Negative outcomes of novel trophic interactions along mangrove range edges.

Tropicalization is a phenomenon that is changing the structure of ecosystems around the world. Mangrove encroachment is a particular form of tropicalization that may have cascading consequences for resident fauna in subtropical coastal wetlands. There is a knowledge gap regarding the extent of interactions between basal consumers and mangroves along mangrove range edges, and the consequences of these novel interactions for the consumers. This study focuses on the key coastal wetland consumers, Littoraria irrorata (marsh periwinkle) and Uca rapax (mudflat fiddler crabs), and their interactions with encroaching Avicennia germinans (black mangrove) in the Gulf of Mexico, USA. In food preference assays, Littoraria avoided consuming Avicennia and selectively ingested leaf tissue from a common marsh grass, Spartina alterniflora (smooth cordgrass), a preference which has also been previously documented in Uca. The quality of Avicennia as a food source was determined by measuring the energy storage of consumers that had interacted with either Avicennia or marsh plants in the lab and the field. Littoraria and Uca both stored approximately 10% less energy when interacting with Avicennia, despite their different feeding behaviors and physiologies. The negative consequences of mangrove encroachment for these species at the individual level suggest that there may be negative population-level effects as encroachment continues. Many previous studies have documented shifts in floral and faunal communities following mangrove replacement of salt marsh vegetation, but this study is the first to identify physiological responses that may be contributing to these shifts.