Foundation Species Shift Causes a Partial Loss of Functional Support for Benthic Coastal Consumers

Foundation species support highly productive and valuable ecosystems, but anthropogenic disturbances and environmental changes are increasingly causing foundation species shifts, where one foundation species replaces another. The consequences of foundation shifts are not well understood, as there is limited research on the equivalency of different foundation species and the functions they support. Here, we provide insight into community-level consequences of foundation shifts in the Gulf of Mexico, where the typical marsh foundation species (Spartina alterniflora) is being replaced with a mangrove foundation species (Avicennia germinans), forcing marsh fauna to rely on Avicennia for foundational support. We evaluated the interactions of two common and ecologically valuable basal consumers, fiddler crabs (Uca spp.) and marsh periwinkle snails (Littoraria irrorata), with both foundation species across sites with different levels of mangrove encroachment. By investigating both physical support, measured as habitat association and co-occurrence, and trophic support, as basal resource diet contributions, we found that Avicennia can physically replace Spartina for some consumers, but is not providing equivalent trophic support. Uca and Littoraria commonly occupy encroached sites and associate with mangroves but incorporate almost no mangrove plant matter into their diets. The ultimate consequences of a foundation shift in the case of mangrove encroachment may include shifting energy flows and resource use and decreased populations of basal consumers. Looking at interactions with foundation species from multiple perspectives is necessary to obtain a complete picture of the effects that foundational shifts are having, especially as such shifts are becoming increasingly common.