A Landscape of Fear
Goldenrod is the dominant competitor in the meadow, and without herbivores to keep it somewhat in check, it eventually overshadowed its fellow plants. Paradoxi-cally, fewer grazers resulted in lower species diversity.
“Had I started working with the jumping spiders, I might never have encountered the behavior-shift story,” says Schmitz.
And he and Hawlena might never have become interested in looking at fear factors. The researchers already knew that grasshoppers reacted to the presence of a sit-and-wait spider by changing their behavior and moving elsewhere. The situation is very similar to what a human would do when walking down a dark street in a less-than-sterling neighborhood—first chance you get, you cross over to a seemingly safer and brighter location rather than stroll by that creepy-looking character hanging out on the corner.
“We don’t understand how grasshoppers detect the location of the spider,” says Hawlena, “but when we subject them to the risk of chronic predation, they react just like we would.”
Hawlena placed each overwrought insect inside a metabolic chamber—a transparent cylinder about 3.5 inches long and three-quarters of an inch in diameter—used to measure carbon dioxide output. “They breathe faster and their metabolism increases,” he explains.
If there’s no escape from a spider—Schmitz and Hawlena modeled this in field and laboratory setups, but it could easily happen in nature if spiders or any other predator, for that matter, were especially abundant—the grasshoppers do something else familiar. They crave carbs.
“Stressed grasshoppers need more energy,” says Hawlena, “and they get it from eating more goldenrod.”
Left to themselves, the grasshoppers would rather spend their time munching on nitrogen-rich grass that stimulates their own growth and reproduction, as well as results in high-quality fertilizer. But with spiders in the picture, their stress increases and their diet changes. “Their immediate needs become more important than the future,” says Hawlena.
They head for the goldenrod, whose chemistry—the carbon-to-nitrogen ratio is tilted toward carbon—provides the grasshoppers with the increased carbohydrates they require. So the insects cope with the metabolic demands of stress, but, according to research currently being reviewed for publication, there’s literally no free lunch. Grasshopper mothers stressed out by chronic predation risk having offspring that are 15 percent smaller than those of mothers living in spider-free zones. Also, mothers in the risk zone produce kids with poorer jumping ability. “This results in offspring that are more vulnerable to predation,” says Schmitz.
There’s another cost, as well—this one to the meadow. When it comes to fertilization, nitrogen trumps carbon. Compared to their grass-fed, living-is-easy counterparts, grasshoppers on the high-carb, anti-stress diet produce excrement that is harder to break down and less nutritious for plants. Grasshopper bodies even reflect this change in diet; in death, the corpses containing more carbon don’t offer the premium fertilizer benefits of those that had more nitrogen in them.
Schmitz estimates that grasshoppers, living and deceased, contribute between 5 and 10 percent to a grassland’s overall nutrient budget. So more spider-induced herbivore pressure on the goldenrod not only can alter the diversity and abundance of plant species in the meadow ecosystem, it can also result in poorer fertilizer. This, along with changes in the composition of the leaf litter and the soil bacteria, may eventually change the kinds and amounts of minerals available in the ground—and the varieties of plants and animals that can thrive.
And so, says Schmitz, “being scared to hell can significantly change an ecosystem’s nutrient budget and transform the way it looks and functions.”
Schmitz has synthesized nearly two decades of experimental and theoretical work in Resolving Ecosystem Complexity, a Princeton University Press book due out this summer. “The beauty of this work is that with the insights we’ve gathered, we can predict what an ecosystem will look like if we change it in some important way,” he says.
In the vast boreal forests of Canada, one company is already putting these insights into practice. Mistik Management oversees the planting and harvest of softwood trees on the company’s nearly 4.5 million acres of land in northwestern Saskatchewan. The typical logging operation, says Schmitz, who has worked with Mistik, involves clear-cutting and then replanting primarily with white spruce saplings. But there’s always been a problem with this strategy. “Aspen trees tend to come back first in a clear-cut and choke off spruce growth,” he says.
Based on his work in the enclosures, Schmitz had an idea. While moose and deer did a fine job of grazing aspen sprouts, the herbivores largely avoided young spruce, which are filled with noxious chemicals. But with wolves frequenting the clear-cuts, the fear-filled moose and deer stayed far away.
In an experiment, which involved acres instead of meters, Schmitz compared clear-cuts with areas in which Mistik cut most of the timber but left patches of standing trees. “These became refuges for the moose and deer,” he says.
The herbivores tended to avoid the open areas, and even when they entered them they were ineffective at keeping the aspen out. Not so when there were refuges. “By changing the way we harvested the landscape, we were able to let nature effectively manage for us,” says Schmitz. “And at no real cost.”
Mistik has since adopted the refuge technique. For this company, looking for a more sustainable way to manage its resources, the landscape of fear has become a landscape of opportunity.