Measuring tolerance to herbivory: Accuracy and precision of estimates madeusing natural versus imposed damage

Tolerance to herbivory (the ability of a plant to incur herbivore damage wi thout a corresponding reduction in fitness) can be measured using either na turally occurring or imposed herbivore damage. After briefly reviewing some of the advantages and disadvantages of these approaches, we present calcul ations describing the degree to which estimates of tolerance will be biased by environmental variables that affect both herbivory and fitness. With na turally occurring herbivory the presence of environmental variables that ar e correlated with herbivory and fitness will result in biased estimates of tolerance. In contrast, estimates obtained from experiments in which herbiv ory is artificially imposed will be unbiased; however, under a wide range o f parameter values these estimates will be less precise than estimates obta ined from experiments in which herbivory is not manipulated.