The herbivory uncertainty principle: Visiting plants can alter herbivory

In 1927, Werner Heisenberg proposed that there are fundamental limitations to the study of subatomic particles, as the act of measuring them affects t heir behavior. Here we show that experimenter-induced uncertainty also appl ies in plant ecology, with potentially dramatic consequences for field biol ogists. We tested whether the simple act of visiting marked plants once per week for eight weeks influenced the intensity of herbivory experienced by six plant species in an old field community. Half of the plants were touche d once per week to simulate taking morphological measures, while the other half were left undisturbed (neither Visited nor touched). After eight weeks , visitation resulted in (1) decreased leaf damage by insects on one specie s, (2) increased leaf damage on a second species, (3) a marginally signific ant increase in survival for a third species, and (4) no effect on the rema ining three species. These results serve as an important reminder that seem ingly benign experimental methods may themselves dramatically affect the pe rformance of experimental subjects. Our results raise concern about studies that use repeated visitation of focal plants either to compare rates of he rbivory among species or to investigate some factor that can either directl y or indirectly be influenced by the rate of herbivory (e.g., seed producti on, competition, etc.). Since The six species in our study responded differ ently to visitation, visitation effects must be accounted for in the design of future field experiments.