Bottom-up effects of mangrove encroachment on basal consumers in the Gulf of Mexico

Abstract

Coastal wetlands are complex ecosystems that are shaped by the interaction of multiple environmental factors. As human activities alter the climate however, the structure of wetlands is changing. One such shift is occurring throughout the Gulf of Mexico where mangrove trees are encroaching into salt marshes as a result of climatic drivers including sea level rise and decreased frequency of winter freeze events. Along the Gulf Coast of Texas, black mangroves (Avicennia germinans), the primary encroaching species, are increasing in abundance and displacing salt marsh plants. The marsh plants being replaced are the primary food sources for many consumers at the base of salt marsh food webs, including fiddler crabs (Uca spp.) and marsh periwinkle snails (Littoraria irrorata). My research aimed to determine how these basal consumers respond to this shift in plant communities and the disappearance of their primary food sources. Through surveys and stable isotope analyses, I identified shifts in the distribution and diet of basal consumers in mangrove encroached marshes. Basal consumers in encroached wetlands were physically associated with mangrove structures but did not consume mangrove-derived plant matter. Lab mesocosm studies examined the trophic interactions of Uca and Littoraria with Avicennia in more detail through food preference and food quality experiments. I found that Avicennia was both an unpreferred and poor-quality food source that lowered the body condition of consumers to which it was fed. Field-collected consumers from mangrove encroached sites also had lower body conditions. Consumers at mangrove encroached sites replaced the marsh plants in their diet with fine organic matter, suggesting that either fine organic matter is less nutritive than marsh plants, or the presence of mangroves has negative non-consumptive effects on basal consumers. This research indicates that mangrove trees are not equivalent to the marsh plants they are replacing and that their encroachment has negative trophic effects on basal consumers. These results have important implications for managing coastal wetland ecosystem functions such as nursery habitat and fisheries support and evaluating the restoration uses of mangroves.