A Landscape of Fear
By Bruce Fellman
Picture yourself in a meadow in high summer. Tall downy grasses, amid a colorful patchwork of buttercups, knapweed, Indian paintbrush and butterfly weed, sway languidly under a sultry sun. Monarch butterflies, hummingbird moths and droning bumblebees skitter atop the flowers in jittery haste for nectar. Thoreau, you fancy, could have been inspired in that meadow to write “The Inward Morning”: How could the patient pine have known / The morning breeze would come, / Or humble flowers anticipate / The insect’s noonday hum ... It is the picture of peace.
Don’t believe it. Just below the brilliant spatter of green, yellow, purple and orange are scenes of wanton violence that often end in sudden death. A spider, its eyes glinting in the light, feels movement and gnashes its jaws in anticipation, waiting to ambush a meal. A nearby grasshopper inches along, calmly munching on grass and a wildflower, unaware of its fate.
It won’t be long.
Or maybe it will. The grasshopper feels something, too, and stops its forward progress. Like a human being about to confront a predator of the same species around the corner, the insect starts behaving in a way we’d find very familiar. Inwardly and outwardly, the grasshopper seems to be afraid.
Dror Hawlena, a Gaylord Donnelley Environmental Postdoctoral Associate at F&ES, is piecing together how grasshoppers react to spiders that are intent on making a meal of insect prey. The grasshopper’s physiological response to potential doom is, perhaps surprisingly, “quite similar to our own,” says Hawlena. “Grasshoppers may have different glands, but their stress reaction involves the same hormones and endocrine mechanisms found in humans. In fact, you see this in all vertebrates—and we’re now finding it in invertebrates, too. It’s one of the most fundamental responses, and the most critical. If you can’t respond quickly in an emergency, you die.”
That response, it turns out, is critical in another way. “How a grasshopper avoids a predator can lead to the transformation of an ecosystem,” says Oswald Schmitz, Oastler Professor of Population and Community Ecology. Schmitz, who oversees Hawlena’s research, along with that of numerous graduate students and undergraduates, is a pioneer in an effort to understand how fear and other so-called nonconsumptive effects (NCEs) help determine the species composition, functioning and structure of the natural world.
At its most basic, Schmitz’s work, now in its 17th year in the meadows of the Yale-Myers Forest, involves monitoring and recording what happens during the summer in a series of screened-in cages, each roughly one square meter in area and one meter high. Inside these microcosms, various combinations of spiders, grasshoppers and plants go about their business.
“This is a wonderful model system,” says Schmitz. But, he quickly adds, the research is about much more than the natural history of the organisms that he and his colleagues have had under consider-ation since 1993. “What we’re getting out of these small enclosures are broad conceptual insights that we can apply to much larger systems.”
Natural resource managers and policymakers alike can use fear factors and other NCEs to predict, with a much greater degree of accuracy than has previously been possible, what our impact on the planet might be. Taking these fundamental but, until recently, overlooked processes into account can, says Schmitz, lead to better ways of carrying out such seemingly unrelated projects as the management of timber harvests in the boreal forests of Canada, the reintroduction of wolves and other top predators to the national parks and the implementation of biological control programs in agriculture.
“You want to avoid unintended consequences,” says Schmitz. For example, when wolves were brought back to Yellowstone, one impetus, besides the restoration of a missing part of the ecosystem, was to put the brakes on an overabundance of elk. However, things didn’t work out quite as planned. To be sure, the wolves killed prey, but in short order the elk dealt with their newfound fear of predation by changing their feeding patterns and moving away from the more open range areas.
“Elk are important for maintaining the species diversity and productivity of grasslands,” says Schmitz. “There’s evidence that both declined after the wolves were brought back.”
That evidence was clearly present in the enclosures in the Yale-Myers meadows. But instead of wolves, which were in short supply in Connecticut, Schmitz worked with Pisaurina mira, the nursery web spider and a dominant predator in the meadows. There were other spiders haunting the grasses, but P. mira, which employs a sit-and-wait hunting strategy, was simply the one he encountered first when he began his work. “Choosing it turned out to be pure serendipity,” he recalls.
The most common grasshopper in the field, and the one Schmitz used in the research, was a red-legged species known as Melanoplus femurrubrum. Its favorite food is Kentucky bluegrass (Poa pratensis). The insect was less inclined to eat goldenrod, which was also very common, or a variety of wildflowers, such as Queen Anne’s lace, that flourished in the area and were well-represented in the enclosures.
When he started tallying up his initial results, he noticed something odd. “The spiders had no net effect on prey density,” Schmitz explains.
If the predators weren’t eating many of the herbivores, then there shouldn’t have been much difference between the enclosures that contained spiders and grasshoppers and those with grasshoppers alone. But he found measurable changes in prey behavior when spiders were present, and, by the end of the season, there were significant differences in the diversity and abundance of grassland plants between the two kinds of enclosures. The spiders, he said, were leaving a “predation signature.”
Schmitz had stumbled upon the unexpected ecological consequences of a landscape of fear.