Foundation species, such as marsh plants, mangroves, seagrasses, corals, and oysters, form some of the most valuable and threatened habitats in the world. The loss of these coastal habitat-forming species often results in significant changes in community structure and ecosystem-service delivery. Therefore, understanding how both biotic (e.g., herbivory) and abiotic (e.g., drought) factors can alter foundation species' structure is critical for promoting resilience to anthropogenic stressors. My dissertation focused on how physical and biological processes regulate salt marshes, and how coastal development, specifically shoreline hardening, affects two marsh ecosystem services: erosion protection and habitat provision for marine fauna. Marsh cordgrass (Spartina alterniflora) is a highly productive foundation species that can inhibit coastal erosion. However, anoxic stress can allow marsh periwinkles to overgraze marshes, resulting in marsh die-off. Burrowing crabs can alleviate anoxic stress (via sediment bioturbation) and thus can potentially sustain marsh productivity. From field experiments (Ch.1), I found that crab bioturbation allowed Spartina to compensate for biomass losses from periwinkle grazing. Unfortunately, shoreline hardening (construction of bulkhead or riprap structures) can reduce marsh access for burrowing crabs and thus increase sediment anoxia, preventing marshes from keeping pace with overgrazing. In Ch. 2, I quantified the prevalence of shoreline hardening in the United States and found that 14% of the shoreline is hardened. Housing density is positively correlated with hardening along the Atlantic, Pacific, and Gulf sheltered coasts. With projected accelerated population growth along U.S. coasts, marshes may be lost to future shoreline hardening. Heightened awareness of adverse effects of shoreline hardening has increased demand for "living shorelines". Living shorelines include marsh plantings with or without offshore sills. My field surveys of different shoreline types show that living shorelines provide superior erosion protection to bulkheads during a Category 1 hurricane (Ch. 3). Further, by sampling marine fauna in hardened and living shoreline habitats, I show that living shorelines provide better habitat for fish and crustaceans than hardened shorelines lacking marsh (Ch. 4). Therefore, although shoreline hardening is still commonly used for erosion protection, marshes may prevent erosion better than bulkheads during storm events, while also serving as valuable habitat.