LATITUDINAL VARIATION IN PALATABILITY OF SALT-MARSH PLANTS: WHICH TRAITS ARE RESPONSIBLE?

Abstract

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, pal- atability of polar and nonpolar extracts, nitrogen content) of high- and low-latitude con- specifics 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. Key words: Atlantic coast (USA) salt-marsh plants; biogeographic theory; chemical defense; herbivory; latitude and plant variation; palatability, affected by multiple plant traits; palatability, plant variaton across latitude; plant chemical defenses, latitudinal variation; plant-herbivore inter- actions.