Latitudinal variation in palatability of saltmarsh plants: which traits are responsible?

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.