Ecological interactions often vary geographically. Work in salt marshes on the Atlantic Coast of the United States has documented community-wide latitudinal gradients in plant palatability and plant traits that may be driven in part by greater herbivore pressure at low latitudes. To determine if similar patterns exist elsewhere, we studied six taxa of saltmarsh plants (Atriplex , Juncus , Limonium , Salicornia , Spartina and Suaeda ) at European sites at high (Germany and the Netherlands) and low (Portugal and Spain) latitudes. We conducted feeding assays using both native and non-native consumers, and documented patterns of herbivore damage in the field. As in the United States, high-latitude plants tended to be more palatable than low-latitude plants when offered to consumers in paired feeding assays in the laboratory, although assays with grasshopper consumers were less consistent than those with crab consumers, and plants in the field at low-latitude sites tended to experience greater levels of herbivore pressure than plants at high-latitude sites. Similarly, high-latitude leaf litter was more palatable than litter from low-latitude plants when offered to consumers in paired feeding assays in the laboratory. Latitudinal gradients in plant palatability and herbivore pressure may be a general phenomenon, and may contribute to latitudinal gradients in decomposition processes.

Ecological interactions often vary geographically. Work in salt marshes on the Atlantic Coast of the UnitedStates has documented community-wide latitudinal gradients in plant palatability and plant traits that may be driven in part by greater herbivore pressure at low latitudes. To determine if similar patterns exist elsewhere, we studied six taxa of saltmarsh plants (Atriplex , Juncus , Limonium , Salicornia , Spartina and Suaeda ) at European sites at high (Germany and the Netherlands) and low (Portugal and Spain) latitudes. We conducted feeding assays using both native and non-native consumers, and documented patterns of herbivore damage in the field. As in the United States, high-latitude plants tended to be more palatable than low-latitude plants when offered to consumers in paired feeding assays in the laboratory, although assays with grasshopper consumers were less consistent than those with crab consumers, and plants in the field at low-latitude sites tended to experience greater levels of herbivore pressure than plants at high-latitude sites. Similarly, high-latitude leaf litter was more palatable than litter from low-latitude plants when offered to consumers in paired feeding assays in the laboratory. Latitudinal gradients in plant palatability and herbivore pressure may be a general phenomenon, and may contribute to latitudinal gradients in decomposition processes.

We examined the response of a salt marsh food web to increases in nutrients at 19 coastal sites in Georgia. Fertilization increased the nitrogen content of the two dominant plants, Spartina alterniflora and Juncus roemerianus, indicating that added nutrients were available to and taken up by both species. Fertilization increased Spartina cover, height, and biomass and Juncus height, but led to decreases in Juncus cover and biomass. Fertilization increased abundances of herbivores (grasshoppers) and herbivore damage, but had little effect on decomposers (fungi), and no effect on detritivores (snails). In the laboratory, herbivores and detritivores did not show a feeding preference for fertilized versus control plants of either species, nor did detritivores grow more rapidly on fertilized versus control plants, suggesting that changes in herbivore abundance in the field were driven more by plant size or appearance than by plant nutritional quality.Community patterns in control plots varied predictably among sites (i.e., 17 of 20 regression models examining variation in biological variables across sites were significant), but variation in the effects of fertilization across sites could not be easily predicted (i.e., only 6 of 20 models were significant). Natural variation among sites was typically similar or greater than impacts of fertilization when both were assessed using the coefficient of variation. Overall, these results suggest that eutrophication of salt marshes is likely to have stronger impacts on plants and herbivores than on decomposers and detritivores, and that impacts at any particular site might be hard to distinguish from natural variation among sites.

No abstract

We evaluated 2 potential indicators of stress, viz. the ratio of dimethylsulfoxide to dimethylsulfoniopropionate (DMSO:DMSP) and foliar metals, in Spartina alterniflora collected from areas affected by 4 different disturbances (sudden marsh dieback, horse grazing, increased snail densities, wrack deposition) across 20 marshes in Georgia, USA. The DMSO:DMSP ratio was a stronger and more consistent indicator of stress than either DMSP or DMSO concentrations alone, with significantly higher ratios occurring in leaves and stems collected from affected compared to healthy areas in all 4 disturbance types. Foliar metal concentrations also differed in affected compared to healthy areas. Of 20 metals evaluated, concentrations of 19 were increased in leaves collected from edge and affected areas. Multidimensional scaling using the entire suite of metals showed separation between plants from affected and healthy areas, but no difference among disturbance types. In contrast, chlorophyll a concentrations were not significantly different between affected and healthy areas, and did not correlate with variation in either of the 2 indicators. These results suggest that the DMSO:DMSP ratio and foliar metal suite are sensitive indicators of sublethal stress in Spartina, capable of identifying stress before there are visible signs such as chlorophyll loss. The fact that both indicators were consistent across a variety of disturbance types suggests that they may be primarily responsive to general oxidative stress and thus, broadly useful tools for evaluating the health of salt marsh habitat in the field. Spartina alterniflora salt marsh near St. Simons Island, GA, with a dieback area along the edge of the tidal creek.