Latitudinal variation in palatability of salt-marsh plants: Are differences constitutive?

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 con-specifics. 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 spiccita 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.