Coastal wetlands are simultaneously some of the most valuable and most threatened ecosystems on our planet today. Understanding the forces that shape them are vital to protecting the services they provide. A major shift is currently underway in coastal wetlands surrounding the Gulf of Mexico where black mangrove trees (Avicennia germinans) are encroaching on salt marshes and changing the vegetation community. It is currently unknown how this community shift may affect coastal wetland food webs, particularly from a bottom-up perspective. Fiddler crabs (Uca spp.) are detritivorous organisms that are important basal species in salt marsh food webs. They serve as prey for organisms at higher trophic levels, and play important roles in nutrient cycling and soil aeration. In order to determine the bottom-up effects mangrove encroachment may have on such an important basal consumer, we measured the behavioral and physiological responses of Uca to different diets they may encounter in an encroached ecosystem. Crabs were presented with a representative marsh diet consisting of plant matter from Spartina alterniflora (a dominant salt marsh plant) and a mangrove diet consisting of Avicennia. Mesocosm experiments were set up to measure time spent in the presence of different food sources, while feeding arena trials were used to directly measure feeding activity. With these experiments, in addition to assessments of physiological condition, we determined the differential responses of Uca to the offered diets.
Results/Conclusions
In the lab, Spartina was found to be both a more preferred and higher quality diet for Uca. Crabs spent 60% of their time on land on sediment enriched with Spartina, and fed more than twice as much on such sediment than on Avicennia enriched offerings. Feeding activity on Avicennia sediment was found to be no different than activity on unenriched sediment. Uca also increased in weight an average of 2% more on a diet of Spartina than one of Avicennia. Taken together, these findings indicate that a diet dominated by Avicennia will most likely not be able to replace a Spartina dominated diet in the field. This may lead to decreased abundance of fiddler crabs in mangrove encroached areas and changes in both marsh sediment characteristics and coastal food web structure.