Driving forces behind latitudinal variations in plant-herbivore interactions in SW Atlantic salt marshes

ABSTRACT: Despite long-standing interest in the hypothesis that the strength of plant-herbivore interactions decreases at higher latitude, supporting evidence is scarce and the data are conflicting. We conducted a field survey and 2 experiments to examine this hypothesis, focusing on herbivory by a stem-borer moth (Haimbachia sp. nov.) on the dominant SW Atlantic marsh grasses (Spartina alterniflora and S. densiflora). Field surveys indicated that herbivore abundances and damage, although tending to decrease simultaneously, are unrelated to latitude. Herbivore abundances were related to latitude-dependent variables such as day length and temperature, and also to latitude-independent variables such as precipitation, salinity, and tide amplitude. Abundances were indirectly related to the effects of these variables and sediment characteristics on plant traits like density, height, and tissue composition. After 33 mo, herbivore abundances and damage to high-latitude plants transplanted to low latitudes were 50 times greater than in plants transplanted from low- to high-latitude sites. In a common garden experiment (38o 56’ S) without herbivore pressure, differences persisted in plant traits from high and low latitudes, suggesting a lack of herbivore-induced effects on these plant traits. The persisting conspecific differences in plant traits translocated along latitude suggest that these variations are under genetic control. Thus, our results provide evidence that although plant-herbivore interactions are more important at lower latitudes, many additional and contingent variables unrelated to latitude can interrupt this geographic pattern.