When James Cahill was studying biology at the University of Pennsylvania a few years ago, he was shocked to learn that some of the plants he had been studying out in the field had died.
About half of them, to be precise.
That's not exactly the sort of thing a budding ecologist wants to see, so he took it up with his faculty advisor, Brenda Casper.
Much to his surprise, Casper said she had seen the same thing with some of the plants she had been studying.
"It was quite troublesome," Cahill says, because he didn't think he had done anything to bring on their demise.
"It was hard for us to believe that 50 percent of the plants died in just three months," the length of the various summer research projects, he adds. They wondered if somehow, just by being there, they had managed to kill the plants.
Human Touch Has Effect
Cahill went on to the University of Delaware, but was haunted by the fact that so many plants had died while he was doing little more than measuring their growth. So he teamed up with Casper, and Jeffrey Castelli, a graduate student, to see what they could learn.
What they learned was quite shocking, Cahill says. All it takes to kill some plants is the touch of the human hand, according to their research, published in the February issue of the journal Ecology. But oddly enough, some others apparently profit from the human touch. And some apparently just couldn't care less.
The researchers studied six different plant species in 12 different plots in an old hayfield in Pennsylvania's Brandywine Valley. Half of the plants were left undisturbed to serve as controls, but the other half experienced a very simple, non intrusive "visitation" by the biologists once each week for eight weeks.
They didn't want to do anything that would dramatically affect the plant, says Cahill, who has since moved on to the University of Alberta in Edmonton, where he is an assistant professor of biological science. So the visit amounted to stroking each plant from the base to the tip, as though they were trying to measure its growth, a standard practice in field research.
What they had expected to find, he says, was that it didn't make any difference if they touched the plants.
"We assumed the data was just going to get thrown out," he says. "We didn't think it mattered if you handled these plants."
But by the end of the project, one species, Indianhemp (Apocynum cannabinum) had been devastated by insects.
"We found some plants with all the leaves eaten," Cahill says. In other words, they were dead. That wasn't always the case, but on average, he adds, the plants that had been handled showed severe damage.
By contrast, plants in the control group, which had not been touched, were healthy.
Oddly enough, another plant, Sulphur Cinquefoil (Potentilla recta) seemed to enjoy the human contact. Its leaves looked better than the control plants, and they were free of insects.
A third species, commonly known as Butter and Eggs (Linaria vulgaris) also seemed to do better, with fewer visited plants dying than their unvisited counterparts.
The other three species, Carolina Horsenettle (Solanum carolinense), Canada Thistle (Cirsium arvense), and Kentucky Bluegrass (Poa pratensis), did not seem to be affected one way or the other.
So what gives here? Was it really the human touch that killed the plants?
A Chemical Release?