Latitudinal variation in herbivore pressure in Atlantic Coast salt marshes

The nature of interactions between species oftens varies geographically.  Past work in salt marshes on the Atlantic Coast of the U.S. has documented that high-latitude plants are more palatable to herbivores than low-latitude plants.  We tested the hypothesis that herbivore pressure was greater at low latitudes, and thereby might drive these latitudinal patterns in plant palatability.  For representatives of four common plant guilds (low-marsh grasses-Spartina alterniflora, high-marsh rushes-Juncus roemerianus and J. gerardii, high-marsh forbs-Solidago sempervirens, and high-marsh shrubs-Iva frutescens and Baccharis halimifolia), a variety of chewing and galling herbivores were more abundant at low versus high latitudes.  Sucking herbivores did not display simple geographic patterns.  Damage to leaves from chewing herbivores was consistently greater at low latitudes.  Transplant experiments with Spartina, Solidago and Iva confirmed that plants originating from high-latitude sites were more palatable than plants from low-latitude sites, and also demonstrated that herbivore damage was much greater for plants transplanted into low- than high-latitude sites.  These results strongly support the hypothesis that herbivore pressure is greater at low latitudes, but do not exclude the possibility that other factors might also select for greater palatability of high- versus low-latitude plants.