Die-offs of cordgrass are pervasive throughout western Atlantic salt marshes, yet understanding of the mechanisms precipitating these events is limited. We tested whether herbivory by the native crab, Sesarma reticulatum, is generating die-offs of cordgrass that are currently occurring on Cape Cod, Massachusetts (U.S.A.), by manipulating crab access to cordgrass transplanted into die-off areas and healthy vegetation. We surveyed 12 Cape Cod marshes to investigate whether the extent of cordgrass die-off on creek banks, where die-offs are concentrated, was related to local Sesarma grazing intensity and crab density. We then used archived aerial images to examine whether creek bank die-off areas have expanded over the past 2 decades and tested the hypothesis that release from predation, leading to elevated Sesarma densities, is triggering cordgrass die-offs by tethering crabs where die-offs are pervasive and where die-offs have not yet been reported. Intensity of crab grazing on transplanted cordgrass was an order of magnitude higher in die-off areas than in adjacent vegetation. Surveys revealed that Sesarma herbivory has denuded nearly half the creek banks in Cape Cod marshes, and differences in crab-grazing intensity among marshes explained >80% of variation in the extent of the die-offs. Moreover, the rate of die-off expansion and area of marsh affected have more than doubled since 2000. Crab-tethering experiments suggest that release from predation has triggered elevated crab densities that are driving these die-offs, indicating that disruption of predator-prey interactions may be generating the collapse of marsh ecosystems previously thought to be exclusively under bottom-up control.

Many mechanisms of invasive species success have been elucidated, but those driving cryptic invasions of non‐native genotypes remain least understood. In one of the most successful cryptic plant invasions in North America, we investigate the mechanisms underlying the displacement of native Phragmites australis by its Eurasian counterpart. Since invasive Phragmites’ populations have been especially prolific along eutrophic shorelines, we conducted a two‐year field experiment involving native and invasive genotypes that manipulated nutrient level and competitor identity (inter‐ and intra‐genotypic competition) to assess their relative importance in driving the loss of native Phragmites. Inter‐genotypic competition suppressed aboveground biomass of both native and invasive plants regardless of nutrient treatment (∼ 27%), while nutrient addition disproportionately enhanced the aboveground biomass (by 67%) and lateral expansion (by > 3 × farther) of invasive Phragmites. Excavation of experimental plots indicated that nutrient addition generates these differences in aboveground growth by differentially affecting rhizome production in invasive vs native plants; invasive rhizome biomass and rhizome length increased by 595% and 32% with nutrient addition, respectively, while natives increased by only 278% and 15%. Regardless of nutrient level, native rhizomes produced twice as many roots compared to invasives, which field surveys revealed are heavily infected with mycorrhizal symbionts. These results suggest that native Phragmites competes well under nutrient‐limited conditions because its rhizomes are laden with nutrient‐harvesting roots and mycorrhizae. Invasive Phragmites’ vigorous aboveground response to nutrients and scarcity of lateral roots, in contrast, may reflect its historic distribution in eutrophic Eurasian wetlands and correspond to its prevalence in New England marshes characterized by elevated nutrient availability and relaxed nutrient competition. These findings reveal that discrete differences in phenotype can interact with anthropogenic modification of environmental conditions to help explain the success of cryptic invaders.

Exotic plant invaders that form monocultures and exclude native plants are often the most detrimental to native diversity and the hardest to eradicate. To generate a monoculture, the invader must garner more resources than resident natives and, once established, persist despite high densities of conspecific neighbors. Coincident with expansion and long-term persistence, successful invaders typically accumulate senesced material, but the role of this litter in mediating the invader's ability to establish and maintain monospecific dominance has rarely been investigated. We used stands of the common reed, Phragmites australis, a prolific wetland invader in North America, to explore the impact of litter on interspecific competition with the native rush, Juncus gerardii, and intraspecific competition among live shoots. In 10 9 10 m areas positioned on Phragmites expansion fronts, we removed litter to isolate its effect from live Phragmites on light availability, aboveground biomass and community composition. Compared to adjacent, unmanipulated fronts, light availability nearly tripled and Juncus biomass increased[170% in litter removal areas after 4 months. Although the positive response of Juncus and native forbs was most pronounced on the leading edge of Phragmites stands, litter removal triggered a 271% increase in native plant biomass even in the interior of stands where Phragmites' live stem density was highest. Litter treatment did not significantly affect Phragmites biomass, but more, shorter stems emerged in litter removals revealing Phragmites modifies stem phenotype in response to local litter and light conditions. These results suggest that litter plays a central role in Phragmites' invasion process, from initial establishment to subsequent monospecific dominance. Thus, prescribed litter removal may be an effective strategy to enhance coexistence of native plant populations in wetlands where eradication of invasive monocultures is not an ecologically or economically feasible option.