Recruitment and herbivory affect spread of invasive Spartina alterniflora in China.

Abstract

How multiple factors interact to mediate the spread of invasive species is poorly understood. We studied cordgrass, Spartina alterniflora, which is invading the entire Chinese coast, occupying mudflats throughout this range, and displacing mangroves in the upper intertidal of southern China. For two years, we studied the roles of propagule pressure, competition, disturbance, and herbivory in the dynamics of this invasion at a typical mangrove habitat, Zhangjiang Estuary, southern China. Seed of cordgrass dispersed to all intertidal habitats. The density of seeds in surficial soils was 20-fold greater in cordgrass than in dense mangrove, the ecotone between mangrove and cordgrass, or the open mudflat. Seed predation or herbivory on seedlings was intense in all habitats. While seeds of cordgrass germinated in all habitats, no unprotected seedlings survived anywhere except on the mudflat in our two-year study. Survival was nil in other habitats where seed predation, herbivory, and shading by dense vegetation killed all seeds and seedlings. Seed dispersal to the mudflat exceeded seed predation and herbivory, however, and a few seedlings survived to grow into large clones of cordgrass. Repeated waves of seed dispersal and rapid clonal growth have transformed the mudflat into cordgrass stands within a decade, despite high consumer pressure. Disturbance by typhoons, freezes, and humans make gaps in mangrove that fill with cordgrass. This process is gradually converting mangrove forests into monospecific intertidal grasslands in China.