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Separate effects of human visitation and touch on plant growth and herbivory in an old-field community¹

²Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9 Canada; ³Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6018 USA

Although animal scientists have long been aware that methods used to measure an experimental system can affect the subject of measurement, similar confounding effects of commonly used field methods have only recently been acknowledged by plant ecologists. Here we demonstrate significant effects of weekly visitation (walking up to a focal plant) and handling (taking morphological measures) on plant growth and herbivory in an old-field community. Of the three species examined, Apocynum cannabinum was the most severely affected by our treatments. For Apocynum, weekly visitations resulted in a positive relationship between initial and final size, which did not occur in the unvisited plants. Visitation also increased leaf herbivory, resulting in a reduced leaf:stem biomass ratio. Handling the plants nearly doubled the proportion of individuals with a stem borer emergence hole. Growth of the other species in this study, Potentilla recta and Erigeron philadelphicus, was altered by either visitation or visitation plus handling. Visiting plants in order to observe them and touching them as one would when making morphological measurements can have important biological consequences. We suggest that plant ecologists treat repeated entry into a natural system as a research method, subject to the same scrutiny and justification as all other experimental methods.

Key Words: experimental methods • herbivory uncertainty principle • old-field • thigmomorphogenesis • visitation effect

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