Latitudinal variation in resistance and tolerance to herbivory of a salt marsh shrub

Submitted by editor on 11 September 2014.

By Steve Pennings and Kazimierz Więski.

A new study by Kazimierz Więski and Steven Pennings contributes to the biogeography of plant antiherbivore strategies. Plants can resist herbivore's attack by deploying constitutive defenses, expressed permanently throughout the plant’s life, and induced, expressed only temporarily and triggered by herbivory. Tolerance is a different take on the same problem: plants may compensate for the fitness decline due to herbivore damage, for example by regrowing new leaves. Since interactions between plants and herbivores often vary on a geographic scale these two strategies, resistance and tolerance, may be expected to vary as well.

According to the Latitudinal Biotic Interaction Hypothesis, biotic interactions increase in strength and frequency toward the equator. LBI hypothesis has along a history in ecology and although recently criticized, it predicts well patterns in tidal marshes on the North American Atlantic coast. Building on that, authors tested a prediction that both types of resistance to herbivory and tolerance increase from high to low lattitudes in a salt marsh shrub Iva frutescens. At first they examined  patterns of herbivory in Iva along the 2000 km length of the Atlantic coast. In the field, average levels of herbivore damage, and spatial and temporal variation in herbivore damage were all greater at low latitudes, indicating that both constitutive and induced resistance should follow this pattern.

Steve Pennings and field assistant observing herbivory patterns in the marsh.

A series of laboratory two choice preference tests with Iva specialist beetle Paria, has revealed that both types of resistance were indeed stronger at lower latitudes. The lack of a trade off between these lines of defense may have been caused by more resources being available to Iva at low latitudes. Contrary to expectations however, tolerance to herbivory in Iva (measured as leaf regrowth after a herbivory damage) did not depend on geographic origin.These results emphasize the value of considering multiple ways in which plants respond to herbivores when examining geographic variation in plant–herbivore interactions.

Kazimierz Więski thinking about geographic variation in plant-herbivore interactions when sampling bugs for his next project.