Biogeographic theory predicts that intense consumer–prey interactions at low latitudes should select for increased defenses of prey relative to high latitudes. In salt marshes on the Atlantic coast of the United States, a community‐wide pattern exists in which 10 species of low‐latitude plants are less palatable to a diverse suite of herbivores than are high‐latitude conspecifics. Examination of proximate plant traits (toughness, palatability of polar and nonpolar extracts, nitrogen content) of high‐ and low‐latitude conspecifics of nine plant species suggested that all these proximate traits had the potential to contribute to latitudinal differences in palatability of some plant species. Southern plants were tougher than northern plants (five species), had less palatable polar extracts (four species), and had lower N content (six species). Experimental evidence linking traits to latitudinal differences in palatability was strongest for polar extracts and lacking for N content. For one plant species, none of the traits we studied correlated with latitudinal variation in palatability. Because palatability differences may change when moving from fresh plants to freeze‐dried plants to plant traits, studies of latitudinal variation in freeze‐dried plants or plant traits are likely to under‐ or overestimate latitudinal variation in palatability of fresh plants. This study has begun to identify the proximate plant traits responsible for latitudinal variation in plant palatability in Atlantic coast salt marshes, but the ultimate evolutionary factors responsible for variation in these traits remain to be determined.

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Summary 1.The species-specificity of litter processing by three species of isopods at the interface between salt marsh and coastal forest habitats in the south-eastern United States was examined. 2. To quantify isopod performance, measurements were taken of feeding, digestion and growth of isopods fed on three litter types ( Juncus roemerianus , Quercus virginiana and Pinus palustris ) and on artificial diets containing one of three classes of model phenolic compounds (simple phenolics and hydrolysable and condensed tannins). 3. To quantify the ecosystem impact of isopods, promotion of microbial respiration, changes in detritus chemistry, and the quantity of litter processed by isopod populations were measured. The results support three hypotheses concerning isopod-litter interactions. (i)Isopod performance on different litter types can be predicted based on chemical litter traits, e.g. phenolic concentrations and C : N ratios. (ii) Fully terrestrial isopods are better adapted to the range of phenolics found in angiosperm litter than are semiterrestrial species inhabiting the supralittoral. (iii) Isopod species differ with respect to their impact on decomposition processes due to species-specific digestive capabilities, different effects on microbial decomposition and different rates of net litter processing. 5. Because isopods are transitional between semiterrestrial and terrestrial habitats, unlike most other salt marsh detritivores, they are likely to play a unique role in decomposition processes and in the flux of materials between salt marsh and terrestrial habitats.

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