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Salt marsh die-off is a Western Atlantic conservation problem that has recently spread into Narragansett Bay, Rhode Island, USA. It has been hypothesized to be driven by: 1) eutrophication decreasing plant investment into belowground biomass causing plant collapse, 2) boat wakes eroding creek banks, 3) pollution or disease affecting plant health, 4) substrate hardness controlling herbivorous crab distributions and 5) trophic dysfunction releasing herbivorous crabs from predator control. To distinguish between these hypotheses we quantified these variables at 14 Narragansett Bay salt marshes where die-off intensity ranged from <5% to nearly 98%. Nitrogen availability, wave intensity and plant growth did not explain any variation in die-off. Herbivory explained 73% of inter-site variation in die-off and predator control of herbivores and substrate hardness also varied significantly with die-off. This suggests that salt marsh die-off is being largely driven by intense herbivory via the release of herbivorous crabs from predator control. Our results and those from other marsh systems suggest that consumer control may not simply be a factor to consider in marsh conservation, but with widespread predator depletion impacting near shore habitats globally, trophic dysfunction and runaway consumption may be the largest and most urgent management challenge for salt marsh conservation. [ABSTRACT FROM AUTHOR]

Secondary succession plays a critical role in driving community structure in natural communities, yet how succession dynamics vary with environmental context is generally unknown. We examined the importance of seedling and vegetative recruitment in the secondary succession of coastal marsh vegetation across a landscape-scale environmental stress gradient. Replicate bare patches were initiated in salt, brackish, and oligohaline marshes in Narragansett Bay, Rhode Island, USA, and allowed to recover unmanipulated or with colonizing seedlings or vegetative runners removed for three years. Seed dispersal and seed bank studies were conducted at the same sites. We found that rates of recovery were 3-10 times faster in brackish and oligohaline marshes than in salt marshes. The fast pace of recovery in oligohaline marshes was driven by seedling colonization, while recovery was dominated by vegetative runners in brackish marshes and by both seedlings and runners in salt marshes. Seed and seedling availability was much greater in oligohaline marshes with up to 24 times the seed bank density compared with salt marshes. In contrast to the facilitated succession generally found in salt marshes, oligohaline marshes follow the tolerance model of succession where numerous species colonize from seed and are slowly displaced by clonal grasses whose recovery is slowed by preemptive competition from seedlings, contributing to the higher species diversity of oligohaline marshes. These findings reveal fundamental differences in the dynamics and assembly of marsh plant communities along estuarine salinity gradients that are important for conceptually understanding wetlands and for guiding the management and restoration of various types of coastal marshes. [ABSTRACT FROM AUTHOR]

Although primary productivity in salt marshes is thought to be controlled by physical forces, recent evidence suggests that human disturbances can drive a switch to consumer control in these ecologically valuable ecosystems. We tested the hypothesis that nitrogen enrichment can trigger consumer control in salt marshes in Narragansett Bay, Rhode Island, with (1) a field experiment in which we manipulated nutrient availability (with nutrient additions) and insect herbivory (with insecticide application), (2) a survey of 20 salt marshes that examined the relationship between marsh nutrient status and herbivore pressure, and (3) insect herbivore removal at high and low nutrient input sites to directly test the hypothesis that nutrient enrichment is increasing insect herbivory in these marshes. Experimental nitrogen eutrophication initially increased plant productivity but eventually led to reduced plant biomass due to insect herbivory, and our surveys revealed that marsh nitrogen supply was a good predictor of herbivore damage to plants. Insects had minimal impacts on primary productivity in pristine marshes, but suppressed primary productivity in eutrophic salt marshes by 50–75%. Thus, eutrophication is currently triggering consumer suppression of primary productivity in New England salt marshes and may ultimately jeopardize the ecological and societal services these systems provide. [ABSTRACT FROM AUTHOR]