consumers (fiddler crabs Uca spp. and marsh periwinkles Littoraria irrorata) to determine the extent to which they occur in encroached marshes, and the potential of mangrove plant matter to replace marsh plants as a food source for them. Transect surveys were used to measure the co-occurrence of these consumers with mangrove trees, and we performed feeding preference and food quality trials to determine the trophic responses of consumers to the presence of mangrove trees.
Results/Conclusions
We have found that both groups of organisms occur in encroached marshes to some extent, and variability in their occurrence appears more closely linked to factors such as elevation and salinity than to mangrove presence. However, laboratory experiments have shown that both organisms preferentially consume marsh plants over mangrove leaf material. Littoraria and Uca respectively consumed an average of 6 times more and 4 times more Spartina plant matter than Avicennia. For Uca spp., individuals raised on a diet of mangrove plant matter were also able to store only half as much energy as individuals raised on marsh plants. This indicates that while salt marsh basal consumers can survive and persist in fully encroached wetlands where marsh plants have all but disappeared, they may need to replace the marsh plants in their diet with other sources, such as microalgae or particulate organic matter. Understanding the extent of this trophic shift will allow us to determine the far-reaching effects that mangrove encroachment may have on wetland community structure and function.