Despite long-standing interest in latitudinal variation in ecological patterns and processes, there is to date weak and conflicting evidence that herbivore pressure varies with latitude. We used three approaches to examine latitudinal variation in herbivore pressure in Atlantic Coast salt marshes, focusing on five abundant plant taxa: the grass Spartina alterniflora, the congeneric rushes Juncus gerardii and J. roemerianus, the forb Solidago sempervirens, and the shrubs Iva frutescens and Baccharis halimifolia. Herbivore counts indicated that chewing and gall-making herbivores were typically > or = 10 times more abundant at low-latitude sites than at high-latitude sites, but sucking herbivores did not show a clear pattern. For two herbivore taxa (snails and tettigoniid grasshoppers), correctly interpreting latitudinal patterns required an understanding of the feeding ecology of the species, because the species common at high latitudes did not feed heavily on plant leaves whereas the related species common at low latitudes did. Damage to plants from chewing herbivores was 2-10 times greater at low-latitude sites than at high-latitude sites. Damage to transplanted "phytometer" plants was 100 times greater for plants transplanted to low- than to high-latitude sites, and two to three times greater for plants originating from high- vs. low-latitude sites. Taken together, these results provide compelling evidence that pressure from chewing and gall-making herbivores is greater at low vs. high latitudes in Atlantic Coast salt marshes. Sucking herbivores do not show this pattern and deserve greater study. Selective pressure due to greater herbivore damage at low latitudes is likely to partially explain documented patterns of low plant palatability to chewing herbivores and greater plant defenses at low latitudes, but other factors may also play a role in mediating these geographic patterns.

Biogeographic theory argues that consumer–prey interactions are more intense, and prey defenses better developed, at lower latitudes. Along the Atlantic Coast of the United States, low‐latitude salt marsh plants are less palatable than high‐latitude conspecifics. To test the hypothesis that latitudinal variation in palatability would occur in the absence of geographically different environmental cues (i.e., that differences in palatability are constitutive rather than induced by climate or herbivore damage), we grew high‐ and low‐latitude individuals of three species of salt marsh plants from seeds (Solidago sempervirens) or rhizome cuttings (Distichlis spicata and Spartina alterniflora) in a common‐garden greenhouse environment, and compared their palatability to herbivores over time. We also quantified leaf toughness and nitrogen content over time in order to help explain results of feeding assays. High‐latitude plants were always more palatable to herbivores than low‐latitude conspecifics. Latitudinal variation in plant traits depended on the plant species. Toughness varied as a function of latitude for Spartina, with low‐latitude plants being consistently tougher than high‐latitude conspecifics. For all generations of Spartina, and for seed‐propagated Solidago, high‐latitude plants had a higher nitrogen content than low‐latitude conspecifics. The fact that latitudinal differences in palatability and traits of salt marsh plants persisted in a common‐garden environment suggests that this variation is constitutive, and likely under genetic control, rather than a plastic response to environmental cues. These results are consistent with the theory that latitudinal differences in herbivory have selected for geographical variation in plant palatability, although we cannot rule out other selective forces that may also vary across latitude.

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.

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