Touching Discovery
by Michael Hofferber. Copyright © 2003. All rights reserved.

Talk a walk through a field and run your fingers across the leaves, or
bend over and lightly touch the seedlings emerging from the ground, and you may make the difference between whether those plants thrive or perish.

That's the implication of the findings by three ecologists in Pennsylvania who discovered that touching plants in the field affected their ability to repel insects.

James Cahill of the University of Alberta, and Jeff Castelli and Brenda
Casper of the University of Pennsylvania were conducting field studies of 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. Plants that they had not disturbed were faring much better.

Could it be that they, the detached and impartial scientific observers,
were making a difference in the plants' environment that affected their

To test their theory, the trio of biologists marked 605 plants in a dozen
plots across an abandoned hayfield in Pennsylvania's Brandywine Valley. They visited six of the plots each week, while the remaining six plots were left unvisited as controls.

When they visited the plants in the six test plots, the scientists touched
and measured the plants just as they would during field studies. They took care, though, not to damage the plants in any way.

One of the plant species in the plots,  Indianhemp, did not appreciate their visits. The Indianhelp touched by the visitors was hard-hit by
voracious insects compared to plants of the same species in the control group.

Sulphur Cinquefoil, however, seemed to enjoy the human attention. Plants of this species experienced less leaf loss when visited than when unvisited. Butter and Eggs (Linaria vulgaris), a third species in the study, also did better when touched. Fewer visited plants died during the study than did their unvisited control counterparts.

Three other species of plants in the study --  Carolina Horsenettle ,
Canada Thistle and Kentucky Bluegrass -- appeared unaffected either
positively or negatively by visitation.

The researchers presented their discovery in a report published in the
scientific journal Ecology (volume 82 number 2). "The long-standing
assumption that field researchers are benign observers is fundamentally flawed," Cahill commented. "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."

Cahill and his colleagues suggest that touch-activated plant responses may be causing of some of the effects experienced by plants in the study. These responses, documented by ecologists in many species, can include physical and chemical reactions to handling.

Touching a plant may result in changes to its structure, architecture and the toughness of its leaves. It could cause plants to release volatile
insect-attracting chemicals.

The scientists also noted that when nearby plants were trampled beneath their feet during visits to the test plots, the plants which were being studied received more light, which could prove beneficial to some 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 test plants more visible and available to plant-eating insects.

"Although questions remain about visitation effects," Cahill said, "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."

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