Predation restricts black mangrove (Avicennia germinans) colonization at its northern range limit along Florida’s Gulf Coast

Abstract

Climate change-driven range expansion of black mangroves (Avicennia germinans) is predicted along the northern Gulf of Mexico, where sea level rise is also driving conversion of freshwater forest islands to salt marsh. While climate-driven A. germinans range expansion has garnered considerable scientific attention, the role of top-down controls on colonization is largely overlooked. We investigated the effects of abiotic (flooding frequency, soil depth, soil salinity) and biotic (predation, herbivory) controls on A. germinans establishment at its northern range limit along Florida’s Gulf Coast by comparing fates of caged and non-caged propagules across four landscape positions (from creek edge to forest island interior) and at three sites along a tidal flooding frequency gradient. Within 12 days, grapsid crab, Sesarma reticulatum, consumed 99% of non-caged propagules. Among caged propagules, establishment increased with increasing flooding frequency; however, cages did not entirely prevent predation, which remained a primary cause of mortality, except in the rarely flooded island. Propagules that survived to seedlings experienced mild to fatal herbivory across landscape positions and sites. This study revealed that while relict forest islands and surrounding marshes can support A. germinans, predation and herbivory strongly suppress colonization, suggesting that mangrove expansion models should incorporate biotic controls.