Touching Wild Plants Might Kill Them

James Cahill (now at the University of Alberta, formerly at the University of Delaware) together with Jeff Castelli and Brenda Casper (University of Pennsylvania) were studying plants in an abandoned hayfield and along a forest floor when they noticed that plants they had marked for study were experiencing extremely high rates of attack by insects. The scientists hypothesized that they, the human visitors, were somehow causing this to occur.

To test their theory, the ecologists marked 605 plants within 12 plots in an abandoned hayfield in Pennsylvania's Brandywine Valley. Six plots were visited weekly, while the remaining six plots were left unvisited as controls. When plants were visited, they were stroked once from base to tip, with care taken not to damage the plant body. This handling was designed to mimic what occurs when scientists typically take repeated measurements of plants in field studies.

One of the species studied, Indianhemp, was negatively affected by visitation, experiencing high rates of leaf area loss due to insects. A second species, Sulphur Cinquefoil, seemed to benefit, as the plants experienced less leaf area loss when visited than when unvisited. The third species, commonly known as Butter and Eggs, also tended to fare better when visited. Fewer plants of this species died when visited than did their unvisited control counterparts. Three other species in the study were not affected either positively or negatively by visitation.

"The long-standing assumption that field researchers are benign observers is fundamentally flawed," says Cahill. "The very act of conducting an experiment can alter experimental results, and the potential effects that researchers may have when visiting plants must be addressed in future field studies."

The scientists also note that when nearby plants were trampled, the plants which were being studied received more light, which could prove beneficial to plants. Competition for light can reduce the growth of many plant species and increased amounts of light can make some plants more vigorous. Trampling the neighboring vegetation could, however, also make the plants in question more visible to plant-eating insects. The ecologists were unable to find any clear pattern of traits which would make an individual species more or less vulnerable after visitation.

"Although questions remain about visitation effects," Cahill says, "we believe it is clear that field biologists working with plants can no longer assume that their activities in the field do not alter the biology of study organisms."

Their discoveries are published in the February issue of Ecology.