Native herbivores enhance the resistance of an anthropogenically disturbed salt marsh to Spartina alterniflora invasion

Abstract

AbstractThe extent by which human activities create an inroad for the invasion of exotic plant species and the mechanisms that drive a disturbed ecosystem's resistance to exotic invasion is largely unknown. We investigated the mechanisms by which salt marsh resists cordgrass (Spartina alterniflora) invasion by two key factors: anthropogenic disturbance by ditching and herbivory by the native crab Helice tientsinensis. In northern China, although abiotic resistance (i.e., hyper‐stresses) inhibited the landward invasion of cordgrass to high marshes, our transplant experiments showed that anthropogenic ditching created windows of opportunity for cordgrass invasion to high marshes by enhancing propagule pressure and providing favorable (i.e., low‐salinity and high‐inundation) microhabitats for colonization. Furthermore, a native herbivore exclusion experiment showed that crab grazing strongly suppressed seedling establishment and growth of cordgrass in ditched high marshes, which gradually created native biotic resistance to cordgrass invasion. We conclude that grazing by native herbivores can enhance the resistance of high marshes to cordgrass invasion triggered by anthropogenic ditching disturbance in northern China. These findings highlight that it is critical to investigate ecosystem resistance associated with anthropogenic disturbance to better understand the multiple mechanisms of exotic plant invasion, and we call for integration of these findings into invasion control strategies.