Oswald Schmitz and I met early on a May morning at Greeley Memorial Laboratory in New Haven, Connecticut. The low concrete building squats like a bunker in a hillside garden looking over a derelict rifle factory. Built in 1959, Greeley is home to the ecologists of the Yale School of Forestry, where Schmitz is the Oastler Professor of Population and Community Ecology. He has been at the school since 1992 and in that time has authored 115 articles and four books with titles like Resolving Ecosystem Complexity and Trait-Mediated Indirect Effects.
Schmitz was wearing his field clothes: khaki pants and a sun shirt, the uniform of a professional bug catcher. He has a graying beard and wears glasses, and he always locks eyes with me when we talk. For the past twenty years, Schmitz has been studying abandoned farm fields in New England. A casual observer of one of Schmitz’s fields might see a beautiful sweep of grasses and wildflowers, probably surrounded by a century-old rock wall; Schmitz sees a landscape shaped by fear.
In the early 2000s, scientists made a series of discoveries that caught the public’s attention. Working in Yellowstone Park, researchers found that the reintroduction of wolves in 1995, along with a series of drought years, had decreased the elk population in the park by sixty percent. Relieved from constant grazing, the willow and aspen trees lining the riverbanks started to grow back, and with the trees came an increase in beavers. The beavers built dams, changing the course of the rivers. The trees reduced soil erosion, stabilizing the riverbanks. Other species experienced changes too: the wolves killed coyotes, resulting in an increase in small rodents and thus in predatory birds. Songbirds came back to the new stands of trees, and bears were attracted to the regenerating berry bushes. Although recent research has complicated this narrative – bears, for example, might kill more elk than wolves – the powerful image of the wolf’s ability to shape whole landscapes remains in the public consciousness.
These “trophic cascades” have featured in ecological discourse for a few decades now. Ecologists proposed in 1960 that the world is green because there are predators to control the population of herbivores; otherwise the deer, cows, and sheep of the world would eat every leaf in sight. However, this initial theory couldn’t explain the Yellowstone case: the wolves didn’t actually eat that many elk. Instead, they changed the way the elk behaved. They created a new mental landscape that the elk had to navigate, with peaks and valleys of risk and safety – a landscape of fear.
Over the decades he’s spent studying the landscape of fear, Schmitz has worked with moose, lynx, snowshoe hares, and black bears – anything that could be found in the boreal forest of Canada. Schmitz studied wildlife management as an undergraduate in Ontario, where he spent his summers tracking black bears. He had to catch the bears in traps before they could be sedated and tagged with radio collars. “You were never quite sure how long the chain was and how much play the bear had,” Schmitz remembered. “Black bears like to do fake charges at you, so you just have to guess how close you can get.” When I asked how close a bear had gotten to him, he held up his hand and spread out his thumb and forefinger. “Once you’ve been charged a few times you don’t get scared anymore.”
But today, Schmitz studies creatures that are quite a bit less intimidating: grasshoppers and spiders. “Some people study biology because they’ve fallen in love with a certain animal and have questions about it,” Rob Buchkowski, a fourth-year PhD student in Schmitz’s lab explained to me. “But Os starts off with questions and finds the animals he needs to answer those questions.” For the past two decades, Schmitz has been studying these insects to understand how the landscape of fear manifests itself across New England, and possibly the world.
In May, I joined Schmitz and Buchkowski on a trip to their field site in northeast Connecticut. The radio was playing country music, as mandated by Schmitz on all trips to the forest.
We were headed to the Yale-Myers Forest, an 8,000-acre property that was deeded to the Yale School of Forestry in the 1930s. A wealthy graduate had purchased the cutover land and abandoned farms for less than four dollars an acre. Today, the forest sells timber at a profit and supports recreation and research.
As we drove, Schmitz and Buchkowski explained our work for the day. We would be sinking cages into the ground to create insect enclosures, where their experiment would take place. The cages were cylinders of mesh screen and chicken wire, a meter tall and half a meter in diameter. “You can do any sort of ecology research with a trip to Lowe’s and some Ziploc bags,’’ Buchkowski joked. “A mass spectrometer is helpful too.”
We pulled off the highway just south of the Massachusetts border and drove on winding roads past small farms and cow pastures. After ten minutes the paved road ended, and white, diamond-shaped signs that said “Yale Forest” began appearing regularly on the trees bordering the dirt road. Schmitz abruptly turned off the road onto a track through the woods, and I saw a hand-painted sign labeled “Xmas Tree Rd.” Schmitz took the truck slowly over the bumpy track and we pulled into the field, trading the shade of the trees for an expanse of blue summer sky.
The forest’s agricultural history was evident in the rock walls that demarcated the edge of the field from the forest. Before its original owners sold out and moved west, the field was probably plowed for corn. Now, it sprouts spruce and fir for the Yale Forestry Club’s holiday sale, and also a collection of black mesh cages, their tops open to the sky with tall grass poking out. Schmitz led us over to the cages, the remnants of last year’s field experiments. We all wore tall rubber boots to keep off the ticks and poison ivy, both of which were abundant in the field.
Each cage enclosed a circle of vegetation. Schmitz rattled off the Latin names of every plant he saw: Solidago rugosa, goldenrod. A wildflower with numerous tiny yellow flowers and saw-toothed leaf margins. Poa pratensis, Kentucky bluegrass, “just like the kind in your lawn.” Hieracium caespitosum, yellow hawkweed. Taraxacum officinale, common dandelion. “Purple vetch, that’s… Vicia, Vicia villosa. I’m getting rusty!”
Schmitz focuses his research on goldenrod and grasses. The goldenrodflower has a sturdy stem and broad leaves that stick out horizontally. It spreads via its roots, so it often occurs in thick patches. Most grasses instead have a skinny stalk and thin, weak leaves. Goldenrod is competitively dominant, which means that if goldenrod grows next to a grass and both reach out their roots for resources and spread their leaves for sunlight, goldenrod will win. “Goldenrod should take over this entire field,” Schmitz explained to me as we inspected the cages, “but for some reason it can’t. I wanted to know why.”
A month later, Schmitz and I were back at the forest. We had set all the cages during a few days of backbreaking labor, in the heat and in the pouring rain. We had taken soil samples, identified and counted plant stems, measured the soil moisture, collected leaves for isotope analysis, and inserted strips that measured rates of nitrogen turnover into the soil. Now we needed to find some occupants for the cages.
“We couldn’t catch here for a while,” Schmitz informed me as we pulled off the main road into a hayfield, “because we caught about two thousand grasshoppers in one summer and crashed the population.” Several different fields have provided the insect source for Schmitz’s experiments – this one is called Strawberry after a long-removed sign that once advertised strawberries for sale down the road. Strawberry Field was newly mown, and as I walked through the grass bugs jumped out of the way of my boots.
Schmitz taught me how to sweep a net back and forth along the grass, as if cleaning the floor, and then quickly close it up to trap bugs inside. Then I carried the net over to him, where he had a tray full of glass jars and an aspirator made of surgical tubing dangling from his mouth. A mesh screen prevented Schmitz from sucking any bugs into his mouth, although he inhaled plenty of dirt. He opened the net and giant brown grasshoppers, black crickets, and bright green katydids immediately leapt out. Schmitz ignored the wave of insects jumping onto him and probed with his aspirator, sucking up grasshoppers until he had enough caught in the tube. He held them up for me to see: five grasshoppers, each about a centimeter long.
“Melanoplus femurrubrum,” he said proudly. “The field is crawling with them! This is the most I’ve seen in a few years.” He deposited them in a glass jar and called Buchkowski back in New Haven to tell him the good news.
Melanoplus femurrubrum, the red-legged grasshopper, is common throughout all of North America. Its legs do not actually turn red until the insect has shed its exoskeleton for the final time. For the young ones Schmitz and I caught, the key identifying features are a rounded head and a white stripe just behind the eye. After spending a day staring into a net searching for Melanoplus,I dreamt of the bugs crawling towards me.
In his first experiment in the fields, Schmitz reasoned that goldenrod was not taking over because it was being controlled by herbivory. If grasshoppers were constantly eating the plant’s leaves, its growth would be repressed, just as the elk in Yellowstone had browsed the trees. The only problem with his theory was that grasshoppers don’t like to eat goldenrod. Its leaves are too carbohydrate-rich; it would be like eating only candy bars. Grasses provided a more nutritionally balanced diet for the insects, but for some reason the grasses seemed to be getting along fine. There had to be a reason for the grasshoppers to eat goldenrod – there had to be wolves.
Pisaurina mirais not exactly comparable to the wolf of Yellowstone – it’s not even a lycosid, a wolf spider. Pisaurinalooks rather delicate, with a long thin body and skinny legs, but Buchkowski informed me that after a few years and a hefty diet of grasshoppers, members of the species could grow to be the size of your hand. I suddenly didn’t feel so safe standing in a field of waist-high grass full of spiders.
Buchkowski and I had come to this field to complete the final step of the experiment. A third of the cages would have no insects in them, a third would have just grasshoppers, and a third would have grasshoppers and a spider. We had to catch enough spiders to stock the cages, and this patch was the only place where we could reliably catch Pisaurinain large numbers. Schmitz had stumbled on it several years ago, and the owner of the field, Ruth, welcomed us enthusiastically when we arrived on her property.
It was easier to catch the spiders than I expected – we swept through the field with nets just like we were looking for grasshoppers, and then trapped the spiders in plastic cups. We drove back to Christmas Tree Field and added the spiders to the selected cages, injecting a dose of fear into the grasshoppers inside.
In Schmitz’s most famous experiment, he stocked the same cages with spiders and grasshoppers for three years in a row. He used two different types of spiders: Pisaurina mira, a sit-and-wait predator, and Phidippus rimator, an active hunter. When he compared his final measurements with the initial conditions, the changes were significant. Cages stocked with only grasshoppers were full of goldenrod. The grass had been eaten out of existence, as expected. However, the same was true in the cages with the active hunting spider. For some reason the grasshoppers had acted as if the spiders weren’t even there. In the cages with Pisaurina, the opposite was true. The goldenrod was significantly reduced, and the grass was thriving. The changes cascaded all the way to the soil level, where decomposition and nitrogen cycling had been slowed by the presence of the spiders.
Schmitz needed to know why the predators didn’t affect the grasshoppers in the same way – if there were a trophic cascade like in Yellowstone, all the grasshoppers should have shifted their feeding habits.
So he and his students set up a behavioral experiment: two tall cages with a grid drawn on them. One had Phidippusand grasshoppers, and the other had Pisaurinaand grasshoppers. After watching each cage for sessions of twelve straight hours and recording the grid position of each grasshopper and spider, he got an answer to his question.
To a grasshopper’s sensory system, Phidippus rimatoris almost impossible to predict. The spider wanders throughout the canopy of the plants, actively chasing down grasshoppers. The grasshoppers don’t feel safe anywhere, so they grazed on their preferred grasses and try to avoid being eaten.
Pisaurina mirais a much more patient hunter. It elongates its body along a stem of grass and blends in, waiting for a grasshopper to pass by. Then it strikes, grabbing the grasshopper with its front legs and overpowering it. This tactic drastically alters a grasshopper’s landscape of fear. Every blade of grass becomes a potential trap, and the grasshoppers seek cover in the large leaves of goldenrod. To deal with the stress of fear, they eat the sugary leaves – Schmitz measured the grasshoppers’ metabolic rates to prove so. He compared it to stress eating before a test. Their eating suppresses the goldenrod and prevents it from taking over the entire field, creating the balanced mixture of grasses and goldenrod Schmitz saw when he first started sinking cages in Christmas Tree Field.
Schmitz and I had to place ninety cages on my first day in the forest. To sink them into the soil, we had brought along dibble sticks, heavy metal spades used to dig trenches for the cages to slot into. “It’s brutal work to sink these in the heat,” Schmitz said. Later that day, as I was sweating and wrestling with a dibble stick in the hard dirt, Rob informed me that ten years ago Schmitz had sworn he would never sink another cage. We both looked over at him. His back was bent as he popped a cage into place and moved on to the next one, muttering curses to himself with a huge smile on his face.
When Adam’s not in the lab grinding up plants and bugs and putting them into tiny tin capsules, you can find him… oh wait, he’s always in the lab grinding up plants and bugs and putting them into tiny tin capsules. You can send him an encouraging email at: firstname.lastname@example.org.