Mima-iype earth mounds, up to 18.6 m in diameter, occur in marsh, transition, and upland areas of the Tijuana Estuary, but are best developed at intermediate tidal elevations. Plant dominants on mounds grade from low marsh to upland species. Upland and high marsh species show steeper species-area relationships than low marsh species, reflecting insular patch dynamics. Mounds are activity centers for several herbivorous mammals that probably influence mound and intermound vegetation. The mounds create a highly patchy transition, and indicate that landward limits of marsh species are more reliable marsh boundary indicators than seaward limits of upland species.