The effects of nutrients and grazing on the estuarine marsh invader, Phragmites australis

Background/Question/Methods

Coastal marshes suffer from a number of disturbances, including habitat destruction, pollution, eutrophication, and invasive species. Despite the importance of these habitats, we lack a complete understanding of the factors that influence marsh communities, especially when multiple stresses act simultaneously. Eutrophication and invasive species are common marsh disturbances that often occur together, but little is known about how these insults influence consumer preferences or the plant traits that mediate consumer choices. The addition of nutrients, as with eutrophication, often increases plant nutritional value, leading to increased herbivory, which can induce plant chemical defenses and depress nutritional quality. Thus, plant-herbivore interactions in this disturbed ecosystem are likely complex, but may be important in influencing marsh community structure. In addition to eutrophication, Phragmites australis subsp. australis, the common reed, has invaded extensive areas of marsh on the East and Gulf coasts of the U.S. and has especially affected the estuarine marshes of the Chesapeake Bay. Despite its prevalence, relatively few studies have investigated how Phragmites is integrated into marsh food webs. Eutrophication has been shown to facilitate the invasion of Phragmites, but no studies have investigated how additional nutrients may impact herbivore preferences for this plant compared to native species.

Results/Conclusions

We conducted field experiments in a brackish wetlands site, where Phragmites comprises 76% of marsh grass abundance. Despite Phragmites’ dominance, only 14.5% of periwinkle snail grazers (Littoraria irrorata) were found on this plant, whereas 29% of snails were found on native grasses. This is likely due to the fact that Phragmites is chemically deterrent to snails, defended by an unidentified compound(s) that deters snail grazing compared to a palatable food (XX). To investigate the separate and interactive effects of grazing and nutrient addition, a field experiment was conducted where Phragmites was grown in 1m² plots with or without snails and/or fertilizer. After one month, Phragmites that had been grazed upon by snails was less palatable than ungrazed plants (XX). Fertilizer addition did not influence snail grazing, however. An additional experiment, run for three months, demonstrated that fertilized plants were grazed upon significantly more than ungrazed plants (XX), most likely due to increased nutritional content of plants. Grazing on snail + fertilizer treatments are still under investigation. Investigating how plant traits may be affected by these disturbances and how this may influence herbivore feeding will contribute to a better understanding of marsh systems and their responses to stress.