Megafauna Utilization of and Impact on Salt Marshes

Abstract

Megafauna (>45kg) are known to inhabit and reshape freshwater, terrestrial, and marine systems throughout the globe through consumptive and engineering pathways. Following long-term conservation, large consumers have expanded and recolonized regions thought to be beyond their niche space. By re-entering these systems, these large consumers may restructure these new habitats. The goal of this dissertation research was to document what megafauna use salt marshes and examine the effects of megafauna recolonization of salt marshes. We first conducted a literature review and found that a variety of megafauna utilize salt marshes, a total of 34 species including sharks, manatees, pinnipeds, crocodilians, sea otters, hippos, and large terrestrial animals, such as lions, bears and water buffalo. Next, we examined the impacts of 2 species of megafauna that recolonized salt marshes in Elkhorn Slough using observational and experimental studies. We conducted a control-impact study to understand the potential effects of harbor seals (Phoca vitulina) hauling out of the water on the structure of salt marsh edges. We found that sites with harbor seal haul outs had significantly more compressed soil, that the foundation plant species were heavily denuded, and that animal diversity shifted strongly - burrowing crabs were almost eliminated entirely from the edges of marshes, while birds congregated in high number. Erosion is a pressing issue in Elkhorn Slough at 0.5 m/yr, and despite the vast differences in regions with seal haul outs and denuded surface, we found no increase in erosion rate, likely due to compensatory effects of an intact root system, lack of crab burrows, and highly compressed sediment. These findings suggest that harbor seals act as ecosystem engineers in salt marshes by hauling out onto marsh platforms and generating large disturbances. Though megafauna can reshape systems and potentially drive erosion, they are not the only major drivers within estuarine systems – eutrophication is another major factor that reshapes salt marshes and is known to cause erosion. Recent studies showed that sea otters (Enhydra lutris) consume root-eating marsh crabs and through this trophic cascade, lower edge erosion rates. Using a factorial experimental design (sea otter presence x nutrient addition) at the edge of the creek banks, we examined the relative impact of otters and fertilization on erosion rates. We did not detect differences in marsh erosion rates or crab burrows or activity from otters or fertilization but did find effects of both main factors on plant communities. These findings do not match previous studies that show significant effects of both otters and eutrophication on erosion rates, but this mismatch likely stems from the interaction of differences in duration, scale, and erosion rates that vary greatly along creek banks. Taken together, this work highlights the importance of studying the impacts of megafauna on salt marshes, and variability in their effects.