Print PageInteractions between plants and herbivores often vary on a geographic scale. Although theory about induced defenses and tolerance is predicated on temporal or spatial variation in herbivore damage, few studies have compared induced resistance and tolerance across a latitudinal gradient.
In 2002-2005 we surveyed 15 salt marshes along the Atlantic coast from Florida to Maine on 8 dates. At each field site we visually scored leaves of the saltmarsh shrub Iva frutescens for herbivore damage. In a common garden laboratory experiment we measured induced resistance in Iva frutescens. Potted Iva from high and low latitude sites (9 sites total) were damaged with an Iva specialist beetle Paria atterima. Before the damage and on several dates after the damage, feeding trials with Paria were performed. In each trial 2 leaves, 1 from a damaged and 1 from an undamaged plant, were offered to replicate Paria. As a proxy measure of induced resistance, we calculated a preference index for each trial. In a second common garden experiment we studied tolerance to herbivory of Iva from various sites along the Atlantic Coast. In 2009 we measured a number of plant traits on plants damaged by herbivores in a factorial lab experiment in the previous year. Tolerance was measured using an index of regrowth 1 year following herbivore damage.
Results/Conclusions
Theory predicts that induced resistance should matter more when herbivore damage is variable but sometimes strong. In the field, temporal variation in herbivore damage on Iva was over 3 times greater at low versus high latitudes, indicating that induced resistance should be stronger at low latitudes. Consistent with this prediction, Iva induced resistance to herbivores following damage, and induced resistance lasted longer in low versus high latitude plants (ANOVA, p<0.005). Theory also predicts that tolerance to herbivory should be greater where average herbivory damage is greater (in our case, at low latitudes); however, tolerance to herbivory in Iva only indirectly depended on geographic origin. Tolerance was greater in smaller plants, which were more common at higher latitudes. As a result of this indirect causal pathway, tolerance was greater in high latitude plants, contrary to our expectations. Our results suggest that tolerance to herbivory is a less plastic defense strategy than induced resistance.
